• Don't overlook our Welcome Package, it contains many links to important and helpful information about functions at VHT like posting pictures and sending PMs (private messages), as well as finding the parts you need.

    AD

1970 SL350 K0 Black Carburetors - SL3A

fxray

fxray

Veteran Member
Joined
May 1, 2022
Total Posts
800
Total likes
895
Location
Central Illinois
Although LDR already started a thread on the SL3A carburetors (and I already walked on it a bit), I decided to let him finish his thread his way. I figured I would start another one to document my journey going from this:



To this:



The work is mostly done, but I took a lot of pictures along the way. As you can see, the fuel tank is still off so that I can install the throttle cables, clean and line the tank, replace the petcock and cap, etc. This will be an after-the-fact story just dealing with the carbs. It might take me a while to tell it.

Ray
 
When I got this basket case, there were a half dozen of these small Rubbermaid containers full of rusty bits and pieces, all of them rare and valuable! The tubs are finally all empty and cleaned up, aside from a couple of them being used to store some new spare parts. One tub contained the stinky black carburetors:





As you may already know, the SL350 K0, introduced mid-year 1969, used CV carburetors (rubber diaphragms controlling the round slides) just like on the other CB and CL 350s. With the first major update, the K1 model, which took place halfway through 1970, the CV carburetors were replaced with flat slide units.

The CV carbs used on the K0 bikes in 1969 and early 1970 were painted black and were designated as SL3A. They were a one-year-only carburetor, making them hard to find and thus worth saving for use on an SL350 K0 bike. Never mind how rough they might be when found!



Evidently glue makes the rubber diaphragm shrink and wrinkle, making for a poor repair.





Whoever put the glue on the diaphragm also used black iron oxide socket head capscrews to attach the top cover. They didn't hold up well in 53 years of sometimes damp storage.



Most everything besides the top cover and the throttle shaft springs were painted black, including the brackets where the throttle cables attach. I have elected to leave these unpainted, but I did blast and replate them with zinc, as you will see later.

Note that the screw head was painted, but the surface underneath was bare metal, indicating paint after assembly. Even the throttle lever was painted, though not its return spring.



More teardown, a lot of which had to be facilitated by PB Blaster:







Even the screws attaching the float bowl were painted, further indicating that the carburetors were dipped or sprayed after being fully assembled. The outside of my carburetors were both encrusted with a brown, scaly coating, like gravel road dust in a film of oil that had baked solid over many years. It was very difficult to remove.



One slide was seized and had to soak in PB Blaster for several days before it relented and started to move. Once it came out, it was in nice shape -- other than the diaphragm.

One float bowl drain plug was nearly impossible to remove. More on that later.



I prefer the 5-sided, roof-shaped bowl gaskets, don't you? These were lovely!





In case you were wondering, there was no rubber O-ring used on the air/fuel screw, and I found it in scruffy condition:



Well, that's enough pictures for now. The forum wants no more than 15 at a time. I will have more later, including:
  • the rest of the teardown
  • the cleanup process(es)
  • what I did with the paint
  • the making of the replacement felt seals
  • the winding of the choke plate springs
  • the new parts used
  • problems with some of the new parts used
  • etc.

Ray
 
Last edited:
LongDistanceRider said:
It's evident the Black ?paint? was done after assembly like you've found.
Click to expand...
Jim, it looks black to me, but I don't know if it is paint. I read on-line that powder coating has been around since the late 1940s. If you google powder coating a carburetor, you will get a lot of hits. Lots of people are doing it. I don't know how to identify for sure the difference between paint and powder coating, but I guess it is possible that Honda did that with these carbs.
 
fxray said:
Jim, it looks black to me, but I don't know if it is paint. I read on-line that powder coating has been around since the late 1940s. If you google powder coating a carburetor, you will get a lot of hits. Lots of people are doing it. I don't know how to identify for sure the difference between paint and powder coating, but I guess it is possible that Honda did that with these carbs.
Click to expand...
I've talked with different coating and painting people trying to ID the coating to no avail.
One day I'll get it figured out I'm sure. :ROFLMAO:
 
Inside the float bowl, one carburetor's float assembly was in good condition; the other carburetor had one side of its float compressed. Engine backfire at some point?





Good:



Not so good:



At least these are available new, so I saved the one I found that was OK, as a spare, and bought new floats for both carbs.

Note the position of the jets and the spring clip plate that holds them in place. I have seen people posting here and there about the proper orientation.



The little rubber plug was present below the slow jet on each carb, but both were as hard as glass. I got new ones from SCI (Sirius Consolidated Inc.).





With the slides removed from the carb bodies, I reached down from the top of the carburetor with a whittled down popsicle stick (Wal Mart sells bags of them as craft sticks -- handy to have around for a variety of purposes) to drive out the needle jet holder. Chopsticks work well for this too, just don't ever use a metal punch.



They were both filthy, but in good condition otherwise. I have seen these all beat up and pitted before, but not here:





Same process to knock out the main nozzle:



A lot of these get destroyed when somebody tries to turn them with a small screwdriver and breaks off the spacer ears -- a definite No No!



