help! 400ex cutting out!

[bc_sandlin]

when i push the gas down real fast (in neutral) my 400ex cuts out. its been rejetted. its got a k&n air filter and t 4 exhaust. can someone tell me the problem?

[GPracer2500]

Originally posted by bc_sandlin
when i push the gas down real fast (in neutral) my 400ex cuts out. its been rejetted. its got a k&n air filter and t 4 exhaust. can someone tell me the problem?

It's not unusual to induce a bog by whacking open the throttle very quickly while in neutral (or at any time the engine is under a very light load). Here's what's happening:

A temporary lean condition (too much air, not enough fuel) occurs when the throttle is wacked open quickly from lower rpms. This happens because it takes the carb a moment to respond to the sudden increase in airflow through the carb. When the carb's slide suddenly goes from closed to open the engine starts sucking lots of air but the carb's ability to deliver fuel lags behind a little. The accerlerator pump (assuming the carb has one--stock 400EX carb does) is there to correct this condition but APs are not perfect. The AP works by squirting a raw stream of fuel into the intake when the throttle is quickly opened. Open the throttle slowly and the AP doesn't activate (because it isn't needed with more gental throttle inputs). Sometimes APs squirt too much fuel, sometimes not enough. APs can be one of the toughest parts of a carb to tune well. Many AP equiped carbs aren't even designed to have the AP tuned. The 400EX carb is one of them.

If it only happens in neutral and not when you're actually riding then I wouldn't worry about it. If it happens other times or is otherwise troublesome, raising the idle speed a little can help lessen the bogs severity. If that doesn't help and you still want to lessen or eliminate the bog than you'll have to tune the AP circuit. Since it's not really designed with any tuning features, it's gonna be tough to do. Probably not impossible, but tough.