Hi Thierry , Koen was correct in saying only adjust or alter one thing at a time , so once you have seen and read all advice or suggestions it might be a good idea to list them in order of doable with ease , if it was me i would first start blowing air into the air inlet pipe on the gasifier while connected to the engine and then with a spray bottle of soapy water start on the gasifier and then go over all pipe work and connections all the way up to the engine bay , if no leaks are detected at all then move onto restrictions that may be causing it to starve of fuel .
You mentioned earlier in the post that your engine was “hunting” revs increasing and decreasing on there own , well if that was running on petrol i would immediately go to the carb and fuel line to check if fuel was flowing to the carb and then check the jets for blockages same with charcoal gas , if the engine is starving of charcoal gas it will do the same thing , at least it has done on one of my small engines a few times .
If no faults so far found i would then look at changing the Nozzle size and test , if better but not still correctly running then look at the pipe work size make sure its allowing the engine to breath in all the gas it needs .
The last thing i have come across is the tap on your air pump the pvc tap i once used always leaked air or sucked in air even when fully closed off so with the tap turned off to the fan please make sure to spay plenty of soapy water around the fan when you pressurize the system to check for leaks .
Fingers crossed Dave