1973 SL350 Carbs - Wisdom Required [SOLVED]

Clutch Cargo

Clutch Cargo

Veteran Member
Joined
May 7, 2021
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Location
Pearland, Texas, USA
Greetings,

I recently acquired a 1973 SL350 that is exhibiting an issue for which I could use some wisdom and insight, before I dive into troubleshooting.

The bike starts and idles perfectly. Ridden at lower RPM (up to 3.5k or so) it runs fine. However, as you push beyond that it will begin to stutter and jerk, with the occasional, very minor, pop/backfire. To me, it seems that the problem comes as the bike transitions off the needle to the main jet. Based on exhaust sound, it feels like it is the right cylinder.

Gut says that it could be a clog in the main jet, possibly leaking seal around the jet. Maybe a leak in the boot between carb and cylinder. Possibly timing advance is not working. Given the numerous "might be" scenarios, I am interested in the wisdom of the group regarding your thoughts.

Thanks,
Darrell
 
Have you gone through all the other aspects of a tune-up to eliminate them? Cam chain and valve adjustments, points and timing, known good battery that holds at least 12.6v at rest? The charging systems on these bikes are break-even or less until above around 3000 rpm so if the battery is weak it can starve one coil under load. A bad condenser can act that way as well as a weak coil that breaks down under higher load.
 
No, not yet. Just gathering my thoughts on where to start and the path to take. Interesting that I realize of the many bikes I have owned, all the multi-cylinder bikes were fuel injected. So, carb syncing etc. is new ground. I considered condenser as well, but typically I have seen those break down as they get hot. This doesn’t act that way. I greatly appreciate the suggestions and feedback. I would probably do well to simply walk through the basics, top to bottom as you mentioned. The only known off the list is battery being brand new, but doesn’t necessarily mean good.
 
Point taken about the battery, though new it could be bad. Modern bikes make things so much easier but it comes with a price in so many ways. We've seen bad condensers from stone cold so it's a possibility, and a close points gap can cause strange behavior too. Did you check the Welcome Package I posted in reply to your intro? Lots of helpful stuff there, particularly the entries from the Honda ID Guide to be sure you know exactly what year, and more importantly for parts sourcing purposes, the K series of your bikes, along with the link to our FSM library.
 
ancientdad said:
Did you check the Welcome Package I posted in reply to your intro?
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Yes, I did. Thanks! Good welcome for new users. Looking through the Model ID, I did not see the SL350 line. Maybe I just missed it. Curious as I understand that there is some issues with the K? designations on the SL lineup. Not the typical count up 1 for each year.

The forum history you sent, I did actually find and read when I first signed up. That and subsequent updates on growth. Impressive increase in membership and activity. Don’t know what the rift was but it seems that the core decided that a new forum was the place to be. Thanks again for the warm welcome!
 
Clutch Cargo said:
Yes, I did. Thanks! Good welcome for new users. Looking through the Model ID, I did not see the SL350 line. Maybe I just missed it. Curious as I understand that there is some issues with the K? designations on the SL lineup. Not the typical count up 1 for each year.

The forum history you sent, I did actually find and read when I first signed up. That and subsequent updates on growth. Impressive increase in membership and activity. Don’t know what the rift was but it seems that the core decided that a new forum was the place to be. Thanks again for the warm welcome!
Click to expand...

I answered your thought about the "why" we started this forum in my most recent reply to your intro... it pays to revisit all of your threads often here, as we're pretty busy on a daily basis and the easiest way to keep up with all of it is to use the "What's New" button. I scanned all the relevant pages of my treasured Honda ID Guide for 99% of the models we encounter regularly here, but somehow missed the SL350 entries. Glad you pointed it out, I will have them up before the end of the day.
 
How close to stock is the SL?
In view of the various bad 'modifications' done in last 10~12 years to most older bikes, it could be simple or it could be butchered by a 'wannabee cafe racer'
The usual cause of bad performance around 4~5K (and above) is lack of stock air-boxes and modifications to intake system.
The advance should be 'all in' by around 2K. Low battery voltage can (and often does) make bike 'drop' a cylinder as coils can't fully charge below about 11V and even that isn't enough as rpm increases.
I'll wait for reply before speculating further
 
crazypj said:
How close to stock is the SL?
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Fortunately, it is unmolested! I was wondering about advance, but if it is “all in” by 2k then I don’t think that is the issue. The problem doesn’t arise until higher up the range. The way it spits and pops, unburned fuel is making it through, so intermittent spark seems likely. Since the previous owner had freshly painted the frame, I am tempted to place my money on him not properly removing the necessary amount of paint for a good ground. Going to start there when I get some time to dig in. My focus has been on the SL100 restoration, but my curiosity compels me to see if the 350 is a simple quick fix. If so, maybe my brain will stop trying to fix it in my sleep!
 
Honda masked under coil mounting bracket and ran an earth lead directly to it.
Battery ground should also be 'bare'
I found it much easier to use star washers though as they will bite through paint and give good contact.
Use dielectric grease and you won't get any corrosion either
 
UPDATE: Went through yesterday and made sure carb slides were in sync. Removed the coils and better sanded the paint to be sure they had a good ground. And, spent a really long time figuring out how to set the point gaps and timing the engine. First time with this double-points setup is a bear! In the end, may have helped a little, but the more I listen to the bike under acceleration, I am becoming convinced that it is a carburetor issue. Isolated to the right cylinder, it seems, as if bogging down from too much fuel. Next step is to pull the carbs and do a thorough inspection, rebuild if necessary, etc.
 
