I just ran into an erroneous reading from a bad compression gauge (taking out the schrader valve (like whats in a tire valve stem) & spraying it all out with cleaner fixed it). If no leakdown tester you could use just the hose from that compression tester & if the schrader valve is in the integral fitting on the end of the hose then take it back out & if the valve is in the gauge head then screw the hose in the plug hole & connect it to shop air. Check several of the worst cyls & watch the rotor to get em at TDC & listen for where the air is escaping; carb (intake valve)/tailpipe (exhaust valve)/radiator (head gasket)/breather on valve cover(rings)