The ignition timing of engines that don't have speed-dependent timing (either centrifugal advance or computer-mapped spark timing) is usually determined by the need to be able to start them without kick-back on the starter. If the flywheel is heavy enough, 10-12 degrees BTDC is usually achievable. That is insufficient for maximum power at high speed, unless the engine has a very fast-burn combustion chamber: 4 valve hemispheric for example. For a side-valve mower engine, the appropriate timing for maximum power would be around 35 degrees BTDC - but then you couldn't start it. Similar outcomes happen for crude 2 stroke engines such as are used in lawnmowers. The burn rate is not as slow as for side-valve 4 strokes, but it's pretty slow due to poor scavenging. Poor scavenging has the same effect that a large dose of EGR (exhaust gas recirculation) has on an emission-controlled car engine at part throttle. If you've ever driven a car with a stuck-open EGR valve, you'll remember the experience.
