Hey everyone, I have read this forum for about 7 years now and I finally have a question I couldn't find. I have a 2000 400ex and last month it finally needed rebuilding. I got a 440 big bore, wiseco 11:1 piston, port and polish, hot cams stage 2, yoshimura comp full exhaust, k&n filter, no lid, ride in florida, and NO jet changes. I just knew I had to go up because it was going to be really lean, but the spark plug is card board brown and the atv never sputters, back fires, bogs. What is the deal with that? I broke it in and rode it pretty good 3 days now and no change. I just don't wanna ruin this new motor. The dealership did all the work and put the new plug in after the mods. puzzled

 

They must have jetted it for you, also. There is no way way it could run for 3 days without a re-jet. Check your main jet size and see what it is.

 

Nope I asked if he did and he said they don't do that. they don't have a dyno or enough room to actually test ride.

 

bump plz help

 

uuu need to rejet that befor ur engine sieses

 

not necessarily...some times it's still ok, especially at the altitude you ride at.

The best thing to do is to get on it in high gear wide open for a couple mile or so, then pull the clutch in and hit the kill switch and coast down. Then check the spark plug. If it's still brown or gray, you're good to go. If it's white, then you'll need rejetting.

does it backfire at all when you let off the throttle and decelerate in gear? if it does, your idle might be a little lean, but that might be all you need to do.

Or you could just bring it to a dyno shop and have them check for sure, its up to you.

 

yea it does kinda pop when i let off the throttle and downshift but not like backfire real loud or nothing. about the dyno, i looked all around for a shop that has one and the closest one is around 3 hours away. that is what i really wanted to do in the first place. thanks for the reply