The following has been done. Both carbs rebuilt and set to specs. Plugs and points wires swapped from right to left to eliminate any possibility of an electrical problem. Problem remains on the right. The engine starts and idles reasonably well. After a mile of running at 2000 to 3000 RPM the right side starts missing. Actually it is more like quitting and starting. Upon return the engine will not idle. However, adjusting the idle set screw will bring it back but the idle RPM is not steady. At this point the right side misses at idle. If I choke it the right side accelerates the engine to 2000 RPM and after a short while it quits. Sometimes the right side starts again. Other times it won't. At this point I have drained the float bowl and found ample gas in both bowls. I'm new to motorcycle engines (pretty good with cars), but out of ideas. Any suggestions would be appreciated.
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Weird happenings on one cylinder
- Thread starter nick
- Start date
New carb insulators?
I removed them. They looked OK. Installed new gaskets.
I'd go ahead and replace them, not terribly expensive. I add HondaBond to both sides of the gasket to insure they get a good seal to the head.
wentwest
Veteran Member
How's the fuel petcock? And the fuel lines? Clean, clear, new, replaced? Does fuel flow equally from both lines?
Also, have you done anything with the spark advance mechanism?
Also, have you done anything with the spark advance mechanism?
mike in idaho
Veteran Member
Check the compression, see if both sides are reasonably close to even.
No. I did nothing to the spark advance but I will inspect it as you are suggesting. However, wouldn't a problem there effect both cylinders? I checked the flow in both lines. Removed the petcock and cleaned the screen and bowl. Drained both float bowls to compare how much fuel each had. About the same.
Hot compression test shows 120 L 85 R.
stl360+450
Veteran Member
Both are low (obviously) and the difference is huge. You did have the throttle wide open for both tests?
Have you checked the valve clearances?
Checked valve clearances twice. The throttle was closed. I'm at 3,400 ft of elevation.
stl360+450
Veteran Member
The compression test should be done with the throttle wide open. Otherwise the air flow is restricted and the readings will be low.
I'm not experienced with higher altitude variances, but I can tell you the compression variance between cylinders (if accurate since the test was done without the throttle wide open) is likely a part of your problem. If both were equal it would run okay at 120 lbs but it would be weak and probably give you idle inconsistency issues. If you do the test again with the throttle wide open for both and the variance is still that large, it needs engine work.
Spec is 170 psi so the right cylinder is 50% and the left is 70%.
New compression numbers done correctly. Sorry for the earlier mistake. L 140 R150. Spec of 170 at 3400 ft elevation comes out to 154.87. Rechecked valve clearance for the third time. All about .001 wider than spec. Plugs are extremely sooty. The right (problem side) being worse than the left. Turned out air screw 2 full turns from where it was.
Note that the mixture screws on the stock 350 carbs are not air screws, they're fuel screws so turning them out more will increase the fuel delivery, not air.
stl360+450
Veteran Member
This is CB350, right? With stock carbs, turning the mixture screw out will richen the mixture.
Edit: Couldn't respond as fast as you, AD!
Edit, Part II: This thought is little out there, but I wonder if the earlier compression readings (with throttle closed) might indicate that the throttle plate on the left carb is open farther than the right, either due to the idle adjuster screw or a lack of slack in the throttle cable. This would tend to allow the left cylinder to breathe better than the right and achieve higher compression. In any case, it's good that the second set of numbers are much closer together.
If you have the 3D carbs the fuel mixture screw should start at 3/4 turn out from very lightly seated.
Thanks for pointing that out. The Clymer manual shows an air screw. Lesson: don't assume anything. I adjusted as Long Distance Rider suggested.
Since the start of this post there were a lot of suggestions. I followed most if not all. I have not yet changed the carb insulator. I did get a can of carb cleaner and sprayed the intakes. Today I reinstalled the tank. The engine started immediately and ran smoothly. After a minor idle adjustment to 1100 and after a couple of blips to about 3000 it settled back at 1100. I shut it down and will take it out tomorrow. I'm not sure what caused such a dramatic improvement. I'll know for sure after I run it and get it to operating temp. Thanks for all the help so far
wentwest
Veteran Member
Sometimes, when something starts to be right after a struggle, you do wonder what could explain it. In your case maybe it's getting the adjustment of the air/fuel mixture screws the same is an answer. Also cleaning and sometimes just looking things over can help. Either way, get out and run it some and see what you really have.
