XJ 750 one black sparkplug

Hi XJ buddies.

My bike used to run just fine, but some time ago I noticed a sudden loss of power. I could barely get 100Km/h (60M/h) out of her. I resorted in just holding the throttle wide open and all of a sudden she came alive! She was fine after that for some weeks when the problem cropped up again. I inspected the sparkplugs, and noticed cylinder 1~3 looks healthy (light brown) whereas no4 is is matt black (not wet). She starts up easily, and idles perfectly. While on idle, I pulled out the sparkplug lead on no 4, and noticed a different rhythm. Putting it back returns the idle to normal, which makes me think I don't have a spark issue. Wide open throttle it sputters, but any given high speed cruise speed it runs just fine. It also seems the problem is a bit 'retarded'. At high speed what I want to accelerate it feels fine for a second, only to start sputtering again.

Carburetors has got new jets and diaphragms, and seems to be in pretty good shape. The black sparkplug on no4 seems to be the culprit area. This may be due to over fueling with wide open throttle. I'm not sure if the float level is within spec, bit it seems OK comparing the cards with each other with float bowls removed.

Any ideas please?

 

Sounds like cylinder no 4 is running a little rich. Could maybe be that the float on that carb is out of adjustment or maybe the mixture screw just needs some tweaking. Did the problem start around the same time you new jets and diaphragms in?

 

Hi buddy.

No it's not a result of new parts fitted. Problem was also initially intermitted, which leaves my dumbfounded. It's now permanent.

 

If it’s always kind of been a problem like you say, I might start with just screwing in the mixture screw on #4 a quarter or a half a turn depending on how rich the spark plug looks and seeing how that goes.