Briggs and Stratton command that their aluminium-flywheel mower engines are never run without the blade plate fitted. The main issue is a safety problem, which I'll explain in tedious detail in the hope that someone will find the information useful. Small B&S engines traditionally have fixed ignition timing. They would give maximum power with a timing of around 30 degrees before TDC, but the most advance they can actually use, is determined by when it begins to kick back when starting. What they have done, is set a timing (about 6 degrees before TDC, from memory) which is at the limit: the most they could use without risk of kick-back, provided the engine either has the cast iron flywheel, or has the blade plate attached. Trust me, you will not enjoy a decent kick-back: if it doesn't break or dislocate your fingers on the pull cord, it at least feels as if it has.
