2011 kx250f won’t start after rebuild

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7/6/2018 6:37 PM

Hey guys I recently rebuilt this guys 2011 kx250f. Brand new cam chain, new piston, gaskets, new head and valves,new spark plug, everything is in spec. valve shimmed, timing is set. It passed a leak down test, but when I go to start it it won’t start, if I try to bump start it, it will start for a second or two and die out even if I give it throttle or don’t. I’ve pulled the gas tank a few times to check out the injector and throttle body. Sometimes it will start for a second right after I reconnect the gas tank but it’s only that one time after reconnecting the fuel system.

The reason the head and valves were replaced is because he had a chip in his intake valve seIt and it was running lean and letting exhaust gas go through the intake in the exhaust stroke

either has to be electrical or fuel system related is my guess

Note it’s getting spark, tried disconnecting the kill switch to see if was shorting but didn’t do a thing

Thanks for the help Everyone


7/7/2018 12:58 AM

I think your valve timing is off by a tooth. Did it run before you rebuilt it? Did you possibly forget the key on the flywheel? Just ideas....


7/7/2018 5:37 AM

Sounds like fuel starvation to me.
Have you tried starter fluid to see if it will run for a few seconds?
If it does, then I suspect fuel pump, filter or injector as the issue.
Do you hear the fuel pump run when you kick it over?

Paw Paw


7/10/2018 4:13 PM

Took the flywheel off and the key is still there, thought it might have got sheered somehow but it’s not.

Triple checked my timing and it’s right on and yes I’m on the right TDC mark

Engine had 80 PSI of compression on a dead cold motor, it’s within spec

Has spark and tried starting fluid, it did nothing

I checked the fuel pump and it pumps fuel when you kick it and the injector is spraying fuel when you kick it

Anyone got any more ideas?

Maybe the bike just doesn’t like me? Lol I’m out of ideas


7/10/2018 7:18 PM

You are missing something....
You have to have fuel, good Blue spark, compression and the cam timing has to be spot on for it to run.
I have seen KX250F's assembled with the cams out of time, as it easy to do. Double check that you are at TDC when installing the cams and that the marks all line up. Use a dial indicator to be sure you are at TDC.
Make sure your spark is Bright Blue in color as any other color spark will not be strong enough to fire the compressed fuel.
Also make sure the valves are set correctly.
Make sure you good pressurized fuel flow.

Paw Paw


7/11/2018 5:12 PM

Make sure the cam gear didn't spin.

When the timing is set both intake and exhaust cam lobes should match degree wise.
See if one lobe is way up or down compared to the other.
Squirt some fuel down the airbox and see if it will run bypassing the tank.
After you line up the cam gear dots check the lobes to see if they match.


One of the above is missing.


7/11/2018 5:47 PM

Thought id post some more info, and im not sure if its relevant on the 11... but on my 09kx250, theres more than one mark on the flywheel. I had mine timed to the wrong mark and it took me forever to figure it out.