Yz80 no compression

2stroke153
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2stroke153
I have a 1990 yz80 I purchased and trying to restore. It had low compression and wouldn't start. I bored and replaced top end and replaced reeds. Still no compression or start. Replaced crank seals. Bike starts after kicking your leg off and runs fine. It will start easy once warm. In a cold state it has less than 70lbs compression. Any help appreciated!!
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Paul_Pitzonka
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Paul_Pitzonka 11/30/2019 2:29pm
11/30/2019 2:29pm Edited Date/Time 11/30/2019 2:30pm
70 lbs of compression is way too low... what’s your ring gap and piston to wall?
2stroke153
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2stroke153 11/30/2019 6:38pm
11/30/2019 6:38pm
Piston to wall clearance about .003 , ring end gap .02
Paul_Pitzonka
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Paul_Pitzonka 11/30/2019 9:43pm
11/30/2019 9:43pm Edited Date/Time 11/30/2019 10:02pm
I’m assuming since the cylinder has been bored it’s in good condition... The piston to wall is a little on the loose end should be closer to .002” and The ring gap’s a little large to me... I know that’s to Yamaha’s spec, but I never understood why they recommend such a large ring gap... I like running two strokes at .0045” to .005” x bore size (industry standard is right around .004”) Say you used the largest oversized piston available 50.00mm (or 1.969”) this lands you at about .009”-.010” If you want a little loser gap because you’re worried about the rings butting I could see opening it up to about .012”... When you are running your compression test is it at full throttle? Also the rings not being completely seated will drop compression numbers slightly... odd question but has any porting been done to the cylinder?
2stroke153
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2stroke153 12/1/2019 4:47am
12/1/2019 4:47am
No port work has been done. I thought clearance was a bit excessive myself. I didn't take anything off the ring gap and was curious to why the gap was so big??? Even if the wall clearance was down to .002 the ring gap would still be on the high limit in my opinion! This is also a wiseco piston kit at the 2nd oversize(49mm) 1.929". This may be a dumb question but the wiseco piston kit said bore .002 clearance. I've also read that the piston is designed with the clearance already taken into consideration and that you bore the exact dimension of the bore (49mm). If this is the case then my wall clearance is pushing .005. Are you following that lol? Thanks for your help!

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Paul_Pitzonka
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Paul_Pitzonka 12/1/2019 6:05am
12/1/2019 6:05am
How are you obtaining your piston to wall measurement? If you think it’s possible you have .005” piston to wall that’s more than likely your issue... always measure a piston before you do any type of change to bore size (It’s good practice to measure any piston before installation in general)... my suggestion would be to pull the top end apart and get a more accurate piston to wall reading...
2stroke153
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2stroke153 12/1/2019 6:29am
12/1/2019 6:29am
Its actually been awhile since I have worked on it so I'm going off of memory. Was getting ready to start up the winter project! I remember that the wiseco piston was barrel shaped so it had run out diameter measurements up to .005 depending on wear you checked the diameter. I agree a good place to start is to tear it down and go from there. Thanks again
Paul_Pitzonka
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Paul_Pitzonka 12/1/2019 6:36am
12/1/2019 6:36am
Widest part of the skirt is where you obtain your measurement from... Manufacture normally lists a height up from the bottom of the skirt...
2stroke153
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2stroke153 1/8/2020 6:09am
1/8/2020 6:09am
Update: Rebore and new piston/rings with correct clearance of .002. Runs like a dream. Starts within 2 kicks now. Thanks for everyone's help and input.
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