RE: HELP! SUGGESTIONS PLEASE - Which fuel pump for a large (45cc) petrol (gas) engine?
You do not have the fuel pulled up to the carb with the lines full of fuel. That is why you need to choke the carbs. This is also why the prime burns off and then will not fire. If nothing else, choke the carbs with your fingers until the fuel is pulled up (lines full of fuel) to the carb. Have the ignition OFF when doing this. Then try to start the engine.
That will really suck if that engine has no chokes. Just sayin'