bluemower wrote: Rpm is set at fast idle.
Technically that is top no load..
AKA High Idle.
However I have seen similar engines that will , eventually, experience premature failure from overheating (burned valves, hot spots on the cylinder, heavy carbon build up, etc) when they are not run to top no load at least some of the time - I'm gonna have to throw together a couple junked mowers and invest a little gas to do a comparison side by side.. when running at less than top no load for extended periods, the ambient temperature is going to be a huge factor in engine cooling - Much like starting up an air cooled motorcycle and leaving it running without moving - how long you think it would run at quarter throttle with little to no air flow?
But as a rule of thumb, the simple and most correct answer to the original question will be "yes" - if you have more information such as the load being applied, the amount of work the engine actually must do, the ambient operating temperature, and maintenance habits, you can then weight that answer out with a "might be just fine for 20 years" to "Thar She Blows, less than a week" - it'll be heavily dependent on the application and how much supplementary cooling the engine may get besides the flywheel fan - Engines are designed and spec'd to run at a specific RPM and that will also include the shrouding and ducting, the application environment, etc.
Engines built to run at extended idle or low RPM may have different or additional tin duct work to more precisely direct the reduced air flow for better cooling.. - again it really is heavily dependent on the application.
How poor are they who have not patience. What wound did ever heal, but by degrees? - Iago (Othello Act II, Scene 3)