I had the same problem with my Generic (see sig.) when it was new from the factory. I also found that the factory was useless to get information from.
I finally checked the intake/exhaust valve clearance. Can't remember for sure what it was but it was in the .030-.040 (thirty to forty thousandths) range. That was WAY to much! No wonder it was hard starting, loud, lacked power and ran on when shut off. A friend who works in the small engine field recommended that I adjust both the intake and exhaust valves to the .004-.006 (four to six thousandths) range.
Viola! Now it starts better, doesn't run on, but it still has lousy voltage/frequency (low power) control, OH WELL never again!
2017 GMC 3500 4x4 Denali Duramax
2019 Outdoor RV (ORV) Timber Ridge 24RKS