there's essentially two different forms of vapor lock that we see on old carburated cars. these symptoms don't occur with fuel injection because those systems are under high pressure. with the low boiling points of todays gasoline, we are witnessing vapor lock and heat soak symptoms that these cars didn't necessarily have from the factory with the stock configurations. if the lines around the fuel pump between the frame and carb are getting excessively hot, whether the line is laying over the heat riser passages on the intake, or the fan is creating a trajectory of hot air on parts of the line, the fuel in the line can vaporize. the mechanical fuel pump is designed to pull fuel, while an electric pump is designed to push, if the fuel isnt liquid around the mechanical pump, then the fuel pump loses its prime. people automatically tend to put an electric pump on, and assume that it solves both types of vapor lock and it only solves this form, which isnt the most common form... electric pumps arn't any more reliable, plus they are noisy... but it isnt the only solution. you can simply run insolating hose around the fuel hose and that solves the problem, 9 times out of ten, or simply route the fuel lines better. otherwise, the only electric pumps that are reliable are expensive holley and carter pumps, which also require a regulator, which is an added expense as well. these pumps MUST be mounted as close to the tank as possible and with a fuel filter in front of them. the other more common symptom is the fuel is vaporizing in the bowl of the carb from too much heat at the carb base. symptoms c with vapor lock, include, choppy idle and stalling when excessively idling or at very low cruise speeds such as traffic jams... not to be confused with the similiar symptoms accociated with flooding. as described in the AVS2 adaption article i've previously written, you can run set screws in heat riser passage, or better yet block off all heat at the intake gasket itself, which is an excellent and preferred choice. at bare minimum, all aluminum base carbs with baseplate heat riser passage exposure, requires a steel heat shield or the base of the carb will also rot out...., the intake runners will still get plenty hot enough to vaporize the fuel in the runners, (which is what you DO want). that also eliminates heat soak. if you you think you are getting vapor lock on the freeway or at cruising speeds, you are misdiagnosing the problem, as vapor lock only happens when the fuel is sitting stagnant in the lines and bowl long enough to get excessively hot. heat soak is the VERY reason why most people i talk to are looking into EFI, when really, its a simple fix. if the carb base is too hot, the fuel will vaporize/ evaporate/ boil in the bowl of the carb immediately after shut off, causing hard starting/ excessive cranking conditions, in as little time as simply getting gas at a gas station. blocking the heat at the intake gasket eliminates that. if you live in very cold icy climates, you will notice slightly more cold blooded symptoms when the engine is cold prior to reaching operating temp. in those situations, you can experiment with punching only a small hole, or two small holes in the block off plates for the intake gasket to find a happy medium. my 401 can sit in the garage for a week and fire up on the first turn.
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