i just bought a 400ex about 3 weeks ago with a 440 big bore and exhaust, jets the quad ran perfect the first week or so now it boggs down in the high rpms, i ghanged the oil, spark plug, cleaned filter, cleaned the carb now im not sure if i should rebuild the carb or if its an ignition problem

 

Hey millertime07 welcome to the forums. There are several things that could be the problem here. You said that you cleaned the carb. I'd make sure the jetting is proper(mine wasn't). 400's are known for stretching the timing chain. It may be out of time slightly. Listen for a slight, consistant ticking noise. I'm no mechanic but maybe this will get you started.

 

no ticking from timing chain, it will run fine for the first 20 min then start bogging down
thanks
any more advice

 

any one else reay need the help

 

check to make sure your choke isnt stuck GOOD LUCK

 

this may sound simple, but is your choke down? the yellow lever on the left side of the carb should point downward.

 

choke isnt on it first thing i checked

 

It's starving for fuel. Either go up a couple sizes on the main or check for a fuel restriction in the petcock or in the carb inlet and needle/seat assy. in the carb.

 

how fast did it go with the 440ex bore?

 

The VERY first thing I would do is a compression test.