Compression check:

Pull the plug.

Crank flywheel with finger.

As piston comes to the top of the cylinder, you should feel resistance. On new engines, you might not be able to get the piston past the top of the cylinder with your finger turning the flywheel. This is good.

On good used engines, the piston should go up past TDC (top dead center) and while getting firm resistance, you should be able to get it past TDC and back down the cylinder.


If you spin and feel little or nothing, you need a new piston or sleeve.

When cold, the sleeve is tight around the piston. On old engines, this can mask a loss of compression. Once warm, the sleeve expands...and in a worn engine, this causes compression to drop....and to drop too much when the piston/sleeve are worn out.

On a Mach, you could get a piston and sleeve for 50 bucks and have what will run link a brand new engine (have to break it in again!). The rebuild takes 30 mins and is definitely worth it.