The float needle seat came out without much struggle:





More to follow.
 
Last edited:
On the end of the throttle shaft where the throttle cable attaches, we have a nut, a split ring lock washer, the cable lever and the return spring. The end of the shaft is flat and the hole in the lever is a matching slot.







The little screws that hold the throttle plate can be removed fairly easily:



But the blind end of the screws have a tiny hole in the end and have been expanded to prevent the screws from vibrating loose and being ingested by the engine. When the screws are removed, they can raise a burr on the backside of the shaft. I dragged a small stone across that part of the shaft before removing it from the carburetor body. Otherwise, the burr would make the shaft difficult to remove and would damage the body as the shaft comes out.



Then I took a picture of the plate as I removed it, so that I could get the proper orientation when I put it back together.



The idle adjustment screw, the spring, and the lever end itself were all painted black:





On the idle speed adjustment end of the throttle shaft, there is this stack-up of parts: removable "E" clip, steel washer, felt washer, plastic bushing.



And, still inside the carburetor body was this plastic washer to be removed and saved for reuse:





With the throttle shaft deburred and removed, it is time to look back at the end where the cable lever had been. There is a small ring, shaped like a freeze plug but with a hole in its middle. This pressed ring needs to be pried out gently for reuse. Underneath it is a felt washer to be replaced.



Next parts to remove will be from the choke end of the carburetor.

More to follow.
 
Last edited:
The first thing that catches the eye here on the choke plate is the gaping hole where the spring loaded relief door is no longer spring loaded! It is loose and flopping around. This is not good, but, sadly enough, it is also not too unusual.

There are many tales of woe on the internet about this. Nobody sells a replacement choke plate unless you can find one on eBay, pay through your nose for it, and then pray that the spring is in better shape than the one you have. This, in spite of the fact that it will be the same age as the one you have.







With the plate removed and flipped over, we can see the tiny spring which has lost a "leg" from the end that used to contact the plate. This is a confounded little mess to say the least! I will revisit this later, with a repair method. Repair because -- like the choke flap itself -- nobody seems to sell a replacement spring that is completely correct for this application. I searched, believe me, I searched! In the end, I made my own springs.



With the plate removed and the shaft deburred, the shaft can be pulled out. Well, mine first required several applications of PB Blaster, interspersed with soak time intervals. Once it started to wiggle back and forth it came out fairly easily.



On the choke end of the carburetor, there are brass collars pressed onto the body of the carburetor, surrounding the openings where the choke shaft enters the carburetor. These can and must be gently removed to access the felt seals for the choke shaft. If you look closely at the above picture and use your imagination to get under the crud and dirt, you can see the brass collars. They will be more apparent in later pictures.

Note that this LH carburetor (as a rider sits on the bike) is the one with the choke control lever. There are two brass collars that are open in their centers on this carb, because the choke shaft sticks out both sides of the carb body. One end has the control lever, the other has a flat, threaded end to accept the linkage that connects it to the RH carb choke shaft.

The RH carburetor only has one "open" brass collar like the ones on the LH carb. The one on the other end of the RH carb is a completely closed endcap. There is no felt washer under the closed endcap, so you can leave that in place if it is in good condition.

The brass collars are very thin and very old, so be gentle. I used a light grip with fairly smooth pliers and wiggled them back and forth till they came loose. Use a soft touch here!

I was able to reuse all three of mine (as well as the endcap which I removed just to see what was inside). ;)

Well, easy peasy, the carburetors are all disassembled and the cleanup can begin!

In my case, before starting any cleaning, I put all the loose parts back into the Rubbermaid tubs shown at the start of this thread. I cleverly labelled one tub as RH carb parts, and the other one as LH carb parts to keep them separate.

Then I stewed for a while about the felt washers and the choke plate springs.

I did a lot of reading on-line about these items. One article that stands out above the rest is this one by Mike Nixon where he discusses the reason for the felt washers. He also has them for sale in case you want to buy some. He has recently become a member on this VHT forum and has been posting some good articles.

There are lots of stories about these felt washers. Some believe that if they are in poor condition, they will allow enough of an air leak to affect the air/fuel mixture. Others say, "NO -- they are only dust seals."

Some have bought felt furniture pads from Ace Hardware and punched holes in the center for the shaft to pass through. Some have cut up old felt hats to make their washers.

I was concerned about the actual sizes, both inside and outside diameter, as well as thickness. Then there is the worry about keeping the inside hole concentric with the outside diameter.

How about density of the felt? What to soak them with? Some say you must use heavy oil or they won't seal and your bike will never again run properly. Others use dielectric grease (or plumbers silicon grease which is the same thing).

I don't want to discount or disparage what others have said and done, and I don't want to start another felt washer discussion. There are tons of those already. I will merely explain what I wound up doing.

More to follow.
 
Last edited:
Nice documentation of each and every step Ray. Like Mike I feel those felts are purely dust seals, still important for shaft and bore protection.
The felts are soaked 90w gear oil. I stopped by an auto repair shop with a small pill bottle and they filled it while laughing.
 
