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"vapor lock" / fuel delivery


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There has been much ramble about so-called 'vapor-lock' problems. It is NOT ENUF to check the vent in the gas cap ONLY. Most gas tanks have a VENT TUBE that runs from just inside of the filler neck near the cap to the TOP (T-O-P) side of the tank while the larger filler tube enters the tank near the bottom of the tank. NOT all tanks are made this way but many are. IF the VENT TUBE (NOT JUST THE CAP) is restricted this can cause fuel delivery problems.

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I dont care what you "check". Even assuming you pre fuel-injection car is in FACTORY FRESH condition, it is most likely going to "vapor lock" on any gasoline meeting modern ASTM standards.

Here's why. The higher the 'Ried Vapor Pressure', the more likely you are going to get an easy start on cold mornings. Problem...higher the vapor pressure of the fuel, the more likely it is to boil into vapor, which will not flow (at least, not as fuel) thru your fuel system in a car using the tradiional suction-type fuel pump.

Modern cars simply cant vapor lock - because NOwhere in their fuel delivery systems, is the fuel being "sucked" / under vacuum. Modern cars have high pressure fuel pumps either IN or AT their gas tanks. With the fuel under pressure all the way to the engine - again....NOT going to vapor lock.

Funny....SOME carburetor-equipped vehicles I have..simply wont vapor lock...others do. Why..? Who knows...lower temperatures in the fuel lines...?

Dosnt matter - bottom line - if you want to be free of vapor-lock in a carbuetor equipped car, get an electric pump, and put it back as close to the gas tank, and as low as is safe. With a modern electic fuel pump, even at modest pressure ( dont go above 5 lbs or you could over-power your carb's needle/float valve seat) you have resolved the vapor lock problem forever.

Another note - if you get vapor lock (again, this is only possible in a car with a carb., standard "suction type" diaphragm fuel pump mounted on the engine)....and you want to stop it in a hurry.....THINK...where is it vapor-locking ! Always amuses me to see people doing all kinds of wierd things to the PRESSURE side of their diaphgram-type fuel pumps. That is not where it vapor locks. You need to cool down the SUCTION side of the pump - the lines between the gas tank and the fuel pump. Doing neat and wierd things to the PRESSURE side...? Sure...it "works".....CAUSE YOU ARE GIVING THE REAL VAPOR LOCKED AREA...A CHANCE TO COOL OFF AND / "CLEAR"......!

Pete Hartmann

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Ried values - Reductio ad-absurdum:

I CURRENTLY have an 88 Ranger, 73 Impala, 56 Packard all with carb and mechanical fuel pump in nearly everyday operation or at least weekly operation under high heat conditions here in Tn. AT NO TIME have i EVER experienced vapor lock in these HIGH MILEAGE, beat to death and in one case rust bucket vehicles. These cars ARE kept in good running condition. smirk.gif

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furthermore:

if the pump pressure gets weak, but still strong enuf to keep some fuel in the carb then the pump cannot pressurize the line ggoing to the carb enuf to keep it from boiling the fuel, especialy since its right up next to the engine in many cases. SO vapor lock can occur ANYWHERE in the fuel system after leaving the tank including the carburetor itself. Vapor lock however ONLY occurs under EXTREME conditions OR poor maintenance such as a heat riser stuck closed.

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I had this problem in spades with my '36 Pontiac 6. If I shut the engine off when it was hot, it would't restart because of vapor lock. I solved it by adding a 1" aluminum spacer between the carb and manifold. That got the carb bowl farther away from the hot exhaust manifold and the problem went away. A local machine shop made the spacer very inexpensively.

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