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Carburetor brass observations

Emlupi

Veteran Member
Joined
Sep 9, 2024
Total Posts
417
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573
Location
CT
Happy Thanksgiving everyone, I hope everybody is having a great day with the family.
Just a couple of observations I made between “kit” and OEM emulsifier tubes for 722A carbs. I realize that this is a sample size of one but here it is for your consideration.

Here are some pictures of the emulsifier for the primary main. Kit is on the left, OEM on the right. It is noticeable that the holes on the kit tubes seem to be all the same size and different from OEM.
IMG_6924.jpeg
IMG_6925.jpeg

Here’s a couple of the secondary main emulsifiers. Once again, kit on the left. Same deal, holes are of different sizes than OEM.IMG_6926.jpegIMG_6927.jpeg

I also took some measurements of the dimensions of the tubes for comparison. In my observation, the “shoulder” portions of the tubes were very consistent between kit and OEM. The main bodies of the OEM tubes were slightly larger than those of the kit tubes by .002-.003”.
You may draw your own conclusions.
 
Look at the pilot jets and you’ll see a similar variation. The holes on the pilots for OEMs are significantly larger.

edit: on the built in emulsion tube portion, that is.

OEM Brass > Aftermarket
 
The side holes for the aftermarket idle/slow jets are known to be undersized, even the Keihin replacements, .6mm instead of .8mm
Looks like the same goes for the emulsion tubes.

Note that the side holes pass air, not fuel, so smaller holes will enrichen the mixture. That's why on some carbs the holes are different sizes in some locations.
 
Out of curiosity I took a look at the idle jets as well and LDR is correct. The kit jets have air holes that are significantly smaller than the OEM Keihin jets. Same as before, kit part is on the left.IMG_6936.jpegIMG_6937.jpeg

Also, the hole specifications stated by LDR are pretty much right on the money or as near as I could measure with implements on hand.
IMG_6935.jpeg
I plan to get some Keihin replacement jets and do some comparisons to see how they stack up against the originals.
 
We've seen so many poor brass parts from kits over the years, we just automatically assume they're not going to be any better no matter which kits you buy. It's unfortunate, in large part because there are so many old carbs out there now that the jets have already been swapped that finding a good used carb with stock parts in it anymore is a crapshoot at best.
 
It doesn’t seem like it should be that difficult to do it right. If you can manufacture them with the wrong size holes, how much harder or more expensive can it be to just make them correct?
Now, I get it, quality control and maintaining tolerances costs money but many of these things aren’t even in the ballpark.
 
Emlupi said:
It doesn’t seem like it should be that difficult to do it right. If you can manufacture them with the wrong size holes, how much harder or more expensive can it be to just make them correct?
Now, I get it, quality control and maintaining tolerances costs money but many of these things aren’t even in the ballpark.
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Agreed, and it certainly doesn't seem like it should be rocket science. Mike Nixon addressed it here in an interesting read.

 
I see this as a bit of a bonus, allowing you to Taylor the tune-up for your particular bike and also for allowing adjustments for todays gas, which is quite different from the gas these things were born on.
My .02
 
ancientdad said:
Agreed, and it certainly doesn't seem like it should be rocket science. Mike Nixon addressed it here in an interesting read.

Click to expand...
Interesting reading, thanks. The engineers who designed these instruments made the specifications for a reason. Manufacturing parts strictly for price at the expense of quality and suitability of purpose isn’t really helpful.
 
OMF said:
I see this as a bit of a bonus, allowing you to Taylor the tune-up for your particular bike and also for allowing adjustments for todays gas, which is quite different from the gas these things were born on.
My .02
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Each to his own.
 
OMF said:
I see this as a bit of a bonus, allowing you to Taylor the tune-up for your particular bike and also for allowing adjustments for todays gas, which is quite different from the gas these things were born on.
My .02
Click to expand...
Except that the variances between jets relates to an unknown result in tuning, so instead of changing jets by known accurately-numbered sizes with a fair idea of the result, you'd be changing to unknowns with unknown results.
 
So, I ordered some new parts from CMSNL to rebuild my carburetors. To continue with what we were discussing earlier here’s a few photos of some the parts.

