yz450 no compression

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 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.

Pull the valve cover, check the valve clearances. Sounds like a valve my of failed.

If it is an 03 or newer, it has automatic decompression, so it will kcik over easily. 03 and newer have a hot start lever on the clutch perch, this could be what you are mistaking as a decompression lever (if the bike is indeed an 03 or newer). It is hard to tell why it would have died without taking it apart. Sounds like you may have to start digging into it to see what you find.

It kicks over way too easily. How does the hot start thing work? The lever I'm talking about is a small handle mounted under the clutch lever. It is basically like a mini clutch lever. It runs down and attaches to the side of the engine on the right side if your sitting on the bike.

To start with, if it has a manual decompression lever, it's extremely unlikely that it's a 450 at all. Yamaha stopped using that system in '03, when the 450 first came out. 2000-'02 models were YZ426's. The manaul system can be added back to the 450, but very few people ever did it.

The hot start valve on the 426 is operated by a red knob on the side of the carb. On the 450, it used a very small lever on the clutch side with a cable running to the top of the carb.

The 10th character of the VIN number is the year model. "Y" was 2000. "1" is 2001, "2" is 2002, etc.

If it has no compression, it has a mechanical problem that requires it be opened up. But check things as it is torn down, instead of just pulling it all apart. Pull the cam cover and check the cam timing and valve clearance. An exceptionally loose valve is bent. An exceptionally tight one may be broken off or sunk in the head. If you find nothing, remove the head and cylinder.

You'll want one of these:




Thanks for the info. I'll have to get the vin..where would it be? It does have the lever though but it does not go to the carb it goes into the side of the head. I am a decent mechanic, but I really have no experience with these bikes, so the information contributed so far has been helpful.

The vin number is on the right side of neck of the bike between the triple clamps.

The vin number is on the right side of neck of the bike between the triple clamps.


look at the front of the bike where the shocks are. On the right side of the frame neck it is stamped there.

look at the front of the bike where the shocks are. On the right side of the frame neck it is stamped there.



not sure if the 0's are 0's or o's.

"o", "i", and a couple of other characters are never used in VIN numbers, for exactly that reason. The tenth character is a "2", so your bike is a 2002 YZ426.

I was way off on what I thought the bike was, I told you I didn't know much about them..lol. Here is the info from the vin:


World region:Asia

Manufactured in:Japan




Body style:Moto Cross

Drive type:RWD

Cylinders:1 Cylinders

Sorry it cut off my post before. Here is the rest.



Body style:Moto Cross

Drive type:RWD

Cylinders:1 Cylinders

That's what I get for not parsing the first 9 :)

Same tear-down and troubleshooting procedure applies though.

I'm starting to wonder if he was holding the decompression lever down while trying to warm it up and caused some kind of valve issue. It really doesn't feel like there is a cylinder/piston problem from the way it cranks.

If the compression release is held, the engine won't run.

What if he pulled it while it was already running? Would that cause damage? It doesn't seem to me that yamaha would design something that way, but I'm just fishing for ideas. I know I'll be tearing the bike down, and I'm just trying to learn a little about it.

The bike manual has warnings all over it about not pulling the decompression lever while the bike is running because it could cause damage.

At lower speeds, like at an idle, there is relatively little danger of damage, but at higher speeds, the risk increases. The exhaust valve will not contact the piston, but the compression release works by contacting the right valve lifter at its outer edge, lifting the valve up off its seat, and/or preventing it from closing. This causes a rocking sort of strain to be applied to the lifter, and the faster the engine is running when it's used, the harder the impact of the lifter hitting the comp release is. Potentially, it can break the lifter and jam the valve.

I don't know if that is what happened or even if he pulled the lever, but the latter is highly possible and he was revving on it.

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