I beg to differ on a small point. If Clymer says it you can assume that you need to verify it.
ausman1000
Veteran Member
Which Hondabond? The HT or 4 type? Thanks
HondaBond 4 is made to be used with gaskets, HT is designed for surface to surface metal(no gasket)
Getting back to the original thread. I started the engine without choke at an ambient of about 70F and it idle at 1100. I added choke to see what it would do. No change to idle RPM. I ran the bike for about 10 min around the neighborhood. When I got home it was it was idling at 2000. Within a minute it went to 3000. I chocked it and it went back to 2000. Both cylinders seemed to be firing normally. I shut it down. I have not changed the carb insulators yet. Does this sound like that might be the problem? Is there anything internal in the engine that could be the problem?
KenCM450
Veteran Member
Idle needs to be set when fully warmed-up. If it starts easily without choke and idles immediately at 1100, the idle is set too high. It's the choke's job to raise the throttle position while cold. The fact that the choke couldn't raise the RPM probably means the idle screw is holding the throttle too high.
ballbearian
Veteran Member
Just guessing you don't have any engine problems to worry on right now but obvious carb work to focus on. Have you pulled the plugs to see if total black and thus stupid rich?
Also said you have fuel in bowls but are the float levels set right? That is fundamental before any adjustments. Then move on to bench synch of throttle plates, etc. I think we got a good sticky here on carb protocol somewhere. The rise in rpm might be the 2nd cylinder kicking in because things are so off.
Also said you have fuel in bowls but are the float levels set right? That is fundamental before any adjustments. Then move on to bench synch of throttle plates, etc. I think we got a good sticky here on carb protocol somewhere. The rise in rpm might be the 2nd cylinder kicking in because things are so off.
Not in this case. Remember we're talking about a CB350 with separate carbs not mounted together with linkage and with manual chokes that have no fast idle setting. I agree that his cold idle speed could be a factor in the reactions when warmed up. I think it's also possible the dynamic timing might be involved too, not sure he's used a strobe and checked advancer operation yet.
The plugs are black but I don't see any way to change the mixture setting except for idle mixture. I set the floats to 23mm.
I will reset the idle to the nominal setting and try again.
I did not check the timing advancer. That's the one item I forgot. What is the correct way to use the strobe to check it?
Just like you would with any strobe timing light, just watch the two unmarked timing marks on the rotor (close together ahead of the static F and LF) and as you bring the revs up, make sure the timing pointer lands somewhere in between them above 3500 rpm.
But you also want to check the springs on the advancer itself to be sure they aren't stretched and loose, which allows the weights on the advancer to stay outward at low rpm and hold the ignition timing above idle specs which will make the engine drift slowly down to idle speed or even keep it from dropping back at all.
Along with everything else, I'm trying to understand the jets. The holes on one side seem larger than the other. The o ring will mount in either groove so they can go in either way. Does it matter which way the jets go in? I'm also not able to find the size stamped on either jet so I don't know if they are the right size.
I'm not much of a carb guy, but if there are no markings on the jets it could mean they're not original. And if true, that would mean someone changed them for the jets in an aftermarket carb kit which typically means they could easily be the wrong size.
ballbearian
Veteran Member
It matters what hole, or position each jet goes. You'll need to reclean if the holes aren't the same on both sides of the same jet and you have to figure out which jet is the larger one to get the correct position. Study the parts diagram and the descriptions and it'll come to you.
Removed advance. Springs are good and it works smoothly. Installed it, retimed both sides and checked with timing light. Advance works as it should. However there is a large miss firing in the right side. The light flashes normally and then goes dark for several cycles and then flashes again. Left side looks good. I'm thinking I should look at the electrical by swapping the plug and points leads and see if it moves to the other side. I had done that before but did not have the timing light set up. What say the forum?
E
Absolutely agree, try that and let us know if the issue followed the wires.
Swapped plugs and points wires. Problem stay with the right side. Swapped plugs even though both are new. Problem moved to the left side. Both plugs are filthy black. I cleaned them and both side are firing correctly. I then turned out the idle mixture by several turns. But another problem persists which was part of my original post. On the right side the idle cannot be lowered below about 1200. The cables are loose as I can make them and I can look at the throttle lever on the right carb and the set screw is not touching the stop. Yet the idle is high even when I run it on that cylinder alone. It's as if the throttle plate is not able to close. One thing I forgot to say. The strobe shows both sides timing are slightly advanced. I will reset those later but I don't think it is enough to make a difference.
stl360+450
Veteran Member
Sooty plugs are likely rich, so you would want to turn the mixture screw in to lean the mixture.
Seems like you would notice an issue with the throttle plate not closing before installing the carb. I would double check cable routing and make sure that the 1-to-2 throttle cable hasn't come loose at the junction. This would add length to the housing that removes the required slack in the cable.
If the right carb butterfly is slightly off center on the throttle shaft (or slightly bent) it won't close fully and that could happen.
Is there anything else that would effect mixture beside the mixture screw?