The parts came in Honda factory packaging

IMG_7239.jpeg

Here’s a photo of the original needle jet from the 722A carbs.

IMG_7237.jpeg

The next two are new Honda needle jets from the parts order.
IMG_7238.jpegIMG_7240.jpeg

Finally, here’s the kit needle jet that was in the bike when I got it.

IMG_7241.jpeg

Dimensionally the parts were very similar with the factory parts being a bit larger than the kit part. We’re only talking about .001” here or there. The biggest difference is in the size of the holes in the jets. The factory parts have noticeably larger diameter holes than the kit parts. Once again, you can draw your own conclusions from this.
 
Great pics. You can certainly see the difference. I spent a lot of time with stronger reader glasses, a magnifier and feeling the wiggle of micro drill bits measured with my mike, but it paid off on my best bike (68 CB350) which runs great.
For what it's worth, the original tube looks slightly better than the new OEM brass, maybe tiny burrs at the edge of the holes.
 
I think that’s just the light but I will certainly take a closer look.
 
IMG_7325.jpeg
Here are a few different jets. From left to right they are kit, Jets R Us, and OEM. They are all dimensionally pretty much the same. I don’t have the proper implements to compare the actual hole sizes.
The kit jets have no identifying marks on them at all. I assume they are factory sizes but for which carb model, who knows?
The Jets R Us brass seems to be a little higher quality and they are clearly marked for size. They are not Keihin branded. I was not particularly impressed with the o-rings that came with these. They are undersized when compared to the items in the OEM gasket set.
The OEM brass in these carbs were toast.
My plan is to use the Jets R Us brass and the OEM o-rings for the rebuild.
 
The kits will work for rebuilding standard spec carbs. In my case I am going to be jetting up for a higher performance engine so will need to get them individually as needed.
 
When you say the jets are toast, what do you mean?

Is there visible damage on the metering orifice (the little hole in the middle)?

Don’t let some tarnish on the outside scare you away.
 
Emlupi said:
The kits will work for rebuilding standard spec carbs. In my case I am going to be jetting up for a higher performance engine so will need to get them individually as needed.
Click to expand...
Metric micro drill bit sets are pretty cheap and a micrometer to verify. Going custom is oft DIY.
 
It's been an issue for at least 12~15 years. The 'problem' is Kei-Hin changed the specifications on new parts so aftermarket copied it.
'Correct' hole sizes for the cross drillings are 0.80mm, but every one I've worked on from about 2008 has been 0.60mm.
I've been recommending people buy pin vice and 'Microbox' drill set (they are actually pretty damn accurate sizes)
A 0.38mm drill bit is too expensive to test CB360 pilot jets (very 'special' around $112 last time I checked) but 0.35mm drills are pretty cheap from Poland or former 'eastern bloc'
If your going to work on 1970's Honda carbs the $12~$15 is well worth the cost.
 
crazypj said:
A 0.38mm drill bit is too expensive to test CB360 pilot jets (very 'special' around $112 last time I checked) but 0.35mm drills are pretty cheap from Poland or former 'eastern bloc'
If your going to work on 1970's Honda carbs the $12~$15 is well worth the cost.
Click to expand...

www.mcmaster.com

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.
www.mcmaster.com www.mcmaster.com

$5.36 for a .38mm bit on mcmaster.

They're well worth it for a sanity check.

Not all of the sizes we need (e.g. .68mm) are available, but damn near close.
 
Ribrickulous said:
When you say the jets are toast, what do you mean?

Is there visible damage on the metering orifice (the little hole in the middle)?

Don’t let some tarnish on the outside scare you away.
Click to expand...
The jets were seized in the carb bodies and I had to grab them with pliers to work them out. The ends are pretty chewed up. They are also standard factory spec so I probably won’t be using them anyway.
 
ballbearian said:
If you're going to drill to a custom size the Scrambler kits are cheap enough to do without drilling original Kehins. Just an option.
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Not looking to go to any custom sizes at the moment. I got some Jets R Us parts in the next two sizes up for each jet. I don’t have any original Keihin jets to drill. The bike came with kit brass in it and the replacement carbs’ jets were seized in the bodies and took a beating coming out. If it becomes necessary to drill some sort of custom size jet I will use the kit parts that I already have.
 