I have a friend who has a small machine shop in his garage. Discussing the felt washers with him, he said, "Well, it's easy. What you need are some hole punches to make the outside diameter sizes you want. Then, you can make a punch that slides through the center of the larger punch to cut the center holes and keep them concentric."

So, with his lathe and some round stock he had on hand, he made me some punches. I then used them to make some felt washers. Here is the process we used:

I bought this felt "blanket" at my local Ace Hardware Store. As you may suppose, this product is "Hecho in China para Ace Hardware Corp., but yet is located somehow in Oak Brook, Illinois, USA. At least it was cheap. It cost about US $5.00 for a lifetime supply of felt washers, leaving plenty of the blanket intact for my estate sale later on.



I measured the carburetor body openings and the throttle and choke shaft sizes as follows. There are two sizes of openings for the throttle shaft, but the openings for the choke shaft are all the same size.
  • Throttle: .490" OD x .300" ID for one end
  • Throttle: .450" OD x .300" ID for the other end.
  • Choke: .428" OD x .265" ID
Before anyone screams that I measured wrong, I sized my washers to provide some "crush" on the shaft, about .010" as I recall. It didn't matter to my friend the punch maker. He just said to tell him what I wanted to the nearest thousandth of an inch.

For those who prefer metric:
  • .490" = 12.45mm
  • .300" = 7.62mm
  • .450" = 10.16mm
  • .428" = 10.87mm
  • .265" = 6.73mm
Here is what the typical punch looks like, with the cutting edge up:



Here it was in process, as I used his hand arbor press to cut some felt pads. You could hit the punch with a hammer, but a press really makes it easy. Note the piece of black plastic sheet under the felt to protect the cutting edge of the punch, which was razor sharp and worked quite well. This wasn't his first rodeo with making punches of a particular size and shape:





The felt is green on top, but white on the bottom because it has a peel-off sheet. The pads are self adhesive when the white part is removed.

I used a handy Allen wrench to poke through the punch body and eject the pads as I cut them.



But, what about the concentric hole in the middle of the washer? For that, he made a second punch of the proper diameter, and longer than the first punch:



He made the smaller punch to be a nice, sliding fit inside the first punch. In this picture it appears that the small punch has a damaged edge, but that is only a little residue of green felt.

I must admit that I clumsily dropped the punch onto the concrete floor at one point, and he was gracious enough to recut the sharp end without berating me.



To make the washers, I first made a bunch of blanks with the large punch, ejecting each one with the Allen wrench. When I had a bunch of them done, I put them one by one back into the end of the big punch, inserted the smaller punch, and cut the center holes.

Here you can see why he made the inside punch longer than the outside punch:



And here you can see a slug put back into the big punch:



Here it has been through the press:





The first felt washers I made were for the smaller OD throttle shaft. I made a good quantity of them, because it was actually kind of fun. When I had enough, my friend modified the outside punch on his lathe to make it the proper size to cut the washers for the other end of the throttle shaft.

I suppose it is a shame that the punch as I first used it is gone forever, but -- hey, we were not going into business to sell washers in competition with others who already sell these things.

He then made a pair of punches for the choke shaft washers and I proceeded to make a bunch of them.

Let me just say that, if you think the prices on-line are high at $5.00/washer, consider the time involved with making, packing and shipping them. I don't have any interest in doing that, and I don't think $5.00 per washer is exorbitant. Of course, at $5.00 /washer, it will run you $30.00 to do the six washers in the two carburetors.

Why did I make my own washers, then? I wanted to make them to my exact dimensions, and I have a friend who was willing to help me. I also enjoyed the process.

More to follow.
 
Last edited:
LongDistanceRider said:
Nice documentation of each and every step Ray. Like Mike I feel those felts are purely dust seals, still important for shaft and bore protection.
The felts are soaked 90w gear oil. I stopped by an auto repair shop with a small pill bottle and they filled it while laughing.
Click to expand...
Jim, I think I agree that they are purely dust seals, but just in case, I wanted them to fit well, with a little bit of crush. If they are only dust seals, a good fit is still good, eh?

I have some 80/90W gear oil around that I use in my BMW and Triumph gearboxes, but I also have some silicone grease. I thought it seemed to be the right consistency for these washers and that is what I used (YMMV).

In one of the old manuals, Honda recommended silicone grease for lubricating the starter clutch rollers. Plumber's grease for use inside water valves, and dielectric grease are both examples of silicone grease. I have some of both on hand.

Honda included this note in the manual:

Characteristics of the silicon grease: (1) Little change in quality from low temperature through high temperature (2) Temperature causes very little change to the friction coefficient on coated metal surface.​
​
Without reopening that whole can of worms, which has been thoroughly discussed on the old forum and maybe on here too, I decided that silicone grease would work just as well, perhaps better, than heavy oil. Again, if we are only dealing with dust seals, does it matter that much?

I should add that some of the felt seals I found in the carburetors pictured in this thread were mostly powder with a few remaining strands of felt intact by the time I took them out.
 
