rebuilt top end NO COMPRESSION!! HELP!

[3or4HONDA]

Heres the deal my buddies 04 yfz had the front sprocket seal go out and lost all his oil while it was running. well it didnt seize but it fried the piston put a hole in the side and fused the rings to the piston as well as scorred his cams and the caps. valve head was alright but caps were junked and cams were pretty bad so he bought a used head with cams off someone who said it was off a perfectly running motor. So he got the new piston in and last night we put the head back on made sure the timing was right and when we turned it over their is NO COMPRESSION. and it doesnt fire at all. took cams back out and double checked timing marks and it all looks correct according to service manual.

im stumped.... unless the manual is telling us wrong the timing is right on, lined up timing mark on flywheel to get tdc and then each punch on the cams are to the outside level with top of the head and the lobes face out.... if someone sees something wrong with this please let me know.

but even if the timing was off it should at least have compressiona and backfire if it was the timing, this thing has no compression what so ever and doesnt even try to fire.

compression stroke should be determined by the cams so i dont know what is wrong....maybe a stuck valve? would that leak that much compression to the point where it doesnt even think about firing idk

any suggestions will help..... thanks

[yfzrider310]

are you sure that you have no compression? Cause that would only happen if you had a hole in the piston or the spark plug was pulled. Maybe check your spark plug for spark and do a compression test. Then let us know what you come up with. Are you sure you installed the piston and cams correctly. Another thing are you sure your decompression plug didn't blow causing all the oil to spill out.

[3or4HONDA]

i know the timing is right and im almost positive that their isnt ANY compression. it has a brand new wiesco piston so i know thats good and installed correctly. i didnt do a compression test but we checked the spark and its good with a new plug. with the plug just sitting in the hole when we crank it over it doesnt even move, their should be enough compression to see i would think. and he had already changed out the factory decompressor plug for and aftermarket one before the sprocket seal went out.

[yfzrider310]

was the spark plug in the clyinder when you tried to fire it up?

[Ride1Rob]

Originally posted by 3or4HONDA
with the plug just sitting in the hole when we crank it over it doesnt even move, their should be enough compression to see i would think.

You lost me here... the plug shouldn't move. And the best way to see if you have compression is to use a compression tester. And yes the timing being off could give the bike a big loss of compression.

[lpmaster]

almost positive your buckets/shims arent on the correct valves. i've already made this mistake mis-labeling mine during a rebuild. even with my cams correctly timed at tdc 2 of my intake valves were stuck open because i had switched the two buckets and ships over them with the exhaust sides which had didffrent shims in them. check your valve lash and i bet its your problem too. i ahd to pull the damn head back off before i figured it out myself .

[3or4HONDA]

ok thanks i will tell him to check them but the head came assembled with the cams and we just took them out to put the head on so the shims and everything stayed in place.

[jrspawn]

A couple things to make sure you do EVERY time you get into the top end. Always Verify your are truely at top dead center. You dont know how many times ive had engines dropped off with issues and they didnt even verify tdc when trying to assemble. If your flywheel key is sheared(which ive seen Many times) going by the mark on the flywheel key will not be correct. Depending on how much the flywheel spun, it will cause no compression through the strokes. Also, take the time to adjust the valves each time the head is apart. Just reusing the old shims and not double checking clearances when assembling can catch up with you in the long run. Also, how did you align the rings?

Thank you
Justin

[3or4HONDA]

well we know the rings are right as far as i know, they were set each one oppostite of the other, but the head came off a different quad and wasnt dissembled because he was told the top end of the quad it came off of was still good. could that be why it doesnt have compression, just becuase it was good on that quad doesnt mean the clearances and everything is good on this one? is that what you are saying....and im 99% sure the flywheel is fine and it is truly at tdc...we rechecked and rechecked. we think there is a valve stuck because the head was sitting around for awhile.

thanks