Bike runs great, then 1 cylinder dies, then great again? SOLVED

This is a strange problem. I have a 1981 xs650 with VM34 carbs, PMA, and PAMCO.
The other night I was riding home from a fireworks show when one of my cylinders started to sputter, and eventually died. I pulled into an empty parking lot with a street light so I could check it out. I realized the right cylinder was out. I switched the plug wires (put left onto right and right onto left) and the right cylinder was still out. After waiting a bit, the bike started and ran just fine on both cylinders. So I headed home again, and didn't get more than a mile and the right cylinder went out again. I stopped, waited a bit, and then it came back. I went another mile, it went out, I coasted home and it was late so I decided to check it in the morning.
Next morning I began my trouble shooting ride. I started it (first kick like usual) and it ran great on both cylinders. I went for a ride on a long stretch of straight away. Went about 4 miles out, no problems, I turned around headed home and one of the cylinders sputtered out and died. Unfortunately I wasn't near any side roads or driveways when this happened, so I had to try and kick start it on the side of the road. I was pretty sure it wouldn't start (it had only sat for about 10 seconds) but, it did start, and I had both cylinders again, so I headed home hoping I would make it, but also hoping it would die again right as I got home, so I could see if it was the right cylinder again. Well, it made it home on both cylinders for some reason, it didn't die again, which was also weird. I put it on the center stand, let it idle, revved it up a few times, and never lost the cylinder again. I did a compression test on a warm engine and got 150 in both cylinders. No weird smoke, or weird smells.
When it runs, it runs great, no symptoms at all. Both left and right seem to run and sound identical. It is acting like one of the cylinders is running out of gas after a few miles, or a few minutes (heat?) not sure why this just started happening though.
My other thought is that something is happening to the electric when the bike gets hot? Do PAMCO's ever mess up under high heat? Not sure what it is yet.

Sounds like a bad right plug.
Found a microcrack in one of mine awhile back.

http://www.xs650.com/threads/what-have-you-done-to-your-xs-today.32082/page-68#post-420142

I concur doctor ! bad plug ...OR something goofy with the right carb....
Bob.....

I had the exact same problem. On mine, one gas line was a bit to long and it was crimping, so at startup the carbs had gas and when the gas ran out that cylinder stopped working. But after sitting for a few minutes enough gas was able to fill up the carbs again. I shortened the gas line and made sure it wasn't crimping and, bingo no more issues.

I was trying to rule out a bad plug by switching them, but It did that weird thing where it decided not to break properly the next time I have an older pair of plugs that I know are working, I will try them out. Also, the kink in the fuel hose is a good theory, because maybe when the bike is cold, the hose is less likely to bend, but when it heats up it finally kinks. I don't want to change both things at once (cause I really want to know what it actually is) so I will start with the plugs. Something I read in TwoManyXS1Bs link about the bike having hick-ups made me think that mine actually started doing that at well, while sitting at a light, it would just "skip a beat" I did't pay it too much attention, I figured it might even be the weather.

So, its not the spark plugs. I am consistently able to recreate the problem. It happens about 5 minutes into the ride, usually on straightaway while cruising.
I removed both carbs and checked them out, everything looks normal.
Put them back on, problem still there. I figured something out though. When the right cylinder dies I can coast or drive to a stop, and it will still be idling on the left cylinder, and will continue to run on the left only. If I turn the air screw in 1/2 turn on the dead cylinder, it comes back! If I try and drive it home like this, the right cylinder will eventually die again. If I turn the air screw in again (it has bottomed out at this point, because it runs at 1 full turn out normally) the cylinder will again come back to life! But it will run crappy, and If I return the air screw to its normal 1 full turn out the bike will be idling on both cylinders again, as normal.
I am stumped. I will re-do the fuel plumbing soon, but I'm not so sure thats where the issue is.

What your describing is dirty fuel at least in that one carb..... turning the idle air screw dislodges the crap and it gets fuel again for a bit
till it plugs up again .......
however if you can get it to do it on demand or a schedule then it's not fuel related ( fuel would be more or less random) so, that has to be electrical
check the coil and all wiring if it's a TCI unit I would pull the TCI open it up and see if any of the components look like their burnt
and re solder every connection on the back side. ( more than likely its the 4 diodes in the TCI..... )
..... Just my opinion, I'm no expert so use your own judgment!
....
Bob........

