THE SHOP

March '20

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60 THE SHOP MARCH 2020 F uel tank vent issues occur on many vintage vehicles, and especially on vehicles equipped with an engine more powerful than the original. An example of this is a 1969 Chevrolet Corvette. The customer tells us that the engine has great power when he gets into the car after it's been parked, but that the same engine lacks power whenever he accel- erates hard after driving at highway speeds for an extended period of time. The problem ends up being a fuel tank vent issue. His gas cap vent was in the 0.080-inch range. We installed a gas cap that had extra vent holes drilled into it and then had the customer take his Corvette for an extended test ride. We could tell by the smile on his face when he returned that the problem was gone. LET'S VENT The fuel tank on most vintage vehicles uses a vented gas cap as the source for outside air to replace the volume of fuel being pumped from the fuel tank to the engine. The vent restriction of a typical vented gas cap from a production vehicle is typi- cally in the 1/16- to 3/32-inch range, while a race vehicle's fuel tank vent system is often in the 1/2-inch inside diameter range. If the fuel tank vent system is too small for the fuel demands of the engine, the fuel pump will cause vacuum to build up in the fuel tank as it removes fuel. As the vacuum builds up in the fuel tank, the pump will have to work harder to supply the volume of fuel the engine needs until the vacuum gets so high in the fuel tank that the fuel pump is no longer able to function properly. The factory fuel tank vents used on most classic production vehicles were good enough for the needs of original engines during normal driving conditions, but if engine performance has been upgraded or the vehicle is to be operated at sustained high-load conditions, then the vent system may need to be upgraded. Even many of the muscle cars of the 1960s used a vented gas cap that did not allow enough outside air into the tank to replace the volume of fuel the engine was consuming when the vehicle was driven under high loads. We recently had a customer bring his freshly built Cobra replica kit car into our shop to have the carburetor and ignition systems tuned, plus diagnosis of why he had two electric fuel pumps fail in just a few hundred miles. When the customer A Corvette gas cap with a 5/64-inch drill in the vent restriction. 60 THE SHOP MARCH 2020 Fuel tank vent issues occur on many vintage vehicles, and especially on vehicles equipped with an engine more powerful than the original. A simple solution for fixing common fuel delivery issues on vehicles with power upgrades. Proper Gas Tank Venting By Henry P. Olsen

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