Hi William,

Am presuming the mower has a Briggs & Stratton engine. If so, I have found they sometimes need fine tuning with the mixture screw to run well. Have a 12HP model on a Ransomes Bobcat mower and even 1/8th of a turn on the screw can make a big difference. Do not think a normal compression tester is the best tool to use on hand start single cylinder petrol engines. They really operate best when the engine is running at least electric starter speed. The correct tester is a leak down type. But if you do not have access to one and I do not. I use the thin latex glove method. Fit the glove tightly over the exhaust and bring the engine onto compression. If it rapidly inflates, its the exhaust valve leaking. Do the same test over the carb inlet and if possible over the oil filler hole. This will test for blowing past the rings. The glove will inflate a bit, but if it rapidly does so, it identifies the problem area. You valve clearances seem a bit small to me. Would check the manufacturer’s recommendations.

Best wishes,

Grahame

Those little Briggs have that odd way of a compression release by a bump on the back of the inlet cam thats a horrible idea, thats why the odd valve clearance 2-5, so you must not get the cam in the wrong place when going to adjust. I once just ground that bump off, this made the engine run like any normal engine, however it could snatch a bit on the pull-start but it went years after that.