This old beast runs horrible when idling. It runs well for some seconds and than it dies. If the choke is used, it runs quite good, but idling is a bit too high. So I guess increasing the idle speed could help the engine to stay alive It's worth the try... But: Where do I find the idle adjustment on this three cylinder carbed engine? Sadly I don't have any technical documentation at all

Hunt around near the throttle linkage. It should be a screw with a spring wrapped around it. At least thats how it is on the hitachis