Compression in a 90cc atv - ATV Enthusiast Community

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Old 10-09-2011, 12:32 PM
Weekend Warrior
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Join Date: Sep 2011
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Default Compression in a 90cc atv

I have a Manco 90cc that tests out at 75psi, (seems low), my daughters 50cc almost compresses out at 100. I would think the 90 should have higher compression. Try new piston and rings? Help plz.
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Old 10-12-2011, 04:22 PM
Join Date: Oct 2007
Posts: 69

Put a few squirts of gear lube in the cylinder, turn it over a few times with the plug out, then check it. If it goes up, it's the rings. If not, probably a valve. May only need the valve clearance adjusted. Let us know what you find.

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Old 10-13-2011, 08:36 AM
Join Date: Oct 2007
Posts: 69

It may have broke a ring. One of my Honda 125's did. Did you just take off the head or pull the jug, too? With the head off and cold, you should show a few thou clearance on the valves. They should be set with the piston on TDC on the compression stroke and the engine warm. Where to find rings? I don't know. Just found the supplier for Loncin parts the other day after a couple of weeks of serious searching. If it's a GY6 clone, parts are out there.

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