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Help! New crank and piston, now no compression

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I just had a new crank installed, and then installed a new wiseco piston, rings, and a full gasket set on my 2001 yz250f. I followed the directions precisely for gapping the rings, installing them with the grooves in the correct positions, and anything else it might have said. I thought the timing was incredibly easy to set up, then I kicked her over to test and i can turn it over with my hand. Theres some compression but not much. I made sure that the auto decompression lever wasnt pressing on the exhaust valve. Finally I decided to take out the cams and turn it over that way. Still the same. So did the rings somehow manage to get screwed up or a faulty head gasket? Never had a problem with the valves before so I dont see why they would be a problem now. By the way this stuff was replaced because of the rod seizing up on the crank. Your help would be greatly appreciated!!

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You need to double check the timing. The TDC mark your looking for on the flywheel is the one beside the H , then you need to make sure the cams are timed right exactly like in the manual. The exhaust cam lobes should be pointing forward towards the front of the bike, and the Intake cam lobes should be pointing towards the back of the bike. Check out the picture here and compare: IMG_4532a.jpg

You will want to double check your valve shims, and also make sure the auto decompression leaver moves freely on the cam.

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Ya i've checked the timing countless times. I've also checked valve clearances and changed some of the shims. But there is no compression with the cams removed either, so its nothing to do with the cams holding a valve open or anything like that.

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What condition are the valves in? Is there any possibilty they are not seating and letting the compression leak out? You could possibly do a leak down test and try to figure out if and where the air is leaking out.

Triple check the decompression valve to make sure it isn't leaking. The leak down test might help find that too.

Good Luck

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What condition are the valves in? Is there any possibilty they are not seating and letting the compression leak out? You could possibly do a leak down test and try to figure out if and where the air is leaking out.

Triple check the decompression valve to make sure it isn't leaking. The leak down test might help find that too.

Good Luck

Great idea. You can do the leak down tester and listen to see if you hear air leaking out the exhaust , or the intake of the head, or in the crank case. It's probably not economical for you to buy one yourself ($100+), so a quick call to a motorcycle shop and probably $20 later you will be able to get it tested.

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Was the bike in running condition when it torn down for a rebuild? Don't take offense but is the piston in the right direction?

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What condition is the cylinder? did you check it for roundness. Also, did you hone the cylinder at all? Could be a few problems, more than likely the valves are completely cut up and need to be replaced and the valve seats probably need to be reground. Also, you will need to put a light hone on the cylinder, if it is in good condition, for the ring to seat properly. If the cylinder is bad or out of round you will need to replace it. As mentioned before, the best way to check exactly where the leak is, is to do a leakdown test.

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Definetly do a leak down test & if you ride & do your own work on your bike or bikes buy a tester. I went together with a riding body and bought one from MatCo for around $100.00

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I will definitely try the leak down test. I just cant imagine it being the valves tho because they were just fine before the rod seized up.

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Here is an obvious question - is the sparkplug installed correctly? If the cams are off and you still have no compression then you are having a massive leak as if the sparkplug is not even in... :p

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Ha, ya the plug is definitely in there!! You can notice a difference in compression when I pull the decompression lever, so there is some compression, just not all of it.

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Made my own little device to get air into the cylinder after not being able to find a real leak down tester. I had air coming out of both the intake and the exhaust. Keep in mind that i dont have the cams installed right now. So this obvoiusly means that the valves arent seating very well. So head needs to go to machine shop?

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Not necessarily. It could just be carbon stuck in there not allowing the valves to close. Either way, I would remove the head off the bike, then pick up some "seafoam" at the autoparts store and pour some into the intake and exhaust and let it sit over night. If it just leaks right out without staying in there, then you may have to submerge the whole head into something. A bucket of de-greaser or something.

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