Before I installed the carb I used some fishing line to make the initial idle setting. As close as I could see the plate closes fully.
I have the tank off so I can clearly see that when I turn the throttle there is plenty of slack in both cables. I have them synced perfectly.
stl360+450
Veteran Member
When you said you turned out the mixture screw, were you referring to the idle adjuster screw attached to the throttle arm or the actual mixture screw in the side of the carb body? It is possible to back out the throttle adjustment screw so that it won't seat.
Addendum: Adding a picture for clarity.
wentwest
Veteran Member
At least on the CB450 the throttle plates have to be installed carefully because the edges are cut so they end up squarely against the walls of the air passage when the plates are closed. There's actually an angle in the edge so it works right. If the plates are mounted backwards the edges don't line up right even though the gap is correct. No clue whether this is true on your bike, but it is one of those subtle engineering choices that can drive you crazy.
Earlier in this thread I was confused about the mixture screw. I now know that screwing it in leans the mixture. My plugs are really black, thick black, so I have leaned it out quite a bit but have not run it since. So currently the throttle cables have slack as evidenced by the slop in the twist grip and visually observing that the carb throttle arms move simultaneously. The plugs are clean and both cylinders are firing consistently as evidenced a timing light. The problem I am having is that with the idle adjustment screw full out so I can see a gap between the screw and the stop, the right side idles above 1200 and can't be turned down. I will run it today and report the results.
Quick check on the throttle plate install, there's the trademark K for Keihin and #33 visible looking at the back of the carb.
Also check that the throttle plates are centered in the bore like this
NOT like this
Also check that the throttle plates are centered in the bore like this
NOT like this
Since I last posted I received a couple of carb isolators which I installed. I noticed that these after market one are not as deep as the ones I removed. I ran the bike warmed up at slow speed close to home for a couple of miles. The choke was not on. As I rode in 2nd I disengaged the clutch to see at what RPM it was idling. It showed around 3k. I turned the bike around to head home. At the top of the rise i squeezed the clutch again. It was down around 1400. I turned around and started downhill and again and checked idle. It was back to 3k. I repeated this a few times getting the same result each time. After returning I removed the carbs and found the spring on the choke plate is broken on the left carb.
I do not have anything on the written on either throttle plate. But everything else looks like your picture.
Attachments
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Better the spring broke than the flap. Broken spring means that side won't be as effective on choke but should be fine.
I guess that means only ride uphill?:lol: Not sure what to say about the idle speed change, I decel in each gear and only hold the clutch disengaged coming to final stop.
I guess that means only ride uphill?:lol: Not sure what to say about the idle speed change, I decel in each gear and only hold the clutch disengaged coming to final stop.
If I start the motor from a cold condition it idles OK. As it warms the idle goes unstable. Usually I have to adjust it up to keep it running even with choke on. As it warms the idle raises a couple of hundred RPM. Eventually it will go above 2k and I have to adjust it back down but it never stabilizes. I'm about out of ideas. I think I have followed about every recommendation in this thread. I think this has to be carb related. Perhaps I got some crappy parts in the carb kit I installed. I think I may get a new kit. At this point I can't think of anything else to do.
Carbs can be frustrating, but using any brass parts from any kit usually creates a problem. It's sad, but most if not all kits have improperly sized jets and float needles and the best way to deal with it is to clean all your original brass and re-use. Of course, at this point in these bikes' lives there's no way to know if the parts in any carb are still original unless you can find original Keihin markings on them. Also, unless you're working with a brand new rebuild on the engine (and even then not necessarily), the typical Honda twin engine doesn't idle well cold and if you adjust it to idle cold it will usually vary significantly once fully warmed up (usually higher idle after warm).
stl360+450
Veteran Member
Skimming back over the thread, I would suggest you check for air leaks around the carburetor insulators. There are various ways to do this, but you could spray starter fluid around the seals (insulator to carb and insulator to head). If you notice a change in engine speed as a result, then there is an air leak. If the engine speed is completely unaffected, then the insulator is doing its job. An air leak allows unmetered air into the cylinders and affects the air-fuel mixture. If you have any air leakage, it will be more or less impossible to establish a good idle.
It may be difficult to notice any effect if the idle is already high (3k+), so ideally you would check for air leaks with the idle around 1200-1600 rpm. I realize that may prove difficult, but aim for the lowest idle you can when checking for leaks.
It may be difficult to notice any effect if the idle is already high (3k+), so ideally you would check for air leaks with the idle around 1200-1600 rpm. I realize that may prove difficult, but aim for the lowest idle you can when checking for leaks.
I just installed new insulators on the cabs. I did check for leaks as you describe and found the idle did change. That's when I replaced both insulators.