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.