With the felt washers sorted out, I started trying to clean the carburetor parts by using a dash of Pine-Sol in water in my ultrasonic cleaner. I ran it for a long time, maybe two hours at high temperature with little results. The bigger surface areas were coming clean, but not the little nooks and crannies, of which there are many.

I scrubbed with various brushes in my parts washer, which is filled with mineral spirits, but the brown dirt imbedded in the black paint would not clean off.

Rinsed the parts with water and put them back into the ultrasonic with a little stronger mix of Pine-Sol.

At some point, the black coating started to come off the float bowl. I thought I would just strip and paint the float bowls but try to save the original black coating on the main carburetor bodies. This was a no-go. The coating started to bubble on the main body of the carburetor, but the brown dirt would not budge.



So . . . out came the paint stripper:





In short order, the dirt and the black coating wrinkled up and was gone. I followed this up with more time in the ultrasonic bath with Pine-Sol and water:



But they need to be black for this K0 bike, so I started thinking about what to use.

More to follow
 
LongDistanceRider said:
Yeah, what to use for Black that is impervious to fuel. I'm thinking I'll be using CeraKote.
Click to expand...
From what I see on-line, Cerakote looks like a good option. My trouble with that is that I cannot do it myself. I don't want to turn these ancient, one-year-only carburetors over to somebody to let them blast and finish them. If something went wrong, they would be too hard to replace.

A google search for black carburetor paint yields lots of products, but I chose to use something I already had on hand. I'll just have to see how it works out.

Eleven years ago, I painted the barrels (cylinder block) on my Triumph TR6R with POR-15 Engine Enamel. Today, 12,000 miles later, they look the same as when they were first done, except with slightly less gloss. Because the fins on those cylinders are so close together, and so deep, I had trouble with spray paint. I couldn't get the base of the fins coated without getting too much buildup on the outer edges of the fins. For that reason, I brushed the paint. It is self-leveling and they recommend either brush or spray application.



From my research back then on the POR-15 Engine Paint:
  • The Engine Enamel that they sell is good up to 350 deg F.
  • It is a gloss paint.
  • The Engine Enamel should be allowed about a week to air cure.
  • It is an option to use POR-15 as a base coat to protect from rust or corrosion, or the Engine Enamel can be applied without primer to properly prepared bare metal.
  • (Proper prep includes use of their products Marine Clean to degrease, and Metal Prep & Ready to derust and lightly etch the surface.)
Still on my shelf, and still seemingly viable:



The thinner is good to have in case you get the paint where you don't want it, like on your skin (even if you wear gloves, accidents can happen). Without their thinner, it is hard to remove. You can also use it to thin the Engine Paint for spraying, but I am not set up to do that. These carburetors are mostly hidden on the bike, so an imperfect paint job will not be noticeable. They do need to be black. I elected to brush them, just like I did with my Triumph cylinders.



The Prep and Ready directions call for keeping the clean surface wet with this liquid for twenty minutes, then rinsing thoroughly with water, and then getting the surface bone dry before painting. Do it outdoors!

I spent some time trying to seal up the carburetor before using the Prep & Ready. I don't know that it would hurt anything inside, but I didn't want to chance it. Here is one sealed up and one to go:



With a carb in a plastic dish, I kept brushing the surface, keeping it wet.





After 20 minutes:





This old hair dryer gets a ton of use when I am zinc plating parts. I like it because it doesn't get as hot as a normal heat gun, yet does a good job of drying. I used it here.



Here's one with a perhaps less than perfect paint job, and one left to go. One coat was sufficient.





While painting, I used the old hardware to attach the old top cover and the float bowl. I did not plan to reuse the old screws or the top cover, so it was good for protecting the top of the carburetor.

More to follow.
 
While the paint was curing, I blasted and plated some of the little stuff. This mixture screw tip is a little pitted, but I think it will still work. I prefer not to paint the part that shows, even if that is not authentic to the way I found it.





Same for the idle adjustment screws:





And the same for some of the other bits that I plated but won't paint. They will be tucked in where they don't show and they are mechanical bits that would probably work better without paint. I didn't have to blast or plate the slide springs. They cleaned up well in the ultrasonic cleaner.



The choke linkage strap was mangled, but luckily I had a spare stashed away. It was rusty and dirty but it cleaned up well and it actually works.

More to follow.
 
Really a fantastic write up and photo documentation. Thank you for all of the details. Look forward to completing the story.
 
The diaphragm in both of these carburetors were toast. They both had been "repaired" with some sort of glue:



When I got my CL350K3 back in 2010, the diaphragms were in worse shape than the ones here! The only reason I can say that is because the PO had dipped both carburetors in a dunk can of cleaner without taking them apart. He dissolved the diaphragms completely, and they were missing altogether. Carb dip used to be powerful stuff!

Back then, I may have been misinformed, but I had read that you had to cut the plastic ring on the slides to replace the diaphragms. The whole process looked a little shaky to me. I wound up going to a salvage shop and buying several Keihin carburetors for spares. Two of them had good, complete slides. Believe it or not, I am still using those slides to this day. I opened those carb tops last year to check them, and they still look like new -- soft and pliable, with no splits, holes, or tears!

