no 3 cylinder plug running very black

[duzatun]

the frustration of problem solving can get to the best of us but once you find it frustration is replaced with accomplishment haha
well i had probs with my bike breaking down at 2/3 s, throttle and with the help of some of you guys i fixed this with the bigger jets but all through this no 3 plug has been running very black even before i jetted up the carbs when the other 1, 2, and 4, cylinders were lean no 3, was still black some one said may have a leak in inlet manifold but i figured this would make the plug show even leaner than the others , i could be wrong i done compresion test and doesnt seem to be much difference in all four cylinders  but as i only had my old compresion meter hmmm lent the good one and wasnt returned lesson learnt wasnt 100% sure  as i was thinking maybe stuck or cracked ring or valve problem possibly a sad spark from the coil even, by looking at the spark with plug out and held against the head it looks just like the  other  3 plugs not an accurate measurement i guess, tried new plugs same result not to keen on taking the barrels off to check out in their before i locate a top end gasket kit not sure what the head gaskets on these are made off but from what i remeber on the old 750,s  a lot of extras such as o rings and such to replace as well. this is the last problem with the motor left for me to fix then i will feel confident enough to take her an a long run looking forward to that
thanks for any info
edd

[matthewmosse]

The 500/550 mill is a tough cookie, as long as it has oil, not much will stop it, it it is a sticky valve or piston ring then a good few thousand miles might just free it off, If you do take the head off then there are a few O rings to replace as well as head and base gasketts. Personally I've found even fairly bad crank oil leaks on stood engines have self healed after a few months of commuting use, just kept the oil topped up, camchain tension right and got rideing, the flow became a trickle, then just dried up. Whatever is causing the black plug might well do the same thing. Since I stopped using my 500/4 outfit reguarly it has become a right sod to get running right (or at all!) 

[Dave M]

I had a similar problem on a CB750F2C where No 4 was flooding, especially when idling so I suspected the float height.

When I stripped it down I found that someone had replaced the non adjustable plastic float with a brass one. I put a plastic float back in but the level was still too high so I ended up putting the brass one back in and stting it to spec and it cured the problem.

Bit of a mystery though as the other 3 plastic floats were fine but the fourth was too high, yet there's no no means of adjustment.