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Carburetor Percolation

By Bob Wilson

Last month I took my 40 Willy’s to a local car show here in the Phoenix area. It was one of those hot days over 100 degrees. I drove it home after the show and parked in the garage. After a couple of hours I went out to the garage and opened the hood of the Willy’s. After doing so I could hear gas percolating in the front bowl of the carburetor. I put my laser temperature gauge on the the front bowl and obtained a reading of 147 degrees. The rear bowl was 145 degrees. The Willy’s is powered by a GM 502 with a Holly 850.

I checked the Blogs and determined that Ethanol (alcohol) blended fuels boil at a lower temperature than non blended fuel. I also discovered that while engine heat is generally not a problem while the car is in motion, heat can become an issue when the car is parked. One of the symptoms is hard re-starts. Everyone knows that big blocks produce more heat than small blocks, and winter fuels are more susceptible to percolation than summer fuels.

I called Holly and they confirmed my suspicions and the research I had done. Holly advised that they had a fix for the problem. While I had a phenolic gasket between the intake manifold and throttle body of the carb I did not have a heat shield. I purchased a kit from Holly that had a slightly thicker phenolic gasket and a heat shield that protects both float bowls from rising engine heat.

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