I read around a little and I can't really find a post that matches mine so here goes. I've got a yz450 not sure what year (it belongs to my uncle) and I'm trying to figure out what's wrong with it. I just picked it up yesterday and haven't had much time to look at it. It has no compression and it has a manual decompressor (at least that's what I think it is, it has the little handle under the clutch lever) that seems to be engaging and disengaging properly. The bike can be kicked and I can put my hand over the tailpipe and feel small puffs of air when I turn the engine over. I was there when the bike gave out, but I was not standing right there when it happened. We were just getting the bikes started and ready to go and my cousin was started the yamaha while I rode around in the parking lot warming up the other bike. Neither of us knows much about 4-strokes</SPAN> and hadn't really ridden that one before. He had hell starting it and he never pulled the decompressor. He got it started and it ran for a minute. He was revving it up (stupid I know) and he said it just gave a "poof" and then quit running. After that it didn't have any compression. You can crank it with your hand and it feels like the motor is turning fine just without compression. I know the piston is moving up and down because of the air coming out the tailpipe. I am just wondering if anyone here has any ideas as to what could possibly be wrong and where they think I should start. The bike is fairly new, probably about an 05-06 maybe.

I was way off on the description of the bike I just looked up the vin here is the info I got:

VIN:
JYACG10CX2A016322

World region:
Asia

Manufactured in:
Japan

Year:
2002

Make:
Yamaha

Model:
YZ250F

Body style:
Moto Cross

Drive type:
RWD

Cylinders:
1 Cylinders

5561 #14 is the decompression assy.

here is a diagram of the decomp .

I am not sure where to tell you to start but yamaha in my opinion has the best valve design for durability and longevity. that is true but you may have snapped a valve or your bike may have been out of adjustment to start with and now it is in desperate need of an adjustment.

I would decide if it is a decompression lever problem or a valve problem , and if it is somthing you cannot do your self I would drop it off at a dealership and have them do what is needed for the top end.

What do you think a dealer would charge for something like that?

What do you think a dealer would charge for something like that?

maybe $50-100 to take apart and diagnose problem .

and I have heard crazy numbers for replacing valves so my guess is worthless without knowing what is broken.

I would ask in advance how much $$$ is involved in diagnosing the problem . most dealers say depending on how long it takes to find the problem, but in your case they may be able to figure it by how long it will take to disassemble the motor. if they think it is required - obviously they need to determine if tear down is necessary