Re: Mechanic stumped!
When they do a compression test they remove the plugs and crank the engine over with the starter and then compare the readings from all the cylinders. This is a good way to single out a bad cylinder and will tell you that your rings are good and the valves are seating properly. However most engines have double valve springs. One spring can be broken and you can get a good compression reading but under higher rpm the valves can "float" (not close all the way). The motor may run good at low rpm's but miss on higher rpm's or under load.
22 and counting...