Quoted:
just don't forget that when sizing a tri-fuel generator, the power output is not constant across the three fuels. typically, a gasoline generator with a conversion kit will produce only about 80-90% of it's rated output when run on propane, and only about 70-80% of it's rated output when run on natural gas.
this is not a defect with any specific generator design; it simply reflects the different energies contained in the fuel itself.
ar-jedi
This is not always true, especially for a smaller inverter generator. A traditional generator head is spun by the attached motor. If the motor has less power because of fuel type the "torque" spinning the gen head will be less causing less output and possibly a stall exactly like you stated. But, since a inverter gen produces power through a different method (see below), as long as the motor is sized big enough in relation to their alternators output (and both the small Honda and Yamaha are) the power output from the inverter will be the same across fuel types because it is a function of electronics not mechanical power generation.
I cut and pasted this explanation which explains it better than I can:
"The deal with inverter generators is that the engine can run variable speed, and there are no brushes. Powerful Neodymium magnets make this possible, you can make a pretty efficient little alternator pretty small, but you can't control the output voltage, so it is difficult to use the output directly. So to solve this they make it a 3 phase alternator which makes really good DC power when rectified and then they have to make that DC back into AC, requiring the inverter."