An observation/life experience by an old timer. Model airplane glow plug engines are Otto cycle therefore R.P.M. is usually desired rather than fuel economy. Once, I was given a two cycle lawn mower because it did not have the power to cut the thick grass in southern Louisiana. After using it a couple of times, it was obvious that full throttle was necessary to cut, and if rpm dropped, retreat, allow rpm to build to full throttle before reentering combat. Upon closer examination, I realised this was simply a large model air plane engine with magneto ignition. The exhaust ports were three 1/2 inch holes. I applied simple model engine hop up techniques by filing the spaces between the holes to leave a simple guides to keep the ring on the piston. My estimate was about double exhaust area. Then I noticed a throttle limiting device, which was removed allowing full intake opening. After reassembly, the next grass cut was much easier, as it was possible to add throttle as needed. Finished, just before clean up, I stood revving the engine open and closed as in a motorcycle, when there was a loud crash and I found myself standing in fire up to my ankles. The rod had broken about 1/2 it's length, allowing the shaft to swing the remaining piece of rod through the aluminum crankcase, cutting it nearly in half. Years later, at a garage sale, I saw a fellow looking at the only other one I have ever seen, and advised him not to get it. With the muffler off, I showed him that only a small diameter was open in the center exhaust hole, the others crusted over with the burned oil from the mix, and warned him of the urge to increase it's power.