I have done some trouble shooting.
It is not the spark plugs, and it is not the fuel.
Would a change in weather make that much of a tuning difference? I raised and lowered the needle clip, and it made a difference, but didn;t seem to stop it from dying out, or wanting to die out at 1/4 throttle, just cruising steady. Bike starts great, idles great at 1 screw turn out. Just when cruising steadily it starts to bog down, and sometimes will actually die. It doesnt seem lean (plugs look good, never backfires or pops), so would hot humid weather make a VM34 run that much richer, to where it actually dies out? Also, the timing is spot on with the timing light, but does hot weather make a difference with timing also?

sounds like you may have the needles for the needle jets set too high that will cause a flat spot in acceleration or running of any sort above idle. they are supposed to be in the center slot , if you changed them put them back ! then tell us what it's doing !
we can't go chasing out tails all over because you change something and come up with different symptoms when trying to find the cause for the original complaint !
and your Assuming the carbs are clean may or may not be correct I had to tear mine apart 4 times to finally find the blocked passage !
most guys here have also done that so I'm in good company, their very picky carburetors ! not the run of the mill Mikuni carb of decades ago !...
......
Bob......

system9 said:

I have done some trouble shooting.
It is not the spark plugs, and it is not the fuel.
Would a change in weather make that much of a tuning difference? I raised and lowered the needle clip, and it made a difference, but didn;t seem to stop it from dying out, or wanting to die out at 1/4 throttle, just cruising steady. Bike starts great, idles great at 1 screw turn out. Just when cruising steadily it starts to bog down, and sometimes will actually die. It doesnt seem lean (plugs look good, never backfires or pops), so would hot humid weather make a VM34 run that much richer, to where it actually dies out? Also, the timing is spot on with the timing light, but does hot weather make a difference with timing also?

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The weather is a mild influence, but wouldn't do what you're experiencing.

Unless, the carb body is thermally expanding at a rate that reveals a problem.

Closing down the air screw is significant. Makes it sound as though air is leaking into some fuel passage, reducing the vacuum required to pull the fuel from the bowl. I'd be looking at the passageways between the air screw, jet, and outlet port. Also check the bowl vents.

A test others have done is to simply swap the VM34 carbs. If the problem follows the carb, that confirms it's the carb.

Do you live somewhere with hills, or is it all flat? One other thing to potentially test, that could point towards a gas issue and the carb not having enough fuel.... when you're on the highway get up to speed, and after you ride for a couple minutes, before it starts to have it's problem, pull in the clutch and coast for a while(where a hill becomes useful). See if doing that a couple times keeps the bike from dying in the same amount of time. If it extends the amount of time before the bike dies, it could mean that the carb is starving for fuel, and when you pull in the clutch it's giving the carb time to get extra fuel in it, and isn't sputtering out as soon.

The fact that you said you swapped the plug wires and the right side remained the problem, and you know it wasn't a bad plug on the right side, leaves me wondering how it could be an electrical problem if it's entirely isolated to the right cylinder.

When it dies, if it takes a few minutes before you're able to restart it, you could also keep a spark plug wrench with you and as soon as you pull onto the side of the road, pull the right side plug and see if it's getting spark before the bike gets time to cool down.

Also... try turning the petcock off while you're riding so the carbs drain entirely, the bike will die, turn the petcock back on and try to restart the bike. See if it starts immediately, or if it takes 5-10 minutes before it starts, would be an indicator that the right side carb just isn't getting as fast as it should.

there is a screen Under the needle valve in the float chamber on BS34CV carbs that screen is probably plugged with crap and can't fill the float bowl fast enough to keep up with the engine.... ( if it has BS34CV Carbs, dunno about the BS38CV's)
.....
Bob........

I may need to reiterate that the problems I am experiencing are with the VM34 carbs. Some of the tips I am getting are for the BS34/38 carbs. The info is useful but I just don't want there to be any extra confusion.
Tonight I removed the VM34 carbs and put on my BS34 carbs. The BS34 carbs were tuned to a different set of pipes than what I am using now, but here are the results.
The bike started on the first kick, and idled great. I took it for a ride, and while a different experience than the VM34 carbs, the bike ran surprisingly well, roll ons were super smooth, and while not as "explosive" as the VM34's, the bike was pretty quick. All in all I could ride it like this and it would be considered "ok"
Now, when I rode it at steady throttle, as before where the VM34 problem would occur, I could sense the same sort of thing happening. It was as if the power was being sucked away. Totally different carbs, same sort of issue. Cylinder never went out like on the VM's. I started to wonder something, could my exhaust pipes+muffler be causing this? My pipes are custom and a rather large diameter pipe. I wanted plain steel pipe, and found a company that made these basic pipes, probably for racing applications. With the VM34's and these pipes the bike will scream up to high speed and bury the needle. But now I'm wondering if at a certain rpm, there is an issue with back pressure (or lack of) That is causing some vacuum effect? Spark and fuel are good. Attached is a picture of the bike, so you can see the pipes and mufflers.

