I have a 93 cbr 900rr w/ 945cc big bore kit motor now has about 4,000miles. the problem im having is that it blows a bit of dark smoke out of the exhaust only when i get on the throttle then it goe's away and it does'nt do it when im at a steady speed.you could also smell fuel too. I have jetted it w/ 120 main. i sync the carbs, changed all air screws except for #3 couldn't get it out i have all 3 carbs 2 and half turns out..also have a k&n air filter...do you think it could be that #3 carb? also if i have the bike in nutural and rev it out it will blow smoke. thanxs for any help!

 

Dark smoke = too much fuel for sure. How lonng has it been since you changed the plugs? Wonder if it could be a weak spark, or if the plugs are just getting fouled. I'd change the plugs an test the coil and wires to make sure they are in specs before touching the carns again.

 

I dont think it has to do with the spark because the run good and pulls hard through all the gears all the way to rev limmiter.

 

Too Rich.....

 

ive come to the concluesion that its not fuel ..its burning oil.. I know it cant be from the rings because then it would be smoking constantly so im thinking from the head maybe vavle gueds. what do you think?

 

My friend's 600RR does the same thing. It has about 14k miles on it...dark smoke, but only between shifts. I figured it wasn't anything to worry too much about. But if it's doing it the whole time you're on the throttle, i'd guess that's a bit more serious.

 

Maybe you can try to read the plugs, I've never had any luck, only in severe cases could I read them, but Black smoke is carbon (too rich), Blue smoke is oil. Can you yell what color the smoke is? As far as the valve guide seals, I was under the impression if they were the culprit, the oil smoke would be at start up, not at full throttle. If you smell fuel, why do you feel it is oil? Have you had to add any oil?

 

valve seals cause smoke at startup and can also cause a little during a shift, depending on how bad they are. Prior to startup oil will seep through them and into the cylinder, then get burned off at startup. It doesnt cause smoke suring steady riding or acceleration because the combustion pressure keeps the oil from seeping. However, during shifts or deccelation a vacuum can occur in the cylinder when the intake calve opens up against a closed throttle plate. No free air coming in but the pistons still drops, causing a vacuum that can pull oil through a bad valve seal.

 

I found out its oil because im losing oil and the smoke is kinda blueish. I just got through putting another set of carbs on over the weekend. saturday i checked the oil level and i had to add.i started it up and went for a ride and it did the same B.S.[:@] its not carbs, so im burning oil somewhere..correct me if im wrong but if it was my rings it would smoke constantly the whole time...thats why im thinking its comming from the head whats the% its the head or rings..Help please...ive been stressed out about it & sick of sinking money into it, the motor has under 5000,miles on the bottom end..the head i bought on ebay they said it came off a bike with 12000,miles so i though it would be a good head so i didnt have it checked out & money was tite at the time. ..so how would i be able to tell were its comming from?

 

Blueish is smoke ofr sure.

It could be from the rings as a lot of engines will burn oil at hard acceleration, but not under normal operation. I'd probably think it was the head though as that is what wasn't worked on. Too bad you'll have to pull the head and get things redone to be sure.