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COMPRESSION LEVEL ON 1977 YZ100?

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Anybody out there know what the compression should be on my 1977 YZ100?It fires first kick,the plug reads good,and it isn't spewing any oil,but I would like to know for sure.I could probably kick it over with my hand,but like I said,it would start,and it runs perfectly.....all top end and NOTHING below that!Thanks!

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Hey 94cr rider! my compression is at 125 with the bike warmed up and holding the throttle wide open,which is how I understand you're supposed to do it.Where did you get your information that it should be at 140/160? If that is accurate,how low is 125?When should the piston be replaced?Thanks alot!

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My bike is a YZ 100 , not a YZ125. Wouldn't there be a difference? Plus,it's a 1977.Where are you getting your specs from?

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Hey! I called the Yamaha Motor Corp. and they called me back after 3 days and told me compression on a 1977 YZ100 should be 80-125.Seems kind of low,but that's what they said.Soooooo....now all you multitudes of other 1977 YZ100 owners out there have a reference point.I'm actually still skeptical,but who would know better than the big "Y" ?

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Let's see. Everything Yamaha would have would be based on the original 1977 manuals. They are iron sleeved cylinders with a cast piston and manufacturing tolerances were not nearly as close as they are today. So they seriously "underbuilt" the bikes of the day to allow for the potential problems of a slightly out of round cylinder, sloppy crank, loose bearing, lower compression ratio, etc.

Most of those older bikes have a lot less compression than a current model.

And in IMHO a static compression test is about as close to meaningless as just about anything. You can use it to help diagnose topend wear, but you need to have a test with a fresh topend to give you your starting point. Then you can test later to see how much you have lost.

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