I hope your fellow racers will give some good coaching on jets, when you get to that point. Same with any secrets that PJ may have.
 
The conventional wisdom is to go up one size for the free flowing exhaust and less restricted intake airflow. The only way to know for certain what the engine needs is to run it and see the results. Running in a spec class where the majority of the engine components are stock will hopefully reduce the variables.
The other riders have been extremely helpful and forthcoming with information when asked.
 
I re-checked my numbers, (that's still a damn good price for precision drill bit), it was a 0.38mm reamer I priced as I was going to make some new jets.
The '3 jet' CV carbs do not respond well to air box removal, SL slide carbs are same as any other though when your working out jetting.
The 3 jet CV can be modified to work with bell-mouth or K&N filters (I've been doing it since 2008) but it isn't as simple as just swapping jets as that will give a massive problem somewhere between 4,000rpm and 6,000rpm, (when secondary main jets start working as slide lifts, depends on engine vacuum)
 
Ok, so how about letting us know what modifications you are making?
 
Unfortunately, the information is one of the very few things I don't give away as it's pretty much my only income since 2012 (apart for making a few starter block off plugs and various small bits in 6061, axle spacers, tach plugs, etc)
It isn't a high return as I'm not basing things on business but allows me to kinda maintain hobby.
I usually spend 12~15 hrs (or more if there are issues) working on a pair of carbs, mainly to keep me occupied (less than $12.00 an hour, more a service for owners getting ripped of in shops that don't actually know what they are doing)
I guess if I'm totally unable to do various modifications in future I'll get Eric to post the information.
 
Ah, so we gotta pay, got it. I don’t know, I see the majority of bikes in the paddock running with pods, stacks, or just wide open.IMG_6142.jpegIMG_5900.jpegIMG_5908.jpegIMG_5902.jpeg

Stan Lipert advises to run open carbs and tune with jetting. If it wasn’t working I wouldn’t think that so many would be doing it that way.

IMG_7337.jpeg
 
crazypj said:
I re-checked my numbers, (that's still a damn good price for precision drill bit), it was a 0.38mm reamer I priced as I was going to make some new jets.
The '3 jet' CV carbs do not respond well to air box removal, SL slide carbs are same as any other though when your working out jetting.
The 3 jet CV can be modified to work with bell-mouth or K&N filters (I've been doing it since 2008) but it isn't as simple as just swapping jets as that will give a massive problem somewhere between 4,000rpm and 6,000rpm, (when secondary main jets start working as slide lifts, depends on engine vacuum)
Click to expand...
I appreciate your comments when I was matching up replacement slides and their lift holes, in my stock CL350. It still stumbles from 3-4k. I still think of sending them to you for the final touches.
Or, what do you charge for tuition, if I sign a non-disclosure? Who am I kidding? I'm tied to the whippin post here.
 
I was teaching at MMI for almost 12 years, got in trouble quite a lot as I had done more than almost every instructor there (many had come from other fields, military, etc. specialised in particular brands, I played with bikes since I was 11 and was pretty obsessive until into my 30's )
Other classes complained they were not getting same education because I went over and above the stock curriculum.
I had to lean most stuff 'the hard way' and very rarely got answers to my questions (even the real smart guys had never seen problems I talked about although a few could BS really well)
I think Tom Keith was about the only person there who had done weirder stuff than me (built a literal 'flying boat' hang glider on a speedboat)
Even today we get on pretty well.
Anyway, for racing, a 'flat spot at ~4K isn't really an issue as apart from starting engine is never at that low an rpm.
On the street it's way more of an issue.
I haven't tried it but heard CB160 needle and needle jet 'cure' the problem but I haven't seen one to have any idea of the various tapers to change fuel delivery. (making needles isn't something I can do, I have tried)
Long reverse cone megaphones give a pretty large power boost around 10K, late 60's 'race kits for CB72 show how long they really need to be, just about to edge of rear tyre, shorter 'fatter' give more mid range
I learned a lot about modifying carburettors in the 80's, LEDAR jet kits were far superior to anything else out there (Leon Moss worked with Honda for the 'Superbike' series and Kawasaki when they were developing Ram Air - vacuum cleaner hoses through fuel tank)
He would probably still be with us if he had stayed away from hang gliders
I've posted it a few times, he was actually developing a project for the Japanese 'Fresh Air Ram Technology System'
It was a few years before American Kawasaki got a look at proposed bikes, 'Kawasaki FARTS' when it was explained to Japanese marketing a graphic redesign was needed (I liked him :ROFLMAO:
 
