A question rather than any help but why if its a mechancial valve timing issue would it correct its self when upto operating temperature.

Also isn't 650 hPa equal to roughly 17 inhg which is on the lower side of ok.

Can't you adapt an airline fitting to fit on your compression tester so at least you can test for air escaping out of valves etc. Done it in the past with good success.

Obviously its better to have a gauge but not essential. Put a gloove over the exhaust to see if it inflates.

thanks
lee