Sorry for the later reply, my hand is still hurting but my spirits are up and going…
My remarks, be it in some strange order:
1: your generator seems to be in a “bad” shape as it sounds strange to me that your voltage drops so easy. Does it have a functioning AVR or how is the voltage regulated ?
2: What is the real output measured , running on gasoline, from this set, increasing load untill it drops 5 Hz
3: is the rpm adjustable by setting screw ?
4: how is the governor reacting to the changing load ?
5: what is your charcoal like ? is the gas smokeless, blue flame ?
6: how is the slag from your gasifier ? sticking glassy mass or more crumbling ?
My old and still running generator for testing ( 5 years abuse now ) has a 7 Hp honda clone engine ( 212 cc ) and does not have a problem to run 1000 watts ( on charcoal gas ) with carb and even 1800 watts load without the carb, both on 3000 rpm / 50 Hz
Adjusting the air/gas mixture is quite essential, depending the engine temperature and ambient air …
Running on lower vacuum should be slightly better coz , if the mixture is well balanced, there should be entering more mixture in your engine.
My rule of thumb for small engines:
Only if the engine runs 100% perfect on gasoline and only then, it will “perform” on charcoal gas, otherwise it will run , but…
1: check fuel / grade / quality
2: engine and filters… The slightest filter fouling will give you “headaches” to find out why are those murphy’s popping up
3: airleaks… ( seen and experienced that a lot, certain when i had to do startup at a show… )
4: smell your fuel when you refill or refresh next day…