Flathead Fever
Well-known member
This is not a vapor lock problem, and I will explain why. When an engine has a mechanical fuel pump it creates a vacuum in the line between the pump and the tank. When you put a liquid under a vacuum it turns to a vapor easier than if it was under pressure. especially if that fuel line is close to a heat source like the exhaust. A fuel pump cannot pump a vapor. Modern cars "never" vapor lock because the fuel pumps are submerged in fuel inside the tanks which keep them cool, and the fuel is under pressure from the tank to the injectors which raises the temperature it takes to boil the fuel.
We had a lot of phone company, 1984 Chevy Suburban's that serviced the mountains you've seen on the news, the ones with the people buried under snow in their homes here in SoCal. The drivers were complaining that the Suburban's were vapor locking in the mountains, above 6000' elevation but never down in the valleys. The problem happens on warm days and at high elevation after the vehicle was driven awhile. They would act like they were running out of gas and if they pulled over and poured water on the mechanical fuel pump the problem went away for a while. We found that A new mechanical fuel pump had 6 lbs. of pressure and the ones that were a couple years old had 5 lbs. That little loss of pressure combined with the altitude and the engine compartment heat was letting the fuel boil either in the pump or the line from the pump to the carb. Those Suburban's were the only ones I can say for sure were actually vapor locking. Those were engines where their symptoms worsened the longer, they were driven, but they fixed themselves when the fuel systems were allowed to cool down.
You have the opposite problem. If it was running fine when you shut it off you had fuel in the carb, where did it go? It either drained into the engine, it evaporated or it's not a fuel problem at all but ignition. You already diagnosed it when you said you need to hold the throttle wide open to get enough air in there to mix with that excess fuel. One or both of those carbs are probably leaking fuel and flooding the engine. When it sits for a long time that leaked fuel will evaporate and the engine will no longer be flooded but then the carbs would now be empty and with a mechanical fuel pump you would need to crank the engine awhile to fill those carbs back up before it would start. But you said you are priming the engine after it sits with an electric fuel pump so you might be filling them back up before you crank the engine. Check your oil and see if it has strong gas smell and or if the oil level is too high.
We had a lot of phone company, 1984 Chevy Suburban's that serviced the mountains you've seen on the news, the ones with the people buried under snow in their homes here in SoCal. The drivers were complaining that the Suburban's were vapor locking in the mountains, above 6000' elevation but never down in the valleys. The problem happens on warm days and at high elevation after the vehicle was driven awhile. They would act like they were running out of gas and if they pulled over and poured water on the mechanical fuel pump the problem went away for a while. We found that A new mechanical fuel pump had 6 lbs. of pressure and the ones that were a couple years old had 5 lbs. That little loss of pressure combined with the altitude and the engine compartment heat was letting the fuel boil either in the pump or the line from the pump to the carb. Those Suburban's were the only ones I can say for sure were actually vapor locking. Those were engines where their symptoms worsened the longer, they were driven, but they fixed themselves when the fuel systems were allowed to cool down.
You have the opposite problem. If it was running fine when you shut it off you had fuel in the carb, where did it go? It either drained into the engine, it evaporated or it's not a fuel problem at all but ignition. You already diagnosed it when you said you need to hold the throttle wide open to get enough air in there to mix with that excess fuel. One or both of those carbs are probably leaking fuel and flooding the engine. When it sits for a long time that leaked fuel will evaporate and the engine will no longer be flooded but then the carbs would now be empty and with a mechanical fuel pump you would need to crank the engine awhile to fill those carbs back up before it would start. But you said you are priming the engine after it sits with an electric fuel pump so you might be filling them back up before you crank the engine. Check your oil and see if it has strong gas smell and or if the oil level is too high.
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