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I've been working on a lawn mower and a rolling leaf blower and both have low compression (~50) but still start and run. How can this be? Are ez-start compression reliefs common in small motors now?

Very common on most engines now due to easier start systems. No 50 would not be out of the question in some cases. I have worked on engines that had 5-30 psi compression but very low leakdown when tested. And they would start up and run normally.

So with an ez start system how can Iwe know what the Operating compression is?

Googled it and most articles say the manufacturers say do a leak down test.

Without knowing which you are working we can only guess at things.

With a powerful enough drill and with the plug wire disconnected spin the engine at least 1000 rpms (in the correct direction of course) which generally overrides the decompression feature. But as said Briggs only suggest doing a leak down test but that only checks the upper top of the cylinder and rings on a L-head (side valve engine). Also with old L-heads valve clearances tends to close up leading to lost of compression. I have even seen some L-head with negative valve clearances.

Now with OHV engines a modified leak down test can be performed which can check the complete piston stroke and cylinder condition. I use this modified test and have found several OHV engines that pass the regular leak down test to fail in the mid and lower cylinder due to tapering and/or being ovaled. Here a low pressure leak down tester is important as anything over 50 psi will send the flywheel spinning which why the flywheel is generally locked in place with most 100 psi testers.

If you have access to a bore scope or endoscope you can view the cylinder wall condition.