Compression testing

[flyingace]

Does anyone know a way to test compression of an engine not in the bike? I have a spare engine and I'd like to determine its condition.

Thanks,
Ace.

[Anonymous]

I think the short answer is that there's no way to get accurate numbers because an engine off the bike cannot get hot. Aluminum dissipates heat well and all but I don't think a torch will do you, either. You need the steel of the pistons, rings and sleeves to heat up to get anything that will tell you about the life it has left in it.

But I imagine you could get a cold test either by cranking the starter or kicking the kick start whether it was on a bike or not. Also, a spirited rotation of the engine by way of a big ratchet on the crank would also be ok as long as you know it's put together right. You would have leakdown problems there but you can seal up the rings with oil (just put some in the plug hole before connecting the tester) to alleviate that.

That won't tell you much, though it will be obvious if the motor has a holed piston or burnt valve. Probably.

[flyingace]

I'm not too concerned about accuracy. Just ballpark numbers are OK. I'm just looking for a way to spin the engine fast enough to get the same kind of readings I would with a starter motor.

[LW_Icarus]

put a starter in it, get a solenoid, any will do, and wire it up. take the carbs off or open the throttles. using a simple switch or jumper wire to trigger the solenoid.

it will at least let ya know if one cylinder is substantially diferent from the others