2-stroke compression test...

[SpannerMonkey]

Ran a compression test on the Tomos (this model has an 8.5:1 ratio) and the results are coming out as 103psi... As far as I remember... this is pretty low, but I want to double check... what do you guys reckon?
How far off "Knackered" is it?
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My bike? A YX140 powered one-off Monkey Bike, something a little different!

[SpannerMonkey]

Warmed up on idle for a few minutes, but engine was warm previous to that anyway... so I'd say it was up to a decent temperature being a stationary air cooled engine
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My bike? A YX140 powered one-off Monkey Bike, something a little different!

[Teflon-Mike]

8.5:1 compression. I take it thats what the book says. Most likely the calculated C/R of swept volume divided by combustion chamber volume, same as you would calculate for a four stroke....
Only a two stroke has ruddy great holes in teh side of the cylinder, and doesn't start compressing anything until the piston is half way up the bore, and then how much it compresses it depends on how much its managed to 'trap' before the ports closed, which tends to depend on how fast the engine is turning.
Quite common for two strokes to give pretty low compression readings, especially a single turned over on the kicker or by hand.
103psi, is about 6.6bar or roughly an actual C/R of about 6.5:1, which doesn't sound far off, think that's about what they quoted the C/R on on a lot of early Yams, where they quoted the 'effective' C/R of swept volume after ports shut, divided by combustion chamber vol.
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[Kickstart]

Hi

103 doesn't sound too bad for a 2 stroke in a very low state of tune.

Quite often European stuff quotes the theoretical compression ratio while the Japanese quote the effective one (ie, after the ports have closed). Hence why an early Aprilia AF1 is something like 15:1 while an RD125 is quoted at little more than half that.

All the best

Keith
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