Kx250f won't start

[Carterz]

I just bought a 2008 kx250f for $600 off a friend. The bike won't start and he didn't want to deal with it. I dont know much about dirt bikes but I know its getting spark and fuel. It feels like it has really low compression. He said he was riding it and it just shut off on him and wouldn't start again. Thinking maybe cracked piston but I have no idea. Planning on tearing it apart and doing top end rebuild but just wondering if anyone has any ideas

[KimPe]

Decompressor could be stuck and cause the low compression.

[Carterz]

1 minute ago, KimPe said:

Decompressor could be stuck and cause the low compression.

Would that cause it to randomly shut off while riding?

[notoriousE-R-I-C]

A WHOLE LOTTA THINGS

You arent very descriptive. But it didn't happen to you so...

Open it and start measuring and electrical testing 

[schobelnate]

Compression test

[notoriousE-R-I-C]

The bike would run on next to nothing compression...if it has ANY comp, and that you can feel with your hand at the muff. Decomp stuck? This wouldn't happen wHILe running. The bike would have to be sitting a LONG time with water in or around the cam. This wont happen in any short amount of time. Just shutting off would be electrical. Boom, off, electrical. Any fuel issue or motor issue youd hear/feel something not shut off quietly. 

If I was you, Id A buy a manual, B buy a tester and test everything. Do you have spark? If motor sounds fine trying to kickstart it than motors good. A valve being gone or a piston ring being gone wouldn't just shut off. All you have to do to find that out is open up the motor and measure EVERYTHING. That manual you just bought will tell you all the specifications.

[Carterz]

Thanks for the help probably gonna tear it down all the way this weekend

[notoriousE-R-I-C]

Do you have spark?

[Carterz]

Just now, notoriousE-R-I-C said:

Do you have spark?

Yea its getting spark and fuel

[notoriousE-R-I-C]

Feel puffs at the exhaust when kicking?

[Carterz]

Just now, notoriousE-R-I-C said:

Feel puffs at the exhaust when kicking?

I havent checked that yet. Its at my buddies house now I'm going there this weekend to actually get in depth with it. Ill let you know

[notoriousE-R-I-C]

Id pull the whole top end apart and measure everything. 

The problem might be obvious. But if its .001" of something youll never see it but a micrometer/caliper will.

[Carterz]

3 minutes ago, notoriousE-R-I-C said:

Id pull the whole top end apart and measure everything. 

The problem might be obvious. But if its .001" of something youll never see it but a micrometer/caliper will.

I really dont know much about dirt bikes so sorry if I sound kind of dumb but you think it could be the valves? I don't think it would just shut off do to them but worth a shot

[notoriousE-R-I-C]

Ok ThumperTalk. If youre not going to be able to post what I just wrote until the next day. Tell me before I type 300 words. Like why even let this screen pull up to type in if everything will just be erased by the next day... i dont even have an iphone

[notoriousE-R-I-C]

Ok lets do this

No I doubt a silent shutdown is valves

You can get a leak down test and not pull apart motor. I dont have that equipment. I have trained eyes and tools so I just go in...which is why I say get a manual. You got a hell of a price now go spend $30 on a manual 

[notoriousE-R-I-C]

First, just taking the valve cover off might visually expose a problem. Thatd be too easy. Second. Your own leak down test. Pull cams, tie up cam chain so it doesn't fall into motor. Now remove head. Lean ex manifold pointing down, in manifold up, valves face rear. Fill up in manifold with gas, look at valves. Are the in valves leaking? Now lean in manifold pointing down and ex manifold pointing up, valves facing front, fill ex manifold with gas and look at valves, ex valves leaking? 

[notoriousE-R-I-C]

Next, remove valves. I just use a big c clamp with a small piece of pvc with a square cm cut out of it. The square cm is at the valve, once you compress the valve use a radio antenna magnet and pull the keepers. Now pull up spring. Measure spring. It has a measurement. Is it squished down shorter than it should be? Like after yeara of squishing isit 1" instead of 2". Drastic but point being. You can do this, then theres a machine that compresses it and tells you if its lost tension. Like now you can pinch it with your fingers, then you wouldn't have needed the c clamp. Again drastic but you get me right? This isn't it though. Then throw away those valve seals and get new ones. You dont pull a condom off and put it back on do you? Now spring, keepers and seal gone. Look at valve stem in its guide. In there nice and snug or can wiggle around? Then remove valves, one by one, remember which valve went where. (R or L) now look at seats. Are they still there? Youd hear noises if they weren't. Are they cracked? Thats it to valves. Nothing here would be a silent shut off

[notoriousE-R-I-C]

Comp test also before taking motor apart. Or just dive in. New to you...get a fresh piston ring set or a whole piston and rings if your piston looks ok. Maybe it has a hole in it. But pull cylinder up. Take piston off, measure piston pin and rod small end. Hows cylinder look? Either get a hone to feel good or if it looks cross hatched still, scrub it with an oily steel wool and put piston back, hopefully new rings, but its not like I haven't just reused the same piston and rings. But I oily steel wool scrub everytime. 

You got a manual right? Read how to check electronics. Now do that.

Hopefully the problem will be obvious. But if not, measure and test.