Hey guys. I bought a beautiful 2002 RK two months ago from an old guy i know. The bikes modded. I believe 95ci heads are done to the max and a big 46mm mikuni. Problem is since i bought it theres a mean backfire that happens sparatically. It happens when I get on the gas in say 4th gear when I should prob be in 3rd....it falls flat on its face....gives a wicked backfire....then takes off like a rocket! Any ideas?

I'm thinking maybe jetted too high!!??

 

Try tweaking the AF mixture before you re-jet. You may be able to tune it out there.

 

Im not a carburetor expert but I'm somewhat knowledgable in air/fuel tuning. If you are getting a backfire that means unburnt fuel is coming out of the pipe causing the backfire. Somehow or another your bike is using the fuel in the higher RPM range but when you get off the throttle the fuel has to go somewhere to escape and its coming out the exhaust in a backfire. We used to have these problems when tuning the new 600 racebikes with the short exhausts. We would tune the Powercommander to utilize the fuel in the higher rpms because most racing is done right about under redline but during heavy downshifts to enter a corner we would get backfire and alot of times straight flames shooting out of the exhaust. Carburetors are obviously alot more primitive than todays fuel injection and we still had the issue with the fuel injection. It seems as though somewhere in your RPM range with a carburetor you are going to be running either lean or rich cause you have a choice of only installing permanent jets. I would talk to a knowledgeable carburetor guy and find a jet thats appropriate for your type of riding and thats sufficient in all types of RPM ranges.

 

I'd think the mikuni may be too large for the motor- when the throat is large, that means that the velocity of the air is low...so the fuel does not atomize as well and big droplets remain intact, unburnt and later vaporize and bang in the exhaust tract.

check the mikuni tuning guides online and start at square 1 as far as tuning and you may get it right
nightrider.com has a good bike tech and carb section

carb tuning is a balancing act, also make sure that the ignition timing is retarding when the throttle is whacked open


for my carb tuning I use an o2 sensor in the exhaust tract which will get me to maybe 95%...subsequent riding helps me dial is the last bit.

while the mikuni has been popular, I don't use them on my Harleys, i prefer the keihin CV carb, which is easily tuned and modded

mike