Question about 1980 Yamaha XS 650 G

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Can't find compression specs for 1980 Yamaha 650G

Restoring this bike, I can't find what the compression should be. I ran a compression test and originally got 120 psi, then I put a little oil in the cylinders and got 240 psi. The engine will not pop with 120 but it did pop ( a little) with 240. Please help.

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  • kirsten689 Jul 13, 2009

    Thanks for the tip. I dug in further, and the valves were adjusted wrong, the camchain was loose and the timing was off. Now I just have to rebuild the carbs, It's got Mikuni Kogyo carbs on it but no part number, very frustrating.

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  • Yamaha Master
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The compression should be around 150psi, but at 120psi it should pop, try to readjust the valve clearance.
Hope this helps!

Posted on Jun 21, 2009

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Posted on Jan 02, 2017

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Hi, Daniel to perform a proper compression test you need a pressure gauge either threaded or push in. You will get a higher more accurate reading when the engine is at normal operating temperature along with burnt fingers if you are not protected or careful, cold testing works just fine and it's the method I use 99% of the time. If your bike has cams and valves you need to make sure your valve clearance is within book specs. Always blow forced air around the spark plug before you remove it to eliminate any uninvited contamination from getting inside the cylinder. Install the compression gauge, twist the throttle grip to the wide open position and kick or hit the starter button long enough for the engine to turn over 5-6 times. Record the reading and repeat the process 3 times to get the average reading. Single cylinder engines must have a minimum of 100 PSI anything less call a Priest for Last Rites, it's time for a top end overhaul. Multi-cylinder engines must have no more than a 10% difference between each cylinder, for you high- performance ******* no more than 10 lbs. per cylinder. To run on today's fuel most bikes have pistons with compression ratio's designed to give acceptable performance using an 87 octane graded fuel, this means the average compression readings on most bikes will be 125-150 PSI. I have gotten readings of 200+ on some big inch V-Twins and Thumpers but these numbers are rare and require high octane fuel to eliminate cylinder ringing or pinging.
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