2004 KX250F Backinfiring

Hi All. I just purchased a 2004 KX250F at auction. Cheep! Stock pipe, stock everyting. It starts after a few kicks easy enough with the choke on. It idles fine. but no matter how long I run it for, as soon as I push in the choke, it runs like crap. Coughs, pops, backfires and dies. So I removed the carb, opened it up and found some junk in the bottom of the float bowl. I sprayed everything with carb cleaner, brushed and blew out all the port holes with compressed air, put it all back together. I checked the air adjustment screw and found it to be 2-1/4 turns out. and installed it back on the bike. Same thing. Started up fine with choke on. I let it idle for about 10 minutes, pushed in the choke and it popped and backfired and the rpm dropped out and it died.

What I didn’t do…

I didn't get #'s of any of the jets. When I found the crap in the carb I figured it hasn't been used in a while. The bike looks like ti has maybe 10 hours on it. I guess I’m going to open up the carb again and check the jet #'s. Anyone know what the stock jets are?

What I don’t know…

It may have had an aftermarket pipe before and the jetting was changed to match the pipe? Perhaps it was taken off the pipe and the stock pipe was installed back on without changing the jetting back? That could possible explain why the jetting is so far out, right?

So my questions are these…

If the jets are stock and everything looks like it should be, what else could it be? Badly warn valves? I checked them before I installed the carb and they were all within tolerance. It has to be the jetting, right?

Your thoughts. Greatly appreciated.

i would suspect that since it runs well when rich, and poorly when you lean it out, that you are needing a bigger pilot, or your hot start plunger is not seated. i would say, the problem lies only in the carb.

First, try a new pilot jet, do not waste the time trying to clean the old one.

Set the fuel screw according to the perscribed procedure for a perfect idle.

Confirm the exhaust is in good condition, that all gaskets are in good shape the pipe installed right. If it is an aftermarket pipe, check the packing, replace if needed.

Keep in mind it is a race bike, not made for trail riding . You are supposed to be hard on the gas and hard on the brakes, never coasting. Hence no ACV (Automatic Coast Valve).

Yes get a new pilot i cleaned mine wich was a #40 and william1 said to me dont clean it just buy i new one i didnt listen and cleaned and put it back in ..exatly the same problem as i had before ..wouldnt idle and backfired ..bought a #42 it runs like a champ now

Thanks Guys. I just wanted to make sure it was just the carb. And Im happy to say i got it dialed in now. I replaced the stock air screw that was 2-1/4 out with an aftermarket one that I can adjust. I set it at 2 turns out and dialed it in now. Runs like a champ. no popping or anything. Very smooth. I believe its set at 3/4 out now. Think I should change the main anyways? It runs really great the way it is now though.

Do not fix what aint broke. Never change jetting 'because', only change it to resolve a problem. Do not change it because some guy, even if that guy is the world champ. A perfectly running bike will not run better. Just because your girlfriend/wife looks good in a mini dress, does not mean you will.

ok, I tried out the mini dress and it wasn't so bad. Thanks TT!


I just bought an 05 from a guy who had owned it since new. It was 100% bone stock and had exact same symptoms. I changed #42 jet to #45 and moved needle clip to middle position and now it runs like a champ! or a guy in a mini skirt, however you look at it, I think they come from the factory lean (atleast for Florida weather)

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