I've noticed that on my 85 K100RS that the #4 spark plug is very sooty compared to the rest. The others all look white. I'm thinking that this is a rich condition possibly caused by incomplete fuel atomization from the injector. Or maybe low compression?

Also of note when trying to solve a rough idle, I sprayed brake kleen on the #4 intake boot. It made no immediate difference but it seemed that the solvent eventually soaked through the intake boot & the engine ran very smooth until all the brake kleen was exhausted.

Thoughts?

hmmm ...kinda sounds like a lot of posts we have seen about the boot rubbers and air leaks .

sooty plug ....could this be lack of full compression (ie valve clearances /seating)
might be a good idea to do a compression test and compare the results (if you have a guage handy )....

the crank case breather tube enters the plenum right above no 4 intake and i wouldnt be surprised to see some gunk buildup sitting in the valve intake area under that stub as well as through the throttle bodies maybe the brake cleaner loosened up some of the crud sitting there in the valve intake area .

im thinking a good cleanup of the area (throttle bodies and rubbers internally) might be a wise idea. new plugs etc ...but dont forget to check your high tension leads ,coils etc



im definatly doing mine as soon as i get a chance ....after new tyres , driveshaft, as so forth..that list keeps getting bigger...lol

Hi there, sooty how? is it black dry and porous or looks oily?
In the first case (assuming all the electricals are ok) you can have a miss balanced trottle causing a rich mixture, most probably at iddle. If oily then a valve seal, guide or a piston ring.
Best regards
JAP

The plug is dry & much darker than the others. Looks like incomplete combustion to me. I plan on doing a compression test when I have the timing chain back on. I'm wondering about a clogged injector?

Regarding incomplete combustión (rich mixture) I agree with you, and it could also be an injector, but had you check TBs synchro at idle?.
I think its easier than messing with the injectors
Best regards
JAP

I have not checked the synchro but intended to do it at some point. I will do the comp. test first because I'll have the tool & when the bike is back together, I'll get a synch tool & check that first.

Thanks!