Topic: #4 plug fouling-what next?

[davesee]

i've been spending lots of time on my bike this summer....in the garage (1980 cb650 stock airbox and exhaust)

right now the bike is fouling the #4 cylinder spark plug after about 25 miles. it seems to run fine for that long with a reasonable idle.

i've cleaned the carbs a few times, have cleaned the tank and am running an inline filter. i have static checked the timing and set the valve clearances. i have bench synced the carbs as well. i have reset the float heights to what the manual says, and if anything, #4 seems lower than the rest. the plug was fouling before the most recent carb clean and float adjust as well.

before i haul the carbs out again (again!!!!), what should i be looking for here? i know that this is usually a symptom of running rich, but i'm at a loss as to how exactly to find the cause. all the other cylinder plugs are a uniform shade of light tan.

any help is appreciated.

thanks


 

[vern401]

What is the compression of the cylendars?
Check the spark on it to see if it is as strong as the others.
Is it powdery black or thick black on the plug? The thick would almost be sticky.

[ARANUC]

i would check your compression and if that is good check to see if your carbs are in sync if #4 is off you might be dumping fuel into 4

[davesee]

compression is pretty even across the board at 115 cold. the fouling seems powdery not sticky.

i synced the crabs roughly with a drill bit/eyeball, but the num,ber four set screw and the lock nut seem to be stuck. i've set the screw to the proper setting (these are mechanical carbs) but maybe it has drifted by not being locked down. can i adjust this while the bike is running? and how will i know if i've got it right?

thanks again

[Spanner 1]

I think you have answered your own question Dave, if 1,2 and 3 carbs are sync'ed... it's possible #4 slide is sitting higher at idle/low rpm and making that cyl. over-rich....... have to undo the locknut and adjust the slides again... the flashlight method seems most accurate bench sync........ a rough method 'on the bike' would be to adjust #4 while holding your hand 1/2 inch from #4 exhaust and feeling the firing pulses to match 'em up with the others.... you have a 4 into 4 exhaust, I think.?

[davesee]

ah, no, the standard 650 is 4-2...i think the custom is 4-4.

i am now an expert at taking the carbs on and off, so i will just yank them next chance i get.

any tips on seperating the screw from the lock nut? the screw has a very shallow slot, and my impact wrench will not fit. is this a case for a blowtorch? i've never tried.

thanks
again

[davesee]

okay, i'm starting to get a little frustrated on this one. i checked the carb sync again, and although i don't have a method for seperating the set screw from the locking nut, the carb pistons all seem to be in sync =/- .5mm or so. i pulled off the carbs and checked the float which might have been a teensy bit high so i bent it down a bit more. i also felt that the spark plug boot and wire were suspect so i replaced them.

i fired the bike up and the took it for a spin. total running time was maybe 7 minutes...and the plug is completely covered in black. it is not sticky or shiny, just sooty.

the air mixture adjust screw is as far out as i dare get it (in fact i lost one by setting it too far out the other day. 14 bucks for a new one.) the choke valves all seem to be operating well, and all the other cylinders are running right on with tan plugs. at warm up idle, the #4 pipe is hot, but not as hot as the others. this continues for the entire ride.

i welcome any suggestions on what to try next...

[vern401]

Ok set all the floats to the same height. Put in a New plug in #4. it is possible the plug may be bad. Make sure ther spark is the same with All 4 plugs.
Sync with guages if possible.
Clean carb # 4 real good use an air compressor to blow out all the ports.
make sure the piston needle in the carb is set the same as the others. some carbs had different settings you could set them to.