Oh I like the pipes, but it may be that you have too much back pressure ? do you have baffels in the mufflers ? if so take them out and see if that sag in power goes away.
....
what is your Spark Plug color with the BS34 carbs on there ? You should check before you go to far ! just to make sure you don't melt a piston !
.....it kind'a sounds like there is a restriction in the fuel line or gas tank, make sure you have a good continuous flow of fuel, like fill a pop bottle with it....
.....
nice looking Bike !
Bob........

Did you try what I said about running the bike, turning off the fuel until the bike sputters out and dies, turning the fuel back on, and trying to start the bike again right away? It was definitely a good idea to try the different carbs, but I don't think that entirely rules out gas because you're only testing half of the potential sources. There could still be a blockage in the tank or in the fuel line that's restricting the gas flow before it even gets to the carb.

upthemaiden said:

Did you try what I said about running the bike, turning off the fuel until the bike sputters out and dies, turning the fuel back on, and trying to start the bike again right away? It was definitely a good idea to try the different carbs, but I don't think that entirely rules out gas because you're only testing half of the potential sources. There could still be a blockage in the tank or in the fuel line that's restricting the gas flow before it even gets to the carb.

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I re-did all the fuel plumbing, as well as replacing the petcock with a high flow Pingel (it was something on my list to do anyway)
I took the bike out, and while it was running good I turned off the gas, the bike started to die out soon after, and I turned the gas back on while still riding at the same RPM, and it came right back to life, so gas is good. For the fuel tests I even removed the inline fuel filter (which is also a high flow filter) just to make sure the gas was really flowing.
The original symptom still seems to be there even with the different carbs, that is what is really bothering me.

Humm, OK have you taken off the TCI unit and opened it up and looked at the board yet ? seams like you've done everything else, done it right and even double checked ! that's the way you have to do these things !
..... It's possible that the TCI unit is giving you problems... not very likely but possible
what usually goes bad in them is the 4 diodes by the output leads they can be replaced with diodes of higher value ( theirs a section on it in the tech section, and if you have the board out it'a a good idea to re heat /solder every connection on the board ....carefully don't bridge any gaps accidentally.....
....Bob.........

Bob Kelly III said:

Humm, OK have you taken off the TCI unit and opened it up and looked at the board yet ? seams like you've done everything else, done it right and even double checked ! that's the way you have to do these things !
..... It's possible that the TCI unit is giving you problems... not very likely but possible
what usually goes bad in them is the 4 diodes by the output leads they can be replaced with diodes of higher value ( theirs a section on it in the tech section, and if you have the board out it'a a good idea to re heat /solder every connection on the board ....carefully don't bridge any gaps accidentally.....
....Bob.........

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I am no longer using the stock TCI. My TCI went bad a couple years ago. I am using a PAMCO and a PMA. I have recently checked the timing, and it is spot on at idle, and at higher rpm's. But, I'm not entirely sure there isn't some electrical problem with the PAMCO that is messing up under load. When the bike is stationary I can check the timing at all kinds of sustained rpm ranges, and it never skips a beat.

system9 said:

I am no longer using the stock TCI. My TCI went bad a couple years ago. I am using a PAMCO and a PMA. I have recently checked the timing, and it is spot on at idle, and at higher rpm's. But, I'm not entirely sure there isn't some electrical problem with the PAMCO that is messing up under load. When the bike is stationary I can check the timing at all kinds of sustained rpm ranges, and it never skips a beat.

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The PAMCO drives the coil. There is only one coil. This coil feeds both spark plugs. Unless there is something wrong with the coil, both cylinders get the same spark at the same time. Of course, I could be wrong. I had a customer tell me that his rear brakes never worked the same after he installed a PAMCO.

pamcopete said:

The PAMCO drives the coil. There is only one coil. This coil feeds both spark plugs. Unless there is something wrong with the coil, both cylinders get the same spark at the same time. Of course, I could be wrong. I had a customer tell me that his rear brakes never worked the same after he installed a PAMCO.

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I've been saying that I don't think it's the spark. Thanks for the reassuring.
As a testimony to the PAMCO, I run my bike really hard, Like to its limits. Hard enough that I burnt up a stock Reg/Rec (the one that came in the PMA kit) in 3 days. But like I said, the timing is still spot on