I've been rebuilding a little Yamaha MX80 and the engine will not run for more than about 30 seconds. It fires up okay with the choke on, is slow to rev up with throttle open but does get to power band quite impressively. Then after half a minute it just dies as if someone hit the kill switch [ which isn't even connected yet ]. Then as soon as soon as you kick it over again it repeats the performance. Now I was thinking fuel blockage or low float level but have cleaned the carby out several times and checked the float. I did notice that with the choke on it seems to run a bit longer, but then I also found the idle screw to be missing. Replaced this with a normal screw to block the hole up but don't know if the needle end of the original is supposed to simply lift the plunger or also restrict fuel/air flow in the threaded section it screws into. Either way, it still keeps dying shortly after starting. Can anyone point me in the right direction, or perhaps steer me to another possible cause, say reed valves or main seals? It's so annoying to have a creature come to life and then die just as you kick it into 1st gear.