I have 2004 yfz 450... It has k&n filter aftermarket exhaust and is to my standards crazy fast, however it won't cold start worth a crap: if its a cold start not meaning outside air temp: it will fire with the choke off and die. if u push in on the throttle it will pop and cough and hard to keep it running. If u apply choke it makes it impossible to run.... however after about 3 minutes and its hot it will idel just right. hop on it and it runs like a top. Any ideas?

 

has it been jetted to match the aftermarket air filter and exhaust?

 

when you cold start your nott supposed to punch the throttle, it ruins the engine then it wont cold start

 

Have you checked your valve clearance? A bike with properly adjusted valves should fire over instantly, assuming that all else on the bike is in proper condition. I haven't used my choke in over a year, and I spend a LOT of time riding in the winter (-30 to +30 temps from November to March).

 

it was rejetted some time ago. a few rides ago this was not an issue. I have changed fuel and plug.... it will not even fire with choke on at all..... if it is running and u hit the choke, just dies..... with no choke without adding throttle it will not run or idle until its up to temp with the fan on...... until its hot if u hit the throttle it justs pops and spits..... where i am puzzled is that it wasnt this way a few rides ago.... it never liked to idle much after the mods until it was warm but, if u choked it it would then when it warmed, take it off and you were good. now its just weird, i may just have to take it in i supose.

 

I have not checked the valve clearance.... i will google and see how to do it...