This time around, I bought new ones from S.C.I. (Sirius Consolidated Inc.). I believe this was following a recommendation I read on here from Long Distance Rider. They are not cheap but they are very good quality parts. Cutting of the ring is not required!



Here's one slide redone and the other one waiting. I probably found it more difficult to remove the old diaphragms than to install the new ones:



Following another tip from LDR, I used the edge of a plastic credit card to help push the raised lip of the diaphragm into the groove above the plastic ring on the slides. This worked quite well, and was made much easier yet by using some P-80 Lube. I have been using from this same bottle for over ten years now, and it has allowed me to assemble things that I am convinced would not have been possible without it.



When fitting the diaphragms, it is necessary to test fit the slides back into the carburetor body. These diaphragms have location tabs that match the notch in the carb body, just like the original ones did. The diaphragm needs to be positioned on the slide such that the tab fits into the slot and the two holes in the bottom of the slide face toward the choke end of the carburetor body as shown here.



One more point in passing: you may be able to see the crosshatch marks on top of the carb body. I used some wet/dry sandpaper on a piece of plate glass to burnish the tops of the carbs and make sure they are flat. This should help them seal better.

At this point, I stowed the slides back into the previously mentioned plastic storage tubs and continued letting the paint cure on the carburetor bodies. I also kept thinking about those little choke flap springs, and about how Jim had induced me to spend over US $52 on slide diaphragms! :LOL: Thanks, Man!

More to follow.
 
Last edited:
You will want to add a drop of glue to the diaphragm to lock it onto the piston. I've found that the piston will rotate sometimes.
I'm working on a viable choke flap replacement part, complete assembly.
 
LongDistanceRider said:
You will want to add a drop of glue to the diaphragm to lock it onto the piston. I've found that the piston will rotate sometimes.
I'm working on a viable choke flap replacement part, complete assembly.
Click to expand...
Thanks, Jim, but I don't think I will need to in this case. The P-80 Lube that I used has an advantage over the use of silicone or other lubes in that, once the parts are in place, the P-80 "flashes off" and sets almost like a glue. This makes it work well on lip seals that have a rubber or plastic outside diameter.

A smear of P-80 around the OD will often allow me to push a seal into position with my thumbs rather than tap it in with a driver. Despite how easily it goes in, the lube quickly dries and keeps the seal from sliding back out as it might do with silicone or Vaseline. It should act the same way here. I did try to spin the diaphragm once in place and it was locked firm.

In any case, the carbs are already back on the bike and I'd rather take my chances than to wrestle with the air boxes and cross bolt again. :confused:
 
Last edited:
^^^ Speaking of stuff to buy, the hardware on my SL350 K0 carburetors was in rough shape, so I ordered this kit from 4 into 1:



I guess I wasn't paying too much attention when I did this. See anything wrong?

All the screws have split ring lock washers on them, which were never present on the original Keihin carburetors. But, no big deal, the kit is cheap. I could just remove the lock washers and throw them away.

Do you all know what "SEMS" hardware is? I was about to learn.
Here is a clear explanation of "SEMS."

So, the lock washers could not be removed. At least the fastener length was extended to make up for the thickness of the washer, which should not have been there in the first place. The screws all fit OK, they just didn't look quite right.

All of this was my own fault. The 4 into 1 site shows the kit clearly and even explains the preassembled washers. I just missed it. The main reason I wanted the kit was to get new drain plugs for my float bowls.

Another mistake on my part. Their site says that the kit fits Honda CB/CL/SL350 twins from '68-'73, but it actually does not. I should have noticed the difference in the drain plugs. Here is what I needed vs, what I got:



Later bowls have a port-contour to accept O-ring plugs. Earlier bowls like mine were flat in the plug seat area to accept aluminum crush washers. The older plugs like what I wanted are NLA and so are the crush washers. Stealers on eBay are asking crazy prices for them. That made this set from 4 into 1 look attractive.

Here's the rub, the big 12mm diameter head fouls the edge of the bowl before the plug can seat properly. The original head was 8mm in diameter. On the later style float bowls, the plug boss extends further out to be flush with the side of the bowl.



This may not seem like a big deal, but I was getting frustrated. The fastener kit was a minor part of my full bill from 4 into 1, which ran to over US $220 for carburetor parts.
This included:
  • Float sets and pins
  • Rubber intake boots
  • Chrome carburetor top covers
  • This screw set
This was in addition to the parts from Sirius Consolidated Inc., which ran to US$88.00 for:
  • Slide diaphragms
  • Float bowl gaskets
  • Rubber plugs for under the slow jets
I wound up deciding to use the screws with the lock washers, even though it voids the original appearance. However, I had to do something about the plugs.

The thread size for the float bowl plugs is M6-.75, which is an unusual pitch. I looked all over at local stores but could not find a tap. I ordered one from eBay which bounced around for a week before it got from New Jersey to Illinois. Ironically, the tap was the same price as the entire little hardware kit I had bought!





At least I already had the right size drill bit to go with the tap.





You may be wondering how an M6-.75 tap will fix a drain plug whose head is oversize?

