my 400ex takes along time to warm up even when its hot outside. My brother has the same exact fourwheeler and it takes no time to warm up. Whats the problem?

 

Do you have any mods done to it? Do you know the jetting in your carberator? Is it hard to start and keep running before its warmed up?

 

Go two steps up in your main jet. Maybe one in the pilot jet.

 

no point in goin up in your main if its stock however switch to a 40 or 42 pilot and see it that helps

 

Maybe just a carb setting, My friends 500 Foreman since new was always cold blooded to start then he drown the ATV on a deep water crossing. He had a guy clean and adjust the carb and it starts up better than it ever did since he owned it in the last 3 Years,its like a different machine he tells me.

 

Well i bought it like 5 months ago and i don't know if has any thing done to it. The only thing i have done to it is an exhaust.

 

If you put on an exhaust and didn't rejet than you have a big problem you need to address NOW.