you may be able to make a limited increase in RPM by advancing the timing, but that is usually limited to changing the RPM with no load on the motor. when you place a load on the motor, the internal pressure in the cylinder changes as well as the stresses on piston rods, etc... what usually happens when you advance the timing is that the motor will overheat when a load is put on it. there also is no such thing as "unlimited RPM". like any rotating machinery, all of the parts of the motor have limits on how fast they can move before the increases in inertia and other mechanical forces begin breaking things. bearings and bushings begin to wear out quickly, and pistons and connecting rods begin shattering. you might be able to increase the motor speed, but it will not be long before the motor shreds itself.