Help! KX250f is hard starting/won't start

Good day everyone, yes I am a noob.  I have a 2006 that I cannot figure out for the life of me.  It has a new topend on it.  Now all it does is backfire, won't start at all.  I checked all the usual suspects:  it has great spark, the cams are timed correctly, valves within spec, new plug.  Cannot get it started at all.  I even took the airbox inlet off to spray ether thru the carb, still won't start.  Any suggestions before i set the bike on fire???

I recently bought my 06 from a guy who floated a valve. He had the top end done by a shop and he installed it when I got it it would kick and backfire every once in a while. After tearing the top end down and redoing it twice I happened to notice that my exhaust cam sprocket had slipped slightly but enough that the bike would not start. When you have the dots lined up the cam lobes should be pointing front and back and slightly tilted up at about a 40 degree angle.   Hope this helps. 

Yes I checked the cam sprockets.  The circular holes in the sprockets match with the lobes, so the sprocket did not move on me.  I was checking through some older posts on here but no mention of the TPS.  Could the TPS be causing the engine not starting?  How do I check the TPS?  

Maybe do a leak down on the top end. Sounds like an exhaust valve is leaking. If not that then possible ignition problem.

had one same problems, exhaust valve seeping fuel into the exhaust pipe and when firing, backfire into the ex

Need more info.  Was the bike running before the new top end?  What was the reason for the new top end?

Here is a few things.  If you tore it down for a bent or dropped valve, then you need to recheck the cams.  One or both sprockets have slipped.

 

If it was runnng fine before the new top end and you replaced worn parts, then your one tooth off on the timing.  Check the cams again.

What happened, as explained to me by the previous owner, is that he flat landed a double(real hard) and the bike quit on him.  After a few minutes he got it started again but that it was hard starting and ran rough; the next time he took it to the track the bike was even harder to start and wouldn't stay running.  So when I got a hold of the bike I went and troubleshoot everything.  I could not get it to start before the top end, just backfire.  I checked compression, it was low so that's why I did the top end.   Before i started tearing down i checked the cams and valves; cams were not damaged, however both intake valves were only at 0015 thousandth.  There's a white hot spark and the ignition components checked good for voltage.  Intake valves now sitting right at 005 thousandth.  Flywheel key not sheared.  I timed the cams using screwdriver method.  Oh and all valves were good, just carboned up.  Cleaned out combustion chamber and valve seats.  The only thing I didn't touch at all was the carburetor or lower end.  To Altune:  did your bike not run at all either, just backfire?

Sometimes casing a jump will partially disconnect the CDI, based on experience. Last thing I checked that one time, lol. I suppose another connection might have come undone, but that's just guessing. Could be that debris in the float bowl was shaken up enough to jam up the pilot jet. Maybe the pressure of gas in the system pushed the float up enough so the level is now too high. Hey, you never know? Maybe the breather hoses got all jammed up if he hit just right.

You should pull the carb and clean it anyway, and definitely check the float bowl level because it matters. Guessing you got a new plug already, but if not even if the spark looks good, get a new plug.

If you clean the carb, which you should, keep carb cleaner away from all the rubber parts, including the AP diaphragm and the o-ring on the throttle valve. Hole of that valve is on the bottom, by the way. Check the AP diaphragm while you have it apart. I also recommend the R&D remote fuel screw to everyone. Think about getting one.

Valve clearances are expressed in inches, so you mean intakes are at .005 inches, right? If so, that's good. Exhaust should be .0067 to .0087 inches.

I think the carb cleaning will fix things, but keep in mind it's better to replace jets than clean them. They are cheap enough and you can't really clean them as well as you might think.

Thanks for the input, Dragon.   I'm going to tear the carb apart today and give it a good cleaning, and check all the electrical connections.  I'm pretty sure it already has the R&D fuel screw in it.  Since it has a new topend on the motor should I adjust anything in the carb?  The last couple of dirtbikes I had were a 96 XT225 and a 89 yz250, I'm not too familiar with the new 4 stroke bikes.  

Hey everyone, I really appreciate the input that all of you have given.  I FINALLY got the darn thing running, it was the carb after all.  After I thoroughly cleaned it with carb spray and my air compressor and reinstalled it, it fired after a few kicks!!!!   WooooHooo!!!!  I am so excited and cannot wait to get some gear so I can enjoy riding it.  I changed the oil and filter after I had it running for a few minutes.  Again thanks to everyone that contributed to this post.  The next thing I'm going to do is weld the cam sprockets.

Hey everyone, I really appreciate the input that all of you have given.  I FINALLY got the darn thing running, it was the carb after all.  After I thoroughly cleaned it with carb spray and my air compressor and reinstalled it, it fired after a few kicks!!!!   WooooHooo!!!!  I am so excited and cannot wait to get some gear so I can enjoy riding it.  I changed the oil and filter after I had it running for a few minutes.  Again thanks to everyone that contributed to this post.  The next thing I'm going to do is weld the cam sprockets.  I'm looking for somewhere to ride this weekend(NOT on a track), I live in Jacksonville NC.  Anyone live in the same area or know someone with property?

Need more info.  Was the bike running before the new top end?  What was the reason for the new top end?

Here is a few things.  If you tore it down for a bent or dropped valve, then you need to recheck the cams.  One or both sprockets have slipped.

 

If it was runnng fine before the new top end and you replaced worn parts, then your one tooth off on the timing.  Check the cams again.

I redid my topend because it had 50 hours on it, it ran fine before now I have trouble starting it and the compression is on the low side but still in the recommended range. it runs fine and everything like befor.  just isn't the same to  start takes atleast 20 kicks its a 2011 kx250f fuel injected

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