I'd have the head off and check for:
Incorrect, dished pistons fitted reducing the compression ratio (already low at 8.5:1 on a 13/60).
New valves fitted 'out of the box' at some stage and never lapped in.
Thicker than standard head gasket.
A bit more work needed but if there's still nothing obvious:
Check the cam timing is correct - I managed to get my six-pot to start and run (very badly) with the cam 180 degrees out, and a very small error can leave the inlet valve open for too long during the compression stroke.
Check if all the piston rings are present - the SCCA Spitfire Competition Preparation Manual recommends not fitting the lower oil rings on a racing engine for more power at the expense of engine longevity.
You could also try a different compression tester or try yours on a known 'good' engine to see if it reads low.
Good luck!
Last edited by Mister Towed; 21st December 2019 at 07:59..
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