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Discussion Starter #1
So what's good compression on a 01 scrambler 500. The carb isnt on mine right now but put my tester on it and it read 150. It stayed at 150 never dropped down.
Let me know your input on this. Look through service manuel. Didnt seem to find it.
Thanks
Ron

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Cranking compression should be around 50-90 psi WOT if the decomp is working and the cam exhaust lobes aren't worn.
If you put 150 psi on in it with no leak down and didn't blow out the crank seals you should be good lol.
The compression info should be in the maintenance section of your manual.
 

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Compression is somewhat as individual as the engine you are checking - generally, on gasoline fueled engines, you are looking for static compression of over 100 PSI for starting purposes. Static compression is the compression a cylinder generates at cranking speed. An electric start engine turns at about the same speed each time the engine is attempted to be started. A kick start engine's cranking speed may vary with the person doing the kicking.

Compression between 120 and 150 is considered normal - compression over 150 to 180 is high and will be found on engines requiring high octane fuels - 180 to over 200 may be found on certain racing engines, but they are few and far between.

Compression below 100 is generally considered to low for easy starting and good performance, but there are certain special purpose engines that may have static compression as low as 60 PSI that start and run well, but those engines may require a special fuel also.

Static compression is the compression a reciprocating engine produces at 'cranking' speed. An accepted practice is with an unrestricted intake (throttle wide open or throttle body removed), at least 5 compression strokes for a reading - some manuals will instruct to crank until compression quits rising. The compression gauge is one way and reads the highest compression generated - if compression falls after a certain stroke, the reading will not change. Sometimes you want to take consecutive readings on the same cylinder an observe that the compression rises on each stroke. If compression only rises on two of 5 strokes it may indicate an abnormal condition.

Dynamic compression is the compression of the cylinder with the engine running, typically 250 to 500 PSI and it rises as the engine RPM increases. Combustion pressures of over 1200 PSI is normal in a well running normally aspirated gasoline engine.

I say the Scrambler engine is healthy.
 

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Discussion Starter #4
Thank you both for the fast replies. You are both wizards

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