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Flooding = too much fuel
Vapor lock = too little fuel (air instead)
Vapor lock is caused by excessive heat somewhere in the fuel delivery system (fuel pump-to-induction system or within the induction device itself [carb]). In the case of external lines overheating, move the pump to the front near the tank and/or insulate fuel lines running close to the heat source. Excess heat causes fuel to vaporize. Fuel vapors compress and don't move liquid fuel.
If it's a later 911, SC or Carrera, there's a fuel pump check valve that could be borderline dying.
Flooding:
If there's too much fuel, check for excessive fuel pump pressure and/or a higher-than-normal float level in the carbs. Excess heat (heat soak) can also force fuel out of the carbs (at rest) and flood the engine. In the case of carbs, many owners install the carb insulator spacers and modify the float bowl vents per PMO to mitigate the situation.
If you have to hold throttle wide open to start, you are partially compensating for excess fuel by allowing more air to enter and mix. Check plugs and see if they're "wet" from excessive fuel intake.
If the displacement is enlarged, the engine is greatly modified and/or the operating conditions are severe, all the above is exaggerated and additional cooling strategies are needed.
Sherwood
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