Before you pull the carbs, take it for another ride and when it hits the spot where it runs poorly, try lifting the choke just slightly to see if it makes things worse
 
Clutch Cargo said:
And if it does?

Currently, when it shifts into the range of running poorly, it feels and sounds as it would if you left the choke on.
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Yeah, if it's blubbering or 8 stroking, so to speak, and the choke makes it worse then it confirms the richness. Did you check the slide needles to see if all was well there and they are set the same? Stranger things have happened, and on those carbs taking the slides out is easy and you can eliminate that aspect quickly
 
ancientdad said:
Did you check the slide needles to see if all was well there and they are set the same?
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No. I started with the points, timing, etc. As of yet, I have not touched the carbs. The person I bought it from just flipped it out of a barn-find estate sale. He said that they cleaned the carbs, but did not rebuild. No telling what the condition really is inside, or how they may have reassembled. Needle clip set to the wrong level. Bad seals. Main jet enlarged from aggressive cleaning, etc.
 
Clutch Cargo said:
No. I started with the points, timing, etc. As of yet, I have not touched the carbs. The person I bought it from just flipped it out of a barn-find estate sale. He said that they cleaned the carbs, but did not rebuild. No telling what the condition really is inside, or how they may have reassembled. Needle clip set to the wrong level. Bad seals. Main jet enlarged from aggressive cleaning, etc.
Click to expand...

Ah yes, the PO stories... "ran when parked", "professionally rebuilt", etc, etc... well, checking the needles is an easy first step but more likely it needs deeper efforts then
 
ancientdad said:
Ah yes, the PO stories... "ran when parked", "professionally rebuilt", etc, etc...
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In this case, the actual PO is deceased. The person I bought it from picked it up as part of a LARGE barn stash, from the POs kid liquidating the estate. So, the bike was a flip with just enough work done to make it presentable. In talking to the guy, some of the folks he has working for him are not bike folks, so no telling what they did when cleaning the carbs. I specifically asked him about rebuilt versus cleaned, and they were only cleaned. I can't imagine that was adequate for a bike that had been sitting for years. Would not surprise me to find one needle clip in position one, and the other in five! The slides weren't even snapping closed at the same time.
 
Clutch Cargo said:
In this case, the actual PO is deceased. The person I bought it from picked it up as part of a LARGE barn stash, from the POs kid liquidating the estate. So, the bike was a flip with just enough work done to make it presentable. In talking to the guy, some of the folks he has working for him are not bike folks, so no telling what they did when cleaning the carbs. I specifically asked him about rebuilt versus cleaned, and they were only cleaned. I can't imagine that was adequate for a bike that had been sitting for years. Would not surprise me to find one needle clip in position one, and the other in five! The slides weren't even snapping closed at the same time.
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Yep, I had the same situation when I bought my 900F late last year, the PO had died of cancer and his friend was selling off the last bike in his collection. He said he didn't know much of anything that had been done to it and just bought a battery (and the wrong one at that, smaller than original) to get it running and it ran... but not too well.
 
ancientdad said:
Ah yes, the PO stories... "ran when parked", "professionally rebuilt", etc, etc... well, checking the needles is an easy first step but more likely it needs deeper efforts then
Click to expand...

“Just needs the carbs cleaned and a new battery”

*cue year long rebuild*


-Ed
1972 CL350
 
On a side note: I cracked up when I saw the user name, it took me back. I just watched Clutch on YouTube.
 
1973 SL350 Carbs - Wisdom Required (SOLVED!!!)

UPDATE - So, I am most happy to report that the problem is solved, and the bike is now running WONDERFULLY!

In other threads, here on VHT, I discussed the cleaning (and design) of the air filter cages, as well as my constructing new air filters for the bike, from bulk UNI foam sheet. Once I had the new set of properly old filters, made from a proper density foam, the problem was cured! Prior to that, the bike was simply drawing too much air, as the RPM rose, and thus leaning out the mixture.

With that said, my already having set the points gap and timing, in pursuit of a solution, was not time wasted. The cumulative steps have resulted in the bike running, and pulling, great throughout the RPM range. I still feel compelled the double-check the valve clearance before putting my mind to rest.

This bike was single-owner barn find, with only 2,672 original miles. Presumably, those miles were early in its life, with the balance having been sitting unused as part of the deceased's collection. As we know, lack of use can potentially be as bad as abuse, with seals drying out, swelling, etc. It is tough for me to blindly accept the condition of the engine internals on faith. However, at this point, I find little justification (beyond OCD tendencies) to tear into the engine. Having just completed an engine rebuild on my 1970 Honda SL100, after it sat for the better part of a decade unused, it allowed me to see how well vintage Honda engines endure such treatment. Fingers crossed the SL350 is just as good.
 
Good to hear. I'd just ride it a bit and do an oil change early to see what you find in the oil, as well as clean the centrifugal filter again (or if you haven't yet) as well.
 
tomeben said:
Fantastic! I love it when this happens, well done!
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Thanks! When I did the test ride, before purchase, I knew there was an issue that would need to be resolved. Had little doubt that it could/would be, however it still hangs over your head as you start working down the likely candidate list, and each step doesn't do it. With the filters in place, once I headed off down the street with it pulling cleanly through the RPM range, I don't know how the smile could have been any bigger!
 
Awesome to hear you have it sorted now, well done. Thanks for reporting back with the solution as well and also updating the thread title to reflect that it was solved. Both of which can help people coming later on that may have the same issue.
 
HerrDeacon said:
Awesome to hear you have it sorted now, well done. Thanks for reporting back with the solution as well and also updating the thread title to reflect that it was solved. Both of which can help people coming later on that may have the same issue.
Click to expand...

Side note - nice new avatar!
 
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