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Understood , all I’m doing is getting a base setup to get started with. I fully expect to have to do plenty of testing and experimenting to keep getting better results. I’m still in the shallow end of the pool.
 
crazypj said:
On the street it's way more of an issue.
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Coming out of slow turns in traffic.
I didn't go as big as you suggested on the slide holes, maybe half. Upped a jet size (which one?, CRS kicking in) as per LDR.
I do have some dirty stock 160 bits to eyeball.
BTW, Classic Honda Restorations has some Keyster 160 kits on sale.
 
ballbearian said:
I didn't go as big as you suggested on the slide holes, maybe half. Upped a jet size (which one?, CRS kicking in) as per LDR.
Click to expand...
Now you’ve let the cat out of the bag, do tell.
 
Emlupi said:
Now you’ve let the cat out of the bag, do tell.
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I can only point to where PJ did a drive by. I then formed a cause and effect theory, but nothing quantifiable; bigger holes=more lift.
Budgeting for consulting fees for him, has been on my backburner until more saddle surveillance has occurred.
I'm with Solomon, seeking wisdom, is most precious and PJ's got some, probably a lot.

Mousetown, my own 73 CL350

There's a number of the diaphragms that are made with heavier rubber and that will delay the piston rise which in turn will probably cause a flat spot in the power curve. You need to drill slide lift holes 0.04~0.06mm larger with the thicker diaphragms. BTW, slide lift holes go to air-box side...
www.vintagehondatwins.com www.vintagehondatwins.com


Or, slide holes become rabbit holes.
 
Yes, after a bit of reading I can see how slide holes can certainly become rabbit holes.
I am going to hold off on drilling out any slide holes until I can get this thing finished and running. Good to know that there is a remedy for this affliction should it become necessary. Thanks.
 
Emlupi said:
Yes, after a bit of reading I can see how slide holes can certainly become rabbit holes.
I am going to hold off on drilling out any slide holes until I can get this thing finished and running. Good to know that there is a remedy for this affliction should it become necessary. Thanks.
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Sound logic and plan. I'm still in the same boat anyway.
To which particular affliction were you referring? I always have collateral maladies in play.

We do need to clone PJ before his grandson takes him out, or he becomes otherwise extinct.
 
I was referring to dead spots in the rev range during acceleration.
 
ballbearian said:
We do need to clone PJ before his grandson takes him out, or he becomes otherwise extinct.
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That is funny, true, but still funny.
I intend to hang around if only to annoy people but, you never know what's going to happen.
Eric, (Trek 97 on DTT) has the info on CB360 carb mods in case anything happens to me, not sure what else I sent him over the years?
I can remember stuff from the 70's~80's better than what I did 30 minutes ago. (or even five minutes in some cases)
The slide lift holes need 0.4~0.6mm not 0.04. It makes slides lift faster and earlier, it ban be a problem if they are too big as slide can lift too early before engine needs or want's extra fuel.
In the past I have drilled out one slide lift hole then fitted aluminium air restrictors but it's a PITA to do because of the needle boss inside
 
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I have a closely related question.
Do the outer diaphragm ribbed edge need to be glued or sealed to the carb body?
I've seen silicone, gaskacinch, even sticky thick grease on some. I did nothing and had new or flat tops and it seemed fine without anything.
 
ballbearian said:
I have a closely related question.
Do the outer diaphragm ribbed edge need to be glued or sealed to the carb body?
I've seen silicone, gaskacinch, even sticky thick grease on some. I did nothing and had new or flat tops and it seemed fine without anything.
Click to expand...
Can't speak to the diaphragms of today, but I never used anything back in the day when I changed them (but of course they were OEM).
 
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