I needed the drill and tap so my machinist friend (remember the guy who made the felt washer punches?) could chuck a scrap of round stock into his lathe and drill and tap it for M6-.75

Then he could screw the drain plugs into the hole he had just made and turn the head diameter down to the proper size.




And the NLA aluminum crush washer? Although the length of the old vs. new plugs are slightly different, they are the same diameter shank. The aluminum crush washer will work with the new plug. I found five of the washers in my spares box from when they were still readily available. Even McMaster-Carr doesn't show this size.

More to follow as the saga continues, but it is nearing the end.
 
LongDistanceRider said:
If you want to redo the screws to the right ones look here https://belmetric.com/screws/pan-head/?FilterSpecification=JIS__B1111||Special
Click to expand...
Thanks, Jim. I have heard that BelMetric is a good supplier, and I see that they sell that elusive M6-.75 tap that I was looking for a while back. Their JIS stuff doesn't seem to have the little identifying dot on the pan head machine screws, so there is that.

If I were concerned enough to get the absolute correct look, I would blast and plate the original hardware, like I did for my Triumph and my BMW Airhead, but I don't plan to do that. I'm OK with what I got from 4 into 1, now that the bowl drain plugs are modified to fit.

As far as the other 4 into 1 hardware goes, the JIS dot is present and the fasteners are zinc plated steel (not stainless), which I prefer. I just thought I would mention the SEM split-ring lock washers for anybody else who might buy that kit. In the grand scheme of things, not a big deal, and my carburetors are already in place.

Next up will be a discussion of the choke plates, but right now it is time to mow the grass.
 



Choke plates on old Hondas have been a sore spot for a long time because of the tiny torsion springs that tend to break. When they do, they get sucked into the air stream feeding the engine and can lead to catastrophic engine failure. Lots of people have posted their method for repair, but I think @Dunk had the most helpful idea.

Click HERE to go to his thread. He did a nice repair on a choke plate for his CB360, and described the wire he used; showed the spring he made; explained how he removed and replaced the pivot pin; but left out one part that drove me crazy for a while. He did not show exactly how he wound his spring. I even sent him a PM to ask, but I don't believe he saw it. So, Dunk, if you see this, thanks for the info you shared, but I am still curious as to your method.

I thought about this for way too long, but I finally went ahead and ordered some music wire. I knew about piano wire, but I had never heard of winding springs from this stuff before I read Dunk's thread. I got mine from McMaster-Carr:





I figured that 12" lengths ought to be plenty long enough for the tiny springs I needed to make, and 100 pieces ought to give me enough to practice and make as many mistakes as necessary to figure it out (plus that is the minimum order quantity).

After doing some measuring, I came up with the following:
  • Pivot rod = 1.5mm diameter, or .060"
  • Outside of coil = 3.0mm diameter, or .118"
  • Wire = 3.5mm diameter, or .014"
  • Number of coils on the old spring = 10
  • Leg length on old spring = 5mm, or .190"
The legs need to be tangential to the body of the spring, not axial, radial, or radial-over center. If you don't know the meaning of tangential, you could watch this short video, or just look at the spring pictured on the left here:


The reason this is important is that we need some way to hold the first leg of the spring as we wind the coils that will keep the first leg tangent to the coil. We can't just have a mandrel with a hole through the middle, where we could poke the wire through the hole and start winding the wire. That would make the leg radial to the coil, which wouldn't fit the choke plate correctly.

I watched this next guy's video and got an idea that I tried, but it wound up as a fail. You might want to watch it anyway, but I recommend watching it on double speed. ;)


This is what I wound up trying to do after watching his video. I dug a couple of metal bits out of my scrap bin, a piece of 1" square aluminum bar stock and a piece of 1"x1/8" steel strip.

I drilled a 1/16" hole in the center of the end of the aluminum bar, and through the flat side of the steel strip. Then I reversed the drill bit in the chuck so the smooth end was down, still centered over the hole in the aluminum bar.




I also drilled and tapped a hole in the steel strip for a small bolt with a washer to grip the end of the music wire. Ignore those other extraneous holes -- remember, I fished this out of the scrap bin,



With the small bolt run in and snugged down, I flipped the strip over and cut/filed the end of the bolt smooth to the surface:



The washer was necessary to grip the small diameter wire to the steel strip. I also had to bend a hook in the wire and place it under the washer. Otherwise, the wire kept slipping out during the winding process.

Next, with the wire captured under the washer, I put the steel strip on top of the aluminum bar and lowered the drill such that the smooth end of the bit went through the the steel strip and into the aluminum. I locked the quill of the drill in this position.



I gripped the other end of the wire with some small vise grips:



Then I got my wife to hold the vise grips and pull lightly on the wire as I held the steel strip flat against the aluminum bar and started winding it around the drill bit.

(Here, the wire isn't quite taut because I had to use one hand for the camera.)



With the wire positioned as shown, the spring would be wound in the proper direction, rotating the steel strip clockwise around the drill bit. The difficult part here is that you have to pass under the long end of the wire each time around. If you let go of the steel strip, the spring wants to unwind, messing up the shape of the coils.

