check the start delay fuel solenoid. (blue solenoid between carb and regulator) I have seen many of them get over loaded with propylene from the L.P. fuel and sludge up when cool. Once the engine runs a little bit the propylene warms up and the diaphram opens and closes properly. If the diaphram sticks open it will dump fuel right into the carb and possibly cause your problem. That solenoid was originally put in place to slow the timing of the fuel into the carb. The early G.M. engines used to fire off too quickly and break off the nose cone on the starter.