Left cylinder misfire

[Kathos]

So I'm really trying to get my bike reliable and little issues continue to become present. I noticed while riding that I felt a miss. I could tell by the exhaust that its obviously the left side. I have a pamco with a new coil. New advancer. Mikuni vm34s. I replaced both vacuum lines going to the fuel peacock. I tried switching plugs. Plug wires. And carbs to the opposite side but the miss continues to stay on the left cylinder. I tried using the starter fluid trick to see if I had a leak, but it didnt do anything. I noticed that the fluid didnt increase the rpm when I sprayed it into the carbs but it decreased it.
When I sprayed it around the boot there was no noticeable difference. I don't know where go. It's not ignition and its not the carbs.

 

[Kathos]

So I did a carb sync test. And I noticed that the exhaust that is misfiring (left), has loads more vacuum when I start revving.
I also noticed when I first started the bike, it seemed like the other side (right) was missing.
Is it possible for your vacuum to be thrown off of the valve gap is not 100% the same on both cylinders? (Even though I tested both cylinders compression and they are both 150)

 

[TwoManyXS1Bs]

Another interesting mystery.

I have no experience with the Pamco unit, but I wonder if it's possible to reinstall the Pamco rotor 180° out. This could left/right swap the triggering action of the magnets.

Could also try to check valve timing and valve lift. Severly worn cam lobe and/or rocker pad can change manifold vacuums. Let's hope it's not this serious...

 

[Kathos]

Well the pamco is actually a waste spark system so both spark plugs fire at the same time every revolution.
I'm almost positive my problem is vacuum related due to the carb sync test. I just don't understand where the leak could be.
I don't think it's timing because its only doing it in one cylinder at a time.
I'm thinking the next step would be to swap the carb boots to the other side to see if that helps the problem.

 

[NONclow]

I'm with Twomany on this, sounds like valve clearance problem. Maybe a sticky intake valve?

 

[Kathos]

Hm. Can you have a still valve with 150 compression?
I looked over the valve gap today and they all seem to be in order. I do have some notes though.
I put a tube up to my engine today to listen for noise and I
Could hear a pretty noticeable tico coming from the timing chain. Is this normal?
I am still confused on my carb vacuum test. At idle. Right carb has more vacuum. To compensate, I open right carb almost quarter turn to even vacuum. When I increase rpm, the left cylinders vacuum is much higher then the rights.
On a good note, I notice that the misfire seems to be less noticeable.
However, when I increase rpm, the pressure felt through the left side exhaust is stronger then the right.
Any other ideas? Why would the vacuum at idle be stronger on the right then left until I increase rpm?

 

[TwoManyXS1Bs]

...I am still confused on my carb vacuum test. At idle. Right carb has more vacuum...

Another way to perceive this is to say: "At idle. Left carb has less vacuum."

So, readjust Right carb back to where it's supposed to be, and find the vacuum leak on the Left side...

 

[Kathos]

Thats where Im confused. It seems to me that if there is a vacuum leak on the left side, then the vacuum will increase at idle when its fixed. And I will still have more vacuum at higher rpm on the left side.

 

[TwoManyXS1Bs]

Yes, that will be true if you remain out-of-sync. When the right side is brought back down, its vacuum will rise. So, if you can, sync the carbs for a higher rpm, say 3000, then let it return to 1200 rpm idle. Any difference in manifold vacuum at this new idle setting will indicate a problem, that would be easier to diagnose...

 

[Kathos]

I will try as you say, but heres my concern. And I havent been able to word it correctly so Ill try.
If there is a leak on the right side cylinder, then it woudlnt have more vacuum at idle.
If there is a leak on the left side cylinder, then it wouldnt have more vacuum at higher rpm.

When I perform a carb sync test at 3000 or so rpm, I believe that I will be able to get the vacuums to even out at that rpm. This will occur by opening the left side throttle (to reduce vacuum at 3000 rpm).
When I drop back down to idle, I believe that the right side will have an even larger difference in vacuum when compared to the left because I just opened the left side throttle more.

The only thing I can think of is that the left side has a really small leak at idle but when I increase the rpm, the leak somehow seals itself, increasing vacuum. Is that even possible?
Also I think that the fact that my left side exhaust pressure is much more aggressive then my right should lead to a clue. I just dont know how to read it.

I would like to mention that my DIY manometer does not have any reducers in it (like pilot jets), so the difference in vacuum between cylinders may seem greater to me then they actually are.

 

[weaselbeak]

Check the float levels and make damn sure one is not low. Use the clear hose method to be 100% certain. How sure are you of the quality of your diaphragms? The last time I had this particular problem I cured it (maybe it isn't right, but it works) by upping my pilot jets one size. Also, instead of an actual miss, you could just be noticing a popping on one side, caused by an exhaust leak. Maybe the gasket at the head.

