Hey I bought a 93 cbr 900 about a month ago, so I havent really been able to ride it too much since its the middle of winter, but when I do get a chance to ride it I have some problems. First off, its hard to start which is normal sinc ots soo damn cold out, but whats weird is even though I have it all the way choked, I still have to give it some throttle to get it to fire up, which I dont like to do cause I dont wanna foul out the plugs. Then sometimes once I get out and start riding, for the first five minuts or so it will run on 3 cylinders, oor sometimes it will run on all 4 right from the start. sometimes it will be running on all 4, then right in the middle of a ride, it will just start running on 3 cylinders for no apparent reason. When its running on 3 cylinders, and you open the throttle and let the revs climb, it will start running on 4 again, until you get back into cruising revs, then it will run on 3 again. do you think its just because its so damn cold out? I'm thinking maybe a weak coil pack, or something else in the ignition system? Its gt a D&D full system, and its jetted too, it runs realy strong when its on all 4 cylinders, so I dont think it running really rich or anything, dont see it blowing any smoke eaither. what do you guys think?

 

Who did the jet kit? Have you started with the simple tune up, new plugs and wires?

 

If it was a weak coil, two cylinders would be affected. If it was mine, the first thing I would check is the ignition timing to see if it advances correctly. Then I would pull all four plugs to see if they all fire and all have the same hot spark. If so, you can assume the ignition is OK. Then I would look at each spark plug. Are they wet, black, or oily? Are they all the same? I would put in new plugs and start the engine. Still runs bad? I would then take out the carbs and clean everything in the float chambers and under the vacuum chambers. I think the carb piston/diaphrams are sticking.

 

thanks for the replies, Yeah< i have changed the plugs, the ones I took out, or atleast one of them anyways, was very wet. It help at first but I think atleast one of the new ones may have been slightly fouled out right away cause the thing is so damn hard to start when its cold out. even with the choke pulled all the way up, you still havwe to give it alittle throtle to get it to start. hopefully with the temps get up to 60 or higher, maybe the problem will go away, if it is due to flooding/fouling of plugs. [&:] oh yeah, as far as the jet kit goes, it was already installed when I bought the bike, as well as the exhaust. I bought it fmo a used bike shop, they said they put it on a dyno and it made 120 to the wheel. its seems to run really strong, my last bike was a 98 tl1000r I was afraid this old cbr would feel weak or slow in comparison, but its really a strong runner.

 

the more I think about it, I think it might be a coil, when I think back to the sounds it was making, it did actualy sound like running on 2 cylinders, not 3. Plus I was reading similar problems other people had and they were coils problems. all on 93 rr's too. where can I find some good coils for this bike though, havent been able to find any.

 

Johnny, say the spark plugs in cylinders 1 and 4 are wet with gas. If you swap your coils and the plugs in cylinders 2 and 3 are then wet, you know that coil is bad. As it is now, you don't know if you have an ignition or fuel problem. If two carbs aren't working it will run on only two cylinders the same as if two cylinders weren't firing.

 

I know it sounds dumb, but I have a suggestion. Make sure your airbox is pressed all the way down on the carbs and seated and sealed.

I had the same problems your having and it drove me nuts. When I found out what my problem was I felt stupid.

We have pressurized airboxes. If there is not a perfect seal then the carbs don't function properly. They run rich and kill the plugs.