Ok, I'm ready to pull my hair out. I bought a 2000 400ex that smoked. I put a new stock cylinder, piston, rings, lapped the valves, cleaned the carb (many times), new carb kit, new stock muffler, and new spark plug. I still can not get it to run good. It won't idle and just dies. If I hold the throttle about a quarter way open, it won't run smoothly and sort of revs up and down and backfires. It seems to run ok (not great) if I run it on choke or 3/4 choke.

I read that the decompressor can go bad, so I took it out. I checked the compression and it is 120 pounds. I re-adjusted the valves at the correct specs. What else can it be? Electrical? Still a fuel problem? I can't figure this thing out. Please help!

 

hmmm... pilot screw should be about 2 1\4 turns out..that maker run bad but sounds more like a clogged jet

runs better with choke on cuz its lettin fuel through

 

Are there any air leaks between your carb and motor?

 

"Are there any air leaks between your carb and motor? "

Good call, whats your intake boot look like? Is the plug tunin wet black?

 

Thanks Guys. I took the carb off and put my hand over the black boot to see if there are any leaks. I didn't hear any leaks and it held good compression. As I thought about it a little more, I decided to re-check the valve adjustment. One of the exhaust valves were a bit tight so I re-adjusted it at the factory specs. The compression jumped from 120 psi to 150 psi. Maybe there were only 3 valves working properly and therefore at an idle there just wasn't enough compression to keep the bike running. Does this make sense??

Anyway, I'm going to put the carb back on and try starting it a bit later.

One other question....How can I test the timing chain. It seems pretty loose before the tensioner is applied. When I lined up the lines horizontally to the head it looks good. But, once the tensioner applies pressure, the lines aren't exactly alligned. Is this typical?

 

YES.. it retards the timing 10* BTDC

Doesnt sound like timing then

if the valves are off, could make that problem..also will sometimes run really hottt