 

[TwoManyXS1Bs]

A small leak can be significant during idle due to the low volumetric draw. At higher rpms a small leak may not pass much air relative to the larger draw.

 

[Kathos]

So I went in, changed both carb boot gaskets. Re checked each float level. Slightly adjusted the timing chain tension. Aggressively attempted to make sure I could adjust the valves for both cylinders as close as possible. Verified the timing. And got back into the idle adjustment with the manometer. I made sure both carbs were opening perfectly in sync. Then I set the idle so the vacuum was even. I think my issue before (besides a slight leak) is that I didn't adjusty throttle again after I adjusted the idle vacuum.
Now, both sides stay pretty damn close the entire time!
I do feel some slight missing on heavy downshift breaking. But under throttle, it's strong.

 

[TwoManyXS1Bs]

Great! Sounds like you're on top of it. When these machines age and start wearing out, synchronizing and diagnosing can become a black art. Imagine the nightmare trying to sync an old wore-out 4-cylinder Honda CB-750, with the owner hopping around "Is it ready yet? Gimme, gimme"...

 

[Kathos]

Yeah I hear you on that. 4 cylinders would have made the guessing game so much worse. Thanks for walking me through another problem. Road into work today, Feels great!

 

[IllusiveJack]

I'm currently having the same issue with the left missing popping and back firing at idle, I completely swapped the carbs out for a second set and it does exactly the same so I'm guessing it has nothing to do with the carbs. the bike does not blow from the breather, no smoking exhausts and both sides have between 116 to 120 psi.

The bike hasnt run for twelve years until the other day, new points and plug caps fitted but not coils or condensors or plugs, although I have tried several plugs and they are fine in the right hand pot.

I have a new dual coil coming and will try that at the weekend and use the right hand condensor to see if that helps, oh the charging is ok as that goes up to 14v + when the bike runs. Also I will do all the usual checks again, cam chain, valve gaps, timing and points gaps. One thing is that when you open the throttle up both sides fire up a treat. Weird.

 

[Kathos]

Make sure, I cannot express this enough. That your carb slides are opening at the exact same time. With VM34 carbs, each carb has a different throttle adjustment so its easy for them to open at different times.
When checking the vacuum on each cylinder, you will notice that ANY difference in the throttle opening between the two carbs will cause a lower vacuum on one side and higher on the other.
When I sync my VM34. I get my idle to where I want it by adjusting the idle screw. To get both sides close, I usually use the round side of a small drill bit (like a 5/32) and place it underneath each slide. If idle needs increased, use a larger drill bit.
Then you start the bike and check your vacuum difference between both carbs. To increase vacuum on cylinder 1, you either need to lower the slide on carb 1 or open carb 2. To determine which of those you should do depends on if you need to lower or raise your idle. You can play with it to keep idle where you want it.
After you have proper vacuum on both cylinders, you need to adjust the throttle cable so that both carb slides open at the same time. As I mentioned before. If one opens before the other, it will throw off the low rpm sync.
Some people use their fingers to tell if they open at the same time. That doesnt work for me as well as listening. I jiggle the throttle at the handlebar just enough so I can hear the carb sliders shutting. If you can hear one carb shutting and the other carb isnt moving, then you still need more adjustment.
After thats all done. You can check to see if both carbs have proper vacuum at rpms by hooking up the vacuum tool again. Apply the slightest throttle and both sides should continue to have equal vacuum.

Ps. If you dont have a manometer, you really need to make one. They are simple, cheap, and very effective.

 

[CDNTX650]

I'm currently having the same issue with the left missing popping and back firing at idle, I completely swapped the carbs out for a second set and it does exactly the same so I'm guessing it has nothing to do with the carbs. the bike does not blow from the breather, no smoking exhausts and both sides have between 116 to 120 psi.

The bike hasnt run for twelve years until the other day, new points and plug caps fitted but not coils or condensors or plugs, although I have tried several plugs and they are fine in the right hand pot.

I have a new dual coil coming and will try that at the weekend and use the right hand condensor to see if that helps, oh the charging is ok as that goes up to 14v + when the bike runs. Also I will do all the usual checks again, cam chain, valve gaps, timing and points gaps. One thing is that when you open the throttle up both sides fire up a treat. Weird.

Carb sync.

A dual tower coil wont work with points. Unless you use that god awful single points setup.

 

[retiredgentleman]

A single dual output coil will work just fine with the stock, dual set of points, and single lobe points cam. The 2 sets of points would need to be connected in parallel.

Just be aware that when using points, its best to use a coil that has 4 ohms or more resistance in the primary. If you use lower than 4 ohms, you may burn the points.

 

[TwoManyXS1Bs]

You can also play switchback games with the connectors that tie-in the coils with the points and condensers. Switching the condenser leads is easy, but, to switch the points leads also requires switching the plug leads. Then see it the problem switches sides.