01 426. Fresh Top end. No compression?

2 replies to this topic

Posted September 18, 2013 - 06:03 AM


Well let me give a little back story on this piece.  Found it for sale on craigslist with a blown head gasket. I've always searched for quick flips. So I bought it.  Quickly ordered a new gasket set. Waited 3 days and got it in the mail.  I did researched on this repair for a week before I even bought the bike. I had printed off instructions, pictures, and even exerts from the yamaha manual. 


I tore into the bike. After removing the head I did in fact see that the gasket looked pretty bad.  I would consider it blown. Got everything put back together the next day. Went to start it, and it had no compression. I could roll through the entire stroke will little effort. Determined that the piston and rings were bad, and ordered a new 95.00 mm Wiseco. 


Once again I tore the motor down, Installed the new piston, and made damn sure everything was installed correctly and torqued to spec. Went to fire it the new day and STILL no compression. Did some research and determined that possibly it was out of time. Took the valve cover off and lined the cams up again. Still no change.


All I can do is get the bike to backfire through the exhaust.  Which makes me think I've got an exhaust vavle hanging open.  I checked and the shims are still in place, and I went through and "bump" tested the valves. They all seemed to open and close properly.  Im just at the point where I want to throw this thing in a lake! :banghead:


Need some kind of advice on this. Maybe somewhere to start looking? As far as I can tell the decomp is working properly.


Thanks for the help!



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  • vic2340

Posted September 19, 2013 - 01:42 PM


I dont think that year has an auto decompressor so that cant be stuck open. What it sounds like to me though if the valves are good like you say they are it has to be the piston fitment. Did you hone the cylinder and get it properly measured for the piston you ordered?

  • Yakka78

Posted September 23, 2013 - 03:51 AM


Sounds to me like the decomp valve is jammed open. Can easily be overlooked. Take out the cams and rotate through 180 degrees. Re-align the timing marks. Refit cam caps. Check timing again. Your away.

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