i just got a 04 400ex...it starts up fine but when you take off it sounds like crap like its really bogged down and then when you hit a mid to high rpm range it comes to life...it is ported and polished aftermarket cam, valves, shaved flywheel new carb and a racing clutch new piston etc...weve cleaned the carb and put a smaller jet back in it spark plug it good we changed the oil and there was gas in it...and the cdi box is good...we dont know what else to do it is still running poor in the low rpm range but runs perfect when you get on it...someone help please...

 

Sounds like a lean air/fuel mixture. Turn the air/fuel mixture screw out a turn and see what happens.

 

weve tried messing with that but nothing changes weve ran it with out the exhaust on it and the cover off of the airfilter box and still nothing...

 

Sounds like bad gas, I had some in my 250ex today, when it burned off it ran fine. Its strange, but there was not water in the gas, it was the quality of the gas itself I got from the chevron.

 

we put fresh gas in it and it still does the same thing...we are totally stumped on this one...the whole thing is is freshly rebuilt...

 

and also when we discovered gas in the oil we changed it and pulled out the spark plug put it back in...it ran perfect for about two minutes then started the same thing over again...weve never had this happen before could the bigger cam be an issue...

 

it may need race fuel......sounds like a low end spark knock to me

 

bigger cams affect the power band. If you want more top end, you'll scrifice low end, and vice versa. Cars are the same way, a high lift, short duration cam builds low end torque, but lacks on top end. A long duration cam works better for top end since the engine is spinning so fast there is only a few milliseconds that the pison is travelling down to pull in air, so the longer duration helps get the most air in the cylinder in this short time. The drawback is at low speeds the long duration makes the valve be open eveh when the piston is coming up for compression so you lose power at the low end, until the engine is spinning fast enough to overcome this.

but it could also be a jetting issue too, also check the timing.