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Old 12-20-2012, 01:02 PM   #1
prichmon OP
Gnarly Adventurer
Joined: Feb 2009
Location: Huntington, wv
Oddometer: 138
1981 Yamaha YZ60 Xmas for my 13 yr old

I have been working diligently for ~2 months rebuilding a 1981 Yamaha YZ60 for my 13 yr old son. I found the bike for $50 90% complete in pieces. We reassembled to find it had 15psi compression. I also found the seals were dried out in the bottom end.

The brgs seem good so we moved to replacing the top end and sealing the bottom. It was already at 3rd OS so I decided to use a KX60 piston kit based on info from another forum. The KX60 piston required the window to be cut to match the YZ60. I got a little carried away with my dremel and made the height of the port a little more than 13/16" rather than 3/4". As far as I can tell the added height should have no effect on the engine compression. After reassembly the bike made 15psi compression with new bore, piston, rings and gaskets.

I returned the bike to the shop who did the work. I was informed the piston was down in the bore compared to the oem YZ piston. The shop milled the jug to bring the compression up. We verified the compression was up to 65psi with 6 kicks but wouldn't start due to flooding from the leaky fuel petcock and poorly routed overflow line.

I can't tell if the rings have seated nor if everything is OK since it flooded over night and I have not seen it stay running yet. The only things left to complete are: the clutch cable seems to be sticking, the kicker is not "catching" but ~50%, the fuel petcock needs rebuilt since it leaks a little. I have a used kicker assembly, idler, rear brake shoes, air box and the chain.

Can anyone explain to me if the height of the intake window would significantly affect compression? Has anyone done this type of conversion or rebuild? What is the best method to loosen up the clutch cable? How many turns on the clutch pivot shaft should be in place?

If anyone has a line on the original heat shield and side panels I would appreciate anyone contacting me.

We'll try to start the bike tomorrow if all goes well... If anyone can help please give me a yell.

Thanks for any help;

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