Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Bonjour Raphael,
Quick question, the carb looks like it is based on an RC model engine carb, these usually have two mixture adjusting screws.  The one you are moving is the main jet, this usually controls 2/3 to wide open throttle and then there is a second screw (the idle adjust) which is usually situated in the centre of the throttle arm.  If you see an adjuster there then i think maybe the low throttle is too rich and to compensate for this you screw in the main jet, so now the main jet meters the fuel at low throttle, BUT when you open the throttle the engine dies because the main jet is not supplying enough fuel.
May i suggest, if you see a low speed adjustment screw, open the throttle wide open, crank the engine and at the same time open the main jet until the engine fires/runs  (the high speed adjustment should be done first) once you get the engine to run at full throttle then gradually close the throttle and adjust the low speed needle until you get the best low speed running.
I hope this helps, i'm only guessing based on experience with RC aeroplane engines.
Best wishes
Ade