Black max generator hard start backfires ,if it does start doesn't stay running

SOURCE: black max 7kw generator high voltage output

Those voltage readings MAY be normal.

The AC waveform may cause your meter to read inaccurately. You would need to use a TRUE RMS meter to accurately read the voltage.

The siple meters actually read the AVERAGE voltage and then are calibrated to mulltiply it by the "form factor" for a sine wave. OFTEN these generators don't produce a clean sine wave. A square-ish wave will read higher on the garden variety meters.

Compare the brightness of a lamp comparing to when run on your utility.

The frequency depends on engine speed ... yours is running a bit fast...check it under at least half load and if off, adjust the govenor slightly.

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SOURCE: Brand new gx390 Black Max

If you can access throttle linkage to the carburetor, slow the engine speed to idle before turning off. As mentioned in the previous answer ... turning off the fuel valve and letting it run dry is good if you are not planning to use it real soon.