Backfire

I have a 2001 400ex & I have done some minor upgrades to it.. One of the upgrades, I put a FMF power core IV & a K&N air filter.. When I am going down hills the engine back fires some.. A friend told me that the carberator needs bigger jets. Should this correct the problem that I am having?

Yes you need to rejet to cure your problem.

ya that backfire is because its running lean. lean pops

REjetting will take care of the poping and there will be a definite power increase. The name of the game it breathing but more air requires more fuel. Get bigger jets!!:)

Yea, you opened both the intake and exhaust sides of your engine and are probably running a little lean, so you need to rejet the carb.

As a side note: Hearing it backfire when going down hills...deaccelerating...isn't always an indication of running lean though, and with a free-flowing aftermarket exhaust you'll often hear this popping even on a properly jetted engine. More often than not this popping is usually caused by running a little rich--what you're hearing is a little bit of unburnt fuel being ignited in the exhaust rather than in the engine. Just remember that whichever circuit you're checking a lean condition is most always noticed by popping or a hard hesitation "under acceleration" and/or skipping at WOT, whereas a rich condition can be noticed since the motor will blubber since it's getting too much fuel.

what nvr2l8 said is true about it's a normal poping due to the unburnt fuel. even a stock brand new 400 ex has some kinda poping but is less heard due to the stock exhaust

Glover,

If you are hearing the backfire when you going downhill and you are off the throttle, it's a very good indication that you slow fuel circuit is too lean. The stock pilot jet is a 38, with the mods you've done, I'd recommend a 42 or a 45 pilot. Every motor is unique when it comes to jetting, but the majority of the 400's that I've done have been most happy with 45 pilot. Improved cold starting will be another advantage of a properly jetted slow circuit.

While the problem you mention is related to the slow circuit, I'd also suggest stepping up the main jet a few sizes with the mods you've done.

Hope this helps!
Bumpy