Ned Simmons wrote:
The patient is a 1000W Yamaha EF1000is inverter generator. The 50cc
four cycle engine drives an alternator that powers an inverter which
outputs clean 120V 60Hz power.

In normal "economy" mode the speed of the engine is controlled so the
alternator's ouput is adequate to maintain output power. With the
economy mode off the engine runs at constant speed, about 5000 RPM.

The engine starts easily and idles smoothly. A little puff of black
smoke when started, exhaust seems clean immediately after that.

By applying load slowly with a variac the generator ramps up and runs
fine 'til about 50% load, after which it dies.

A bad spark plug could be the cause, or perhaps a
bad magneto coil. Will it run indefinitely at 50%
load? If it runs a while, then sputters and dies
at 50%, that is a strong sign of a bad magneto.
Another possible cause is a plugged-up jet in the
carb. Up to 50% the idle/low-speed circuit gives
an ignitable mixture, above that, the high-speed
jet is needed, and isn't working. If you run it
at 50% for an extended period, with a really lean
mixture, it will fry the engine.

Jon