Kxf250 2004 not starting

Right I got a kxf250 2004 had a new topend with new stage 2 hot cams. Been running great apart front the jetting but I got that sorted. I took it to a track once I come home I jet washed it

Dried it put it away, cleaned the carb changed spark plug and it wouldn't start just backfired. We now have really cold weather and bikes in the garage so it's freezing its been about 1 week since its last started just backfires, iv been told it could be the valves it hasn't got a lot of compression so in starting to think it need valve clearance checking, but what's confused me is its only been 1hour since the rebuild so could it be the vales ?? please help! I can't think what it is!

I wouldn't think it would be anything from the rebuild since it was running well before you washed it and changed the plug and cleaned the carb. I would pull the valve cover off and check the timing to make sure but after that it is a process of elimination. If the timing is good, I would just backtrack with what you have worked on and double check that. If the fuel mixture is not right you can kick these bikes until your leg falls off and they won't start. Good luck.

did you have the valves replaced when it was rebuilt? early kx250f's are notorious intake valve eaters. even the new ones dont go past 100hrs of hard use.

Thanks for replying, ill check the carb air mixture screw, and yeah the valves where done with the rebuild I heard there bad for eating valves but I would expect that after 5hours of riding but it's only been one.

I checked the timing seems right, iv turned the mixture screw still nothing iv tried a 40pilot jet and a 45/ 50 they all just backfire getting really stressed out with it iv been kicking from TDC aswell

So the bike won't run at all? Just backfires periodically out the exhaust? If you only pressured washed the bike after a ride and cleaned the carb I would check ignition connections. Maybe you got water in a connection and it is fouling up the ignition. Take apart each connection carefully and blow them out with low psi air. Also, when you cleaned the carb did you stuff any rags in the air boot to keep foreign debris out? I've forgotten to take a rag out of the boot before and the bike will definitely not start and you really don't want it to if that happened.

Yes just backfires iv tried bunping it today turned the air mixture screw changed jets when I cleaned the jets I had a cover over the topend where the carb connects only there didn't touch airboot, ill do what you said by checking all the connections. Thanks for a reply.

yea check the cdi and al the wires presure washing can be very hurmful to the wires

Iv checked all the wireing CDI plug all seams fine, when I put my foot on the kick start the kick start will push down pretty easy without me phrting any force onit and there will be a noise coming from the air filter. First thing what came to my mind is a valve stuck open. Iv got bad memory so I can't remember if that noise had always been there but I shouldn't think it was meant to be doing that. However when I had the topend done I bought a new topend cylinder which had valves and springs already in it which is what's still Init now. So I don't know how long they've been in there, iv got new stainless steel valves think its time to get them fitted. Plus i have been kicking it like he'll to get it started so they might if warn down. Just a thought it could be that haha.

The noise in the filter is just vaccuum as the valves open and suck air in. If it makes noise on the exhaust stroke than your valves are tight and leaking by when they should be slammed shut. If you have the valve cover off check the valve clearance with a feeler gauge and see if the valves are indeed tight.

Ill get the new stainless steel valves and sprigs put in. Hopefully that will sort it.

Who cut the seats on them? Titanium valves went back in?

i also have a 2004 kx250f. did the same thing to me. ended up buying a new cdi box

I see you cleaned the carb. Is the throttle plate in upsidedown? Also I wouldn't just assume the valves are bad. Check your clearances first.

04 kxf and rmz are known for cdi's going bad.

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