help diagnosing misfiring cylinder

Hi all, I was hoping you might be able to help me diagnose a misfiring cylinder on my 1980 KZ650 F1.
1) Cylinder #4 is not firing on many, maybe most, firing cycles
2) Exhaust head for #4 gets nowhere near as hot as the other heads
3) Pulling spark plug cable for #4 when the bike is running stalls the bike after about 5 seconds, but for other cylinders (#1 for example), stalls it after a second or so. This leads me to think that it is firing somewhat, but not always
4) The spark plug for #4 is foulded slightly wet black.
5) Switching the sparkplug cables for #1 and #4 does not fix the problem - #4 is still the one misfiring. Note that my spark plug cables are directly attached to the coil - emco junk.
6) Switching the sparkplugs between #1 and #4 does not fix the problem - #4 is still the one misfiring.
7) Spraying brake cleaner on the carb boots and the air box manifolds around #1 does not change idle, so no obvious air leak issue.
My head gasket is slightly leaky, as determined by leaking oil, but the bike ran for a season that way.
9) When I put one finger on the top of the sparkplug boot for #4 and another on the chassy, I get a bit of a shock, though not very hard. This persists on #4 after switching both sparkplug cable and sparkplug with #1


Any advice? Thoughts? Thanks

You didn't mention what happens when you spray the carb boot of cylinder #4. Are you able to do a compression test of all four cylinders to see if #4 is significantly lower?

ayeckley wrote: You didn't mention what happens when you spray the carb boot of cylinder #4. Are you able to do a compression test of all four cylinders to see if #4 is significantly lower?

I held off on responding to this thread because it took me a bit to do a compression test. With regard to your first question - I mistyped, and head meant to say that nothing changed when I spray the carb boot of cylinder #4.
The compression test showed compression to be fine on #4. I don't totally trust the tester, and was having difficulty getting #3 above 90psi, but #4, the cylinder that is definitely misfiring, held 140psi, which seems great and is in line with what it was when I last checked it two years ago. So, where to go from here?

Welp, sounds like ignition and compression can probably be ruled out. A bad mixture ratio from an intake leak can also pretty safely be ruled out. That pretty much leaves just a bad mixture ratio from something *other* than an intake leak. Float levels all consistent using the clear tube method? Stock airbox?

Edit: or the carb synchronization for #4 could be off. The problem with that one is sorting out if a different vacuum level is the *cause* of the misfiring, or just a side effect of the misfiring. If it had previously run correctly and the adjustment hasn't been messed with then it's probably OK to disregard that as a possibility. I'm assuming these are not CV carbs.

ayeckley wrote: Float levels all consistent using the clear tube method? Stock airbox?

It's a stock airbox, and you're right that they're not CV carbs, but you know what, you may have pointed me in the right direction. I didn't use this bike very much last season, and reviewing the notes from when I last worked on it, it seems that I played with the float levels on 4. I'm going to give it a check as soon as I can, and it'll be interesting to see if that's the issue. Haven't mucked with the sync at all, so I don't expect it to be that.

I appreciate that this is a bit academic, and the first thing to do is to just go check the float levels, but what should I expect to see there? The sparkplugs on #4 are fouled, so it's running too rich to fire, at least sometimes. I can understand the floats being set too low such that there's not enough fuel coming in, leaving the mixture lean. Is it possible that too-high floats would give a too-rich fuel mixture?

Is it possible that too-high floats would give a too-rich fuel mixture?

yes. And a plugged pilot circuit can also give you a rich condition. But the clear tube test would be in order first if you messed with the float level.

Try a fresh plug in #4? Check how the plug fires outside the engine on #4 (not near the plughole).
Then check for air leaks in the intake boot.

aenikolopov wrote: Is it possible that too-high floats would give a too-rich fuel mixture?

Yes. The "head height" of the gas is too small, and more fuel is drawn out of the bowl than should be. Usually there are other symptoms as well, but you don't seem to be having them so it's not an air-tight diagnosis. Usually ( maybe I should say "sometimes") if the float levels are too high then there will be weeping of the gasketed joint between the float bowl and carb body, or even gas flowing out of the overflow port. There can be afterfiring as well (not to be confused with backfiring).

weeZee wrote: Try a fresh plug in #4? Check how the plug fires outside the engine on #4 (not near the plughole).
Then check for air leaks in the intake boot.

Thanks, I've done all that, hasn't lead me anywhere, unfortunately

ayeckley wrote:

aenikolopov wrote: Is it possible that too-high floats would give a too-rich fuel mixture?

Yes. The "head height" of the gas is too small, and more fuel is drawn out of the bowl than should be. Usually there are other symptoms as well, but you don't seem to be having them so it's not an air-tight diagnosis. Usually ( maybe I should say "sometimes") if the float levels are too high then there will be weeping of the gasketed joint between the float bowl and carb body, or even gas flowing out of the overflow port. There can be afterfiring as well (not to be confused with backfiring).

So , I went ahead and checked the float levels on #4, and in fact the fuel was too low, and I corrected that, but was surprised when I turned on the bike that it hadn't made anything of a difference at all. There were a few specs of black crud in the bottom of the #4 bowls, I wonder whether that implies there might be something stuck in the jets, though the issue really seems to be happening at idle as well as at full speed, or I believe it is at least, so it would have to be pilot and main jet.

Your mileage may vary, but I chased my tail for a long time on a similar issue that I eventually traced to a poorly-sealing float valve. I adjusted the float heights so that everything was perfect (using the clear tube method) but still had bad performance after a few minutes of riding. What was happening was that the leaking valve(s) would mimic a higher fuel level than would actually occur when the engine was running. I would start with empty float bowls, but by the time I ran the test the levels were higher due to the leakage. I lapped the valves into their seats (for a second time, and with more gusto) and also modified my test technique so I was looking at the initial fuel level, not the 5+ minute fuel level. Between those two changes I was able to resolve my poor running (carb boots had already been replaced, stock air box, clean OEM jets, carbs had been synced, valves adjusted, ignition checked thoroughly, etc). At some point I also discovered clogged "undocumented" filter screens after removing the float seats, but that was on BS34 carbs, not VMs like you probably have. Maybe something like that is happening here, although it sounds like your symptoms aren't changing over a 15+ minute time span like mine were. Beyond that I'm afraid I'm out of ideas.

Edit: I shouldn't say I'm out of ideas, just good ones. Everything else I've got becomes a science experiment that you probably don't want to launch into. At some point the easiest thing is just to try a different set of carburetors and see if the issues persist.

ayeckley wrote: What was happening was that the leaking valve(s) would mimic a higher fuel level than would actually occur when the engine was running.

I took a look, the main thing I noticed is that when the engine is running, the fuel level drops about an extra 5mm, using the clear tube. That seems to be normal though (?), because that happens on both #1 and #4, but only #4 is misfiring Any if I'm right about that being typical?

Also, I noticed again, that I am easily shocked when I ground myself with one hand and touch #4 at the bottom of the boot, right by the engine. Now maybe that's not terribly surprising, but it doesn't happen on any of the other plugs/wires. As I stated before though, I've swapped both the plugs and the wires on this cylinder before (with cylinder #1) , to no avail.

I should add - adjusting the air screw all the way in and all the way out has no effect. reset it to where it was when I started.

Ayeckley, I'm happy to hear about any experiments you have brewing in your head, because I'm not sure where else to go, other than tearing out carb #4 for a cleaning.