Topic: weird plug readings

[carl550k]

drove my 76 550k on the highway at 65-70 mph for about 65 miles round trip. later that night i pulled my plugs and found that 1 was badly carbon fould, but still fired. the other 3 looked perfect. what would cause this?the bike ran a bit sluggish but gave me no troubles on the trip. no misfireing.the mixture screw contols the air, it is out 3 1/2 turns compared to the other 3 being out only 3/4 of a turn.is this a carb sync problem, they have been bench syned. thanx in advance, carl

[eldar]

The mix screw should be turned close to the others. There should not be a 3 turn different unless there was some modification to that particular carb. You should also sync them properly. That 1 cylinder is running very rich and should be cleaned and readjusted otherwise the problem will only persist and probably get worse.

[TwoTired]

There is no valid reason one carburetor should be adjusted differently from the others.  There is likely a problem with the carburetor associated with the sooty spark plug that someone tried to bandaid by turning the air screw out farther than the rest.
Here's a list of things that might cause your problem:

-Main jet o ring damaged, main jet fallen out, drilled too large, wrong one, etc.
-Jet needle clipped in different position than others, wrong profile needle, worn needle, etc.
-Main jet emulsion tube cross drilled holes blocked.
-Main air jet restricted or blocked.

-Float bowl fuel level too high from:
-Leaking float valve.
-over flow tube and or stand pipe blocked.

-Very poor bench sync allowing one carb to be open father than the rest at all throttle positions.

-The slow jet fallen out of position (uncrewed) ?  wrong one or buggered.
-Slow air jet blocked.

-And maybe the spark plug IS carbon fouled part of the time.

I think I would drop the float bowl on the affected cylinder and check what I could of main, slow, and float systems.

Tell us what you find.
Cheers,

[carl550k]

thanx for fast responce, its the number 1 carb, float level is correct, no blocked tubes or passages as they have been cleaned and blown dry. could be the sync, have tried a hotter plug in that cylinder, only helps a little. doesnt really effect drivability. could be an air leak since the bike has always never idled well. either revs to 3000 or dies under 1000 rpms. am going to try auto hose clamps after dinner tonight sincr the originals are still on the bike, almost 30 years old. usually during plug checks i'll rotate the fouled one to another cylinder, runs great for only a couple of days. will keep you posted. carl

[eldar]

Leaks cause a lean condition. This problem will be too much fuel. I would really look at switching the float valves between a couple of cylinders and see what happens. If the problem moves or goes away then the valve was bad or not seating properly.

[cub]

Long shot. Soothy sparkplugs can also be caused by weak ignition. Check the spark on it as compared to the others. Maybe the cable got a crack somewhere?