I really thought this would work, and it almost did. After a half dozen attempts that ended with the wire pulled loose or broken after only a few turns, I managed to make one spring. It was a rough first attempt, but was not usable:



As you can see, the coils were too loosely wound next to each other, and the spring was too wide to fit properly. In the above picture, the leg pointing to the right needs to nestle down into the recessed area in the choke plate between the two screw holes. Otherwise, when installed onto the choke pivot bar, the spring leg would be damaged or cut.

Although this attempt failed, the relief door did spring properly, so I figured I was on the right track. I just needed to improve the spring winding process.

Next up: a better method.
 
Last edited:
Here is a better attempt:



Comparing this spring to the one in the previous post, you can see that the coils are closer together, making the spring short enough to fit the choke plate properly. The leg of the spring fits into the little relief trough in the choke plate and will not be damaged when the plate is installed onto the shaft.

Inspiration for this method came from watching this man's video:


He used a power drill and captured the first leg of the spring between the mandrel and one jaw of the Jacob chuck. I decided to use an old hand-cranked drill for better control, and mounted it in my vise:



I still used the vise grip pliers to hold the loose end of the music wire. I could hold the pliers and turn the drill crank at the same time. It was pretty easy to count the coils as the short end revolved, but I may have wound up with only 9 coils due to spring-back. I called it "close enough."

This method was easy. I made six springs in short order, kept the two best ones for my carburetors, kept the two next-best for the spares box, and pitched the two worst ones. There is plenty of straight wire left for future use.

I followed Dunk's method for D & A of the choke plate pivot pins, and followed his lead to use some JB Weld to close up the "open" post on the choke plate.

In this picture, the springs are OK, but the one on the left needs to be slid a bit to its left on the pivot pin before installing the plates onto their bars.





More to follow.
 
The good part about my taking so long with those tiny springs was that the black paint had plenty of time to cure. :LOL:

I am still hoping that the springs I made were the correct way to go. I strongly considered eliminating the springs altogether and permanently closing the choke plate flaps with some JB Weld. Then I would use the choke very briefly when starting the engine, and perhaps not close it fully. I still think this would be an option.

Googling to ask why Honda used the spring-loaded flaps in the first place, one theory that I saw more than once was that these were "relief valves" to protect the carburetors from being blown clean off the cylinder head in the case of a backfire. :ROFLMAO:

This is one example of the sort of BS you can read on the internet. Take a look at the placement of the choke plates and you will find that a backfire would tend to close, not open the spring-loaded flaps, as well as tightly closing the entire choke plate if the choke was already in use. (See pictures further down in this post.)

I also considered soldering the flaps shut, with the notion that this would be reversible later if desired. That is not the way to go. I know this because I actually tried it on a choke plate from a parts-only junk carburetor to see what would happen. Just as the solder started to melt, so did the choke plate itself! :rolleyes:

Once over the hump with spring manufacturing, I started to reassemble these SL3A carburetors.



I made this float gage a long time ago, to measure a 26mm float dimension. It sits on the bottom surface of the carburetor body, and clears the jet clip plate, while touching the top of both floats simultaneously. A couple of years ago, I modified it to 27mm and adjusted my CL350 carburetors accordingly. I think that works better for both starting and running.



Both of these are 350 float bowl gaskets.



The "house shaped" ones are easier to install. I think I got them from Sirius Consolidated Inc. (SCI) as a set of four pieces (because they list it for the four cylinder bikes as well as twins). I would include a link, but their website doesn't seem to work that way. The number on the kit I got is:

18-2634 = NTMOR-030 = K10BF
Listed to fit:
XL250 1972 - 1976
CB350 Twin
CL350
SL350 K0 1969 - 1970 (check your application)
CB500 K Four 1971 - 1973
CB550 K 1974 - 1976
CB500 F SUPERSPORT 1975 -1977

The round ones will work, but it is tricky to keep them in the groove on the float bowl as you install the bowl while compressing the clip plate that holds the jets in place. Then you have to hold the bowl in place while you get the screws started. It is sort of a juggling act. The round gaskets, I believe, are some old aftermarket ones from the Honda dealer.

I seem to remember that the Honda OEM sets are good, and include the slow jet plug and the drain plug gasket, but I can't say for sure. Partzilla shows the float bowl gasket as NLA and David Silver shows a picture of the round O-ring.

The rest of the reassembly went pretty easily. I guess I didn't take any pictures of the brass jets all laid out in a row, but I did get them all cleaned up and was able to reuse them. The idle jets were the worst. They required extended soak times in my ancient Berryman dip can, ultrasonic cleaning, numerous pokes from individual strands of wire cut from a piece of lamp cord, and lots of air blasting. They were both plugged solid.











