2005 400ex running crappy when warm

[Espo720]

I have a 2005 400ex with a yoshimura slip on, big gun rev box and moose filter. It ran great all winter and now that its warm out it runs crappy. When it gets hot it starts to pop and the only way to build rpms is to keep revving it up until it starts to smooth out. I just changed the oil and cleaned the carb today, there was some crap in the jets but not much. The oil had a slight greyish tint to it. I adjusted the intake valves to .004 and the exhausts to .005. They were stupid tight prior to me adjusting them.

I bought the machine back in November with the above mentioned parts already installed and haven't had any problems. The previous owner toasted the swingarm bearings so he ended up selling it to me after he couldn't get em out. Just thought maybe someone has had this running problem before? I am hoping my valve seals aren't shot.

[DragonGunner]

I have a 2005 400ex with a yoshimura slip on, big gun rev box and moose filter. It ran great all winter and now that its warm out it runs crappy. When it gets hot it starts to pop and the only way to build rpms is to keep revving it up until it starts to smooth out. I just changed the oil and cleaned the carb today, there was some crap in the jets but not much. The oil had a slight greyish tint to it. I adjusted the intake valves to .004 and the exhausts to .005. They were stupid tight prior to me adjusting them.

I bought the machine back in November with the above mentioned parts already installed and haven't had any problems. The previous owner toasted the swingarm bearings so he ended up selling it to me after he couldn't get em out. Just thought maybe someone has had this running problem before? I am hoping my valve seals aren't shot.


What size jets do you have? Stuff in the jets isn't good, you used crab cleaner right? Did you tear down all the carb….excelerater pump, air adjustment screw, spring, take the floats out and clean that….? With popping its usually running lean, but I've had a dirty air filter that played havoc also. Not sure on your oil, contaminated oil, like getting water in it will turn it milky. If your seals go bad it will smoke on start up. Get that carb straightened out and you should be ok.

[Espo720]

I didn't take notice on the jet size. I took the whole carb apart, cleaned with carb cleaner. I didn't touch the air adjustment screw because it is a weird half moon shape and figured it was stock from factory. Air cleaner seems fine. Napa sold me the wrong plug so I still need to change that, it was pretty black and cooked. Just odd how it runs very well until it gets hot then it starts acting up.

[samm3020]

I know it sucks to take apart what you just put together.....that being said sounds like you could be jetted rich...black plug...runs good in cold weather but not in warm. Best thing to do is pull the carb and check out what you have for jetting in there. If you find out main jet size, pilot jet size, needle clip position, air fuel screw position you can get back on here, do a search and come up with where you should be for jetting. Or I'm sure someone should tell you if your in the ballpark. IMO