the best way to check for poor compression is to carry out a cylinder leak test,using an airline adaptor into each bore with valves rocked untill closed,if air is heard past the oil cap or dipstick then rings are probably to blame, try opening throttle inlet and liten in the exhaust pipe for air noise if any is heard then valves are the cause. have you tried squirting a little oil down the bores before the test if compression raises then this is also a good sign rings are in poor condition. hope this helps.