an additional thought for compression test is to have the throttle wide open when performing your test
I was under the impression that you don’t need the throttle wide open for a compression
test on a two cycle engine. Perhaps because of the porting? I don’t
understand the difference, but hopefully someone can explain.
If the throttle is closed, a four-stroke has a vacuum at the intake valve on the intake stroke, so the cylinder doesn’t completely fill with air to be compressed.
A two-stroke is different. When a two-stroke’s intake ports open there is a PRESSURE pulse present, coming from the compressed air in the crankcase. At the same moment, the exhaust ports are already open, so the incoming pressure pulse simply blows through the cylinder and out the exhaust ports, equalizing the pressure in the cylinder to atmospheric pressure. This is called scavenging and is required for the engine to even run.
Short story: A four-stroke with a closed throttle will start it’s compression stroke with a partial vacuum in the cylinder. A two-stroke with ANY throttle position will start it’s compression stroke with atmospheric pressure in the cylinder.
I’m getting tired of explaining it and people challenge it anyway. Stop and think how a two-stroke works and you will understand.
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