Topic: CB650 Only running on 3 cylinders.

[sloppydawg123]

didn't sync the carbs, that was on my "to-do" list with a homemade vacuum tester, but seems rather pointless if #2 isn't firing. And if the slides were too low, would that stop them from running above idle? Seems like when I held it around 4k for a good while they would have warmed up quite quickly. but i could still grab the header off 3.

[sloppydawg123]

Just did a compression test, running 130-140 cold, so plenty of compression. Guess I'll do timing, next, because if it's got compression, and the spark, and the timing is on, seems like it has to be a carb dilemma. Any suggestions would be great

[sloppydawg123]

Seems I forgot the timing light at home, but the 1-4 was dead on, and it says only matters for the custom, but I measures the air gaps as well, the 1-4 was .034" and the 2-3 was originally .011. Also the 2-3 seemed a little off for timing, so I moved it a bit, and also set the gap a little further back to around .024, which is in spec with what it said for the customs (.0160-.026 i believe.). Guess next step it check the valves. And if all that doesn't fix anything, take the carbs back off

[cb650]

Customs and standards use the same iggy.  So what your reading from is ?.   Also being a "CDI" the timing dont need to be spot on.  The gap is the biggest thing.  Except for some experimenting when I raced the only thing I have had to change is the gap. Usually because I bumped it while turning the motor over.

[sloppydawg123]

Well timing should be on, i started taking ohm's readings on the coils, couldn't find what they're suppose to be, but they are 2.5 and 2.4 across the secondaries and 16K and 12K across the plug side. But the 12K one is the one that is working, so maybe the coils are bad? I'm beinging to think it's a spark issue because I bought a spark plug test kit, and it  only jumps very small gaps. The battery is on a charger around 12.2 volts, maybe it has something to do with that being low, and it's reading around 11.6 at the coils. Seems like since it does spark, and does so at the right time the pulse generator and spark units must be fine, and the caps read ~5ohms on all, so think the caps and wires are fine. Really running out of ideas here, gonna do some more playing, just to see if I can get 2/3 running by swapping coils or something, and if not probably pull the carbs off or something. Not sure what else it can be, it's wearin me down.

[BVCB650]

That is always my first diagnostic process (swaping parts with known good ones) and never try to trace a wiring-volt issue without first verifying a fully charged battery.

[sloppydawg123]

1&4 run off either coil, and 2 runs off either coil, so now looking into maybe a carb issue? Took them off, bench synced them, still have leaky floats (holes in tubes) apparently epoxy won't hold in gas, so on to new ideas, but that's a separate issue than the "running on three cylinders" dilemma, leak is very slow, and would seem to be irrelevant to the problem.

[Hush]

You don't have leaky floats do you? no I think you have carbs leaking from being over full.
My #2 plug kept fouling and the bike always started on 3 cylinders with the #2 chiming in as it warmed up.
The only difference from any of the other carbs to the #2 is that accellerator pump, if the tiny "o" ring is buggered gas floods # 2 first then #1, gas also floods back into the air filter if you have the stock setup.
If you do take the carbs off please do yourself two favours, #1 replace that "o" ring and #2 replace the standard screws that hold the bowls on with allen key/hex key screws, makes removing the bowls in-place a breeze in the future.

[sloppydawg123]

no leaky floats that i can tell, holes in the overflow tubes. and gonna clean the carbs again, fix the holes, set floats, bench sync, cross fingers, and retest

[Buber]

I still vote for the ignition. If swapping coils doesn't work, then swap the impulsers (or whatever they are called, under the seat), and check the power supply to the coils (you can try swapping that too, remember only to swap the plug cables too).

It would be extremely rare for both carbs, and on that, those two carbs to fail at the same time. Hence I opt for ignition.

[juntjoo]

so what happened here? if you remember that far back. srill alive? I gotta know why that #2 carb was cold!