I just bought a new K&N air filter and ran my 400EX. The quad seemed to run great, but it is idleing very high. I still have the stock jetting in it, could this be the problem? I also have the end cap off the silencer, but no other air box mods. Even messing with the idle screw won't solve the high idleing. Also sometimes it back fires when I shut it off. Does backfireing mean it is to lean or to rich? Will I have to richen up the jetting? If so, will I have to change the pilot and main jets or just one and to what size? Will turning my air mixture screw in help this out? Thanks for your help.

 

Quad 999, are you sure your choke is off ALL the way? It is backfiring when you shut it off because the idle is so high. Are you sure your throttle cable is routed correctly, has enough play?

 

Increasing the amount of air (either in or out), without a rejet will make the bike run lean. That can destroy your engine!! I wouldn't ride it again until you rejet!!

 

You will need to rejet for best performance, but as long as your compression ratio and ignition timing are stock, I would say don't worry about engine damage from lean mixture on your 4 stroke engine. I can't see that the modifications you have done would cause your high idle problem (you should be able to get an adjustment via the idle speed screw). If you think so, pop your stock stuff back on and see if the problem goes away. Also if it is lean, you would want to turn your mixture screw out to richen. Don't confuse the idle speed and mixture screws. good luck

 

Oh yeah Quad, make sure a backfire didn't partially dislodge your carb from the intake boot too.

 

quad,

sounds like you might need to adjust the trottle cable a little.......

 

you don't need to rejet a 400EX just because you put a K&N airfilter on it

I'd check you throttle cable, choke cable, and idle screw.. to solve you problem