Eventually, those stackable Rubbermaid containers that once held dirty, stinky carburetors were empty and cleaned out:



There was one minor issue when I went to install the carburetors back onto the bike. I thought I had plated all four clamps, but I had only done the ones for the air filter end of the carburetors. The ones for the inlet boots still looked like this:



I only need two of these, with the other two as spares, but I might as well do them all at once. Here they are blasted:



And zinc plated:



And finally back in place:





 
Last edited:
With the carburetors back in place, I dug out my NOS, gray, eBay throttle cable from the box on the shelf. It probably should have been put on sooner, but I fiddled around with it and got it installed tonight. Fortunately, it seems to be the right length for this bike. I bought it a long time ago at a high price, and the window for returning it has long been closed.







With the cables hooked up to the twist grip and to the carburetors, I could synchronize the carburetors. I backed off both the idle adjustment screws to where they were .002" feeler gage gap away from their stops.



Then, with my right hand on the twist grip, and a finger of my left hand on the LH throttle lever, I watched the throttle lever on the RH carburetor. As I rotated the twist grip, I was striving for the point where I could feel the LH lever move my finger at the same instant that I could see the RH lever move. To accomplish that, I adjusted the cable ends where they screw into the arms on the carburetors.



When it was right, I locked down the cable adjuster jam nuts.

Then I went back to the idle speed adjuster screws:



I turned them down to where they touched their stops and then gave them an additional turn and a half. The mixture screws are one turn out from bottomed. This should be enough to get the engine to start and run.

This method is not original with me, I learned it long ago from our friend, Bill Lane (RIP).

Notice the blue cap in the picture. There is one on each side for use when (if) this engine runs and I can put a vacuum gage on it. The blue is because it is a high temperature silicon cap. The black nitrile caps don't survive this close to the cylinder head, but these are good to about 600°F.

One weakness of the CB/CL/SL350 engines is the lack of vacuum ports for balancing the cylinders. My machinist friend and I added these before the head went back on:









With the high temp silicone caps, I can leave the vacuum taps installed, and not have to remove and plug the ports.

So, at this point the carburetors are installed and synchronized, but not yet balanced.

When the engine is operable, I will adjust the individual mixture screws to achieve the highest vacuum. I will adjust one carburetor while the other port is capped. Then I will cap the one I just did and adjust the other one. My vacuum tool has a single cylinder "gage" setting for doing this step. The readout is in inches of mercury vacuum. The number to the right, 1020, is engine rpm.



Next step will be to put the tool in "balance mode" and hook up a hose to each port. I will then adjust the idle speed screws such that the cylinders are balanced at idle speed.



In this picture, the tool is in balance mode and is telling me that the cylinders are pulling equal vacuums to within 1 millibar. The number to the right of the display (1120) is a very accurate readout of engine rpm. The tool gets this reading by counting vacuum pulses. This is a good check to verify the bike's tachometer. I will also adjust the final idle speed at this step.

Note the little happy face on the display. When the cylinders are balanced within about 5 millibars, the tool says that is good, and gives you the happy face.

Finally, I will elevate the speed to about 3,000 rpm and lock the twist grip there by engaging the Vista Cruise Control.



I will then balance the cylinders at this elevated speed by adjusting one of the cable adjusters (and leaving the other one alone), until the cylinders are balanced at elevated rpm.



A final check will be to test balance again at idle speed.

I hope that explains the difference between synchronizing and balancing. I often read about "bench synching" these 350 carburetors, and there is really no such thing possible.

When these engines are synchronized and balanced as described here, there is night and day difference between doing it this way and doing it by ear. Sadly, the gage I showed here is no longer available for sale. It is the Grok Harmonizer, which you may have read about on the advrider forum. However, the principle is the same when using other similar gages.
 
Last edited:
Looking at this thread, it occurred to me that you didn't do a full thread in Projects and Builds for this bike. With the quality of all your work, it would have been great to see it all come together.
 
ancientdad said:
Looking at this thread, it occurred to me that you didn't do a full thread in Projects and Builds for this bike. With the quality of all your work, it would have been great to see it all come together.
Click to expand...
Well, Tom, that's nice of you to say. Here is my rather long answer as to why I didn't write a build thread on my SL350 K0. The main reason is that this bike came to me with no title. I know the two previous owners. Both are friends of mine, and I believe the title was lost by the owner before them, if you can follow that.

I don't believe this bike was ever stolen because it didn't show up on any lists as such in a search. I think the original owner used and abused it and put it away years ago and simply lost the title. These 350 Hondas were considered expendable by many people back then.

When I got the bike, I recognized it as a unique, one-year-only model, and thought it was worth saving. In Illinois, to get a title, the bike has to at least look like a viable motorcycle. I was told that it doesn't have to run or be ridable, but it needs to look like it could. Otherwise, the officials won't mess with just a pile of parts.

So, what does lack of title have to do with a build thread? I got the bike like this:



I didn't want to invest too much money and time if I wasn't sure I could title the bike. In putting it together, I walked a fine line between trying to make it look like a viable bike, yet skipping some relatively external things that I could come back to later if I was able to get a title. Doing a non-sequential build like that does not make for a logical build thread. That is why I decided to only do some modules about things like the unique K0 exhaust system, the unique SL3A carburetors, the starter motor, etc.

I am happy to say that I now do have a clear Illinois title in my name and am now ready to go back and fix some things that I would not have done if the bike was to be for display only.
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom