250F 2003 yz250f not starting

Okay, I have some weird problems with my bike. I recently had to tear it apart to fix a gear in the transmission. I put the engine back together and it started right now and i ran it for about an hour around my neighborhood. It was working perfectly. Last weekend I took it to the track and it started and about half of lap one it just died. Would not start at all afterwards. It sounded like no spark/gas. I tried to bump start it down a hill, it tried to start but nothing.

Here are the things i checked.

  • Spark plug looks clean and has spark
  • I cleaned the jets and made sure it was getting gas and bowl not stuck

It feels like it has compression, but I may get a pressure check to confirm this...

But what other things should I look at? My guess would be timing but that wont make since. It was running perfectly a few days before then now nothing?

I need ideas where to look for the problem.
You might check to see if your tensioner came loose and let the cams jump timing. Compression is a good place to start and I think I would take the cover off and see if you have clearance while you check your valve timing.

just to make sure it isnt the victim of a plugged up carburetor you may want to drop a little bit of gas down the spark plug hole and see if it starts, even if just momentarily.

It has happened very few times to me, but I generally disconnect the kill button at the coil if it still doesn't start. I have had two or three times where I got spark with the plug out, but couldnt make it fire with the plug in. a momentary interuption at the kill device was the culprit. Just thinking in print.
Thanks for the tips, when I get home I will check those things. Just mad that it happened 10 mins on the track... Once I get it back running I am selling the damn thing..
I have to think I would shy away from a really 'experienced' 4 stroke. I took a yz400 apart in 2003 that had about as many hours on it as my 87 cr500. there sure wasnt as much of it that I felt comfortable reusing as the cr500. a nice yz250 two stroke would be a good rig to move up to.
I am one who does not like 2 strokes, just my personal opinion. I actually find working on 4 strokes to be pretty simple and straight forward. I am just stumped on what is causing my bike not to start... :smashpc:

When I replaced my 3rd gear, I check all the other parts and they were within spec. Something weird is wrong or I fucked up somewhere when putting it back together. After rebuilding 4 4-stroke engines this is the first one I have had problems with :banghead:
oh after rebuilding some from under 3 cubic inches out to 855 inches I can say they all have ways to come back and haunt you. I dont mind either one, I have always had four strokes and two strokes. You will find out what is wrong with this one. My feeling about the worn out ones is an old one, dating from the 60's. If I have my choice between a high hour 4 and two stroke, I will probably take the two all else being the same. The yz400 and the derivatives has the same transmission that the 250 two stroke did/does. It takes a much bigger beating in the 4 strokes. pluses and minuses abound with both.
Before you start spending money....

pull the spark plug out, put your thumb over the spark plug hole press with all your force and hold the kill switch, and have someone kick it over. If it makes your thumb lose seal, You should have had enough compression to keep it going.

Pull the stator off and make sure the keyway is good. Just because its firing doesn't mean its right.

Thats just what i would do.
I got it started, However it seems like it runs very hot really fast. There is oil and coolant. Going to change the oil again tomorrow. I sent a video of it to my friend (snapchat) and he said it sounds like a sticking valve or something.

There is a ticking? sound when it runs. Would the bike run if it was not timed right? Causing a valve to hit?

I don't want to be into this a lot of money and I don't know where to start looking to fix these problems :/

what causes a valve to stick? if the exhaust valves stick would it make it run hotter? I only ran it at idle for 5 mins and the radiator was super hot so was the engine. I can't remember if that is normal.
Timing specs, I just aligned the timing marks with tdc like the manual says.. and I forgot about the air part lol

I did check the clearance, however i only have the 5mm,10mm,15mm,20,25mm ones :/

the exhaust valves are between 15-19mm and the intake are within 10-14mm...

I need to get a better tool with different sizes to double check.
I would do a leakdown test over a compression test.
It actually pressurizes the cylinder and allows you to record the leakage, and even hear where the air is leaking.
The kits are available at harbor freight or other online suppliers, and you need a compressed air source. -BIG DAN:thumb:

Check the valve and timing specs. 5 mins at idle is along time with no air going through the rads.
I was going to say check to see if the valve spring was broken. Not allowing the valve to fully shut. :noidea: Or the cam is wasted not allowing fuel in or out. But, still feeling like it had good compression. Causing the bike to load up.