400ex cuts out top end

duden89 · 08-09-2010, 08:29 PM

Hey guys my 400ex is cutting out above 3/4 throttle. When I first bought it, it would pop sometimes at full throttle but quickly got worse and just doesnt even fire above 3/4 throttle. It currently has some bad valve stem seals and burns a lil oil (getting fixed tomorrow), but it runs perfect up until you give it alot of throttle. Its got a new spark plug and a white bros rev box, so is it maybe just a dirty carb? Never messed with a 400ex carb before so I thought id get your guys advice before tearing it apart.

Helmut · 08-09-2010, 08:38 PM

Could be a dirty carb. Also check and make sure the accelerator pump is working. You should be able to look into the back of the carb, when you push the throttle, a stream of gas should shoot toward the cylinder. It should do this every time you push the throttle. You may have to push the throttle quickly to get a good stream.

Dirtlover · 08-09-2010, 08:44 PM

Big Gun Rev Box Issues? - ThumperTalk
This is where i was just reading about the CDI boxes and posted my question due to this page..this guy says White Bros and Yoshi..Bg gun are all made by the same company and are all crap and cause break up..very little change in revs,etc..if anyone else knows please chime in..sounds like you got what hes talking about tho..

grasshopper007 · 08-10-2010, 04:15 PM

duden89 do you have the lid on your airbox? If you do take it off, if it goes away your main jet is to big. I had the same thing happen to me when I first got my 400 and thought it was the cdi bought a new one and made no change. Took the airlid off and it ran bad ass. Also check your valve lash.

duden89 · 08-10-2010, 05:24 PM

Airbox lid is on for now, waiting til my outerwear comes in to take it off. Accelerator pump works, it was shooting a stream through the carb when I hit the throttle. What size main jet should I be running? Its been bored to a 408, has a HMF pipe, white bros rev box, and K&N filter, and the airbox lids soon to be off. I tried taking the side plate off the carb to unhook the throttle cable, but the two screws are in tight as can be and wont come undone without stripping. About to head back out and remove carb, exhaust, and the head.

duden89 · 08-10-2010, 06:50 PM

Also, how exactly am I supposed to remove the cam? I need to take the head off, will I need to re-time it no matter what? The allen head bolt holding the cam sprocket on is incredibly tight and I am gonna have to hold the sprocket tight somehow.

I tried taking the cam sprocket bolt out using a 6mm wrench on the 6mm allen key. Ended up breaking the wrench in half (used a second wrench for extra torque). Im guessing theres loctite on that bolt, if so is it ok to take a small tip and use a torch to heat up that bolt and burn off the loctite? Or is there an easier way to do this? I've ran into my fair share of PITA bolts before and this one is definately at the top of the list.

Helmut · 08-10-2010, 07:56 PM

I would use an allen socket and an impact wrench, but make sure the allen socket fits real tight, because if you strip the head of the bolt it will be a lot harder to get off. Also, make sure the allen socket is a good brand or it will just end of messing the head of the bolts up.

tap out · 08-10-2010, 09:24 PM

i just picked up an allen socket set from sears, is this a good brand to use with an impact wrench? it sure has come in handy without an impact wrench.

Helmut · 08-10-2010, 09:39 PM

Quote:

Originally Posted by tap out

i just picked up an allen socket set from sears, is this a good brand to use with an impact wrench? it sure has come in handy without an impact wrench.

It will probably work OK, just make sure it fits real tight.

duden89 · 08-11-2010, 05:31 PM

Are the exhaust gaskets copper? I have a top end rebuild gasket kit and it has two copper washers i think that would fit for the exhuast but im not sure and theres no instructions.