Harley Davidson Carburetor Tips

Harley Davidson Carburetor TipsWhatever brand of carburetor you run, there are things you need to know if you expect to get efficient performance from your carb. A carburetor is a simple device. It takes air (a gas) and gasoline (a liquid) and mixes the two together in an “aerosol” or fog. The air/fuel mixture can exist in different proportions. If there is more fuel compared to constant air, the mixture is said to be “rich.” If there is less fuel compared to air, the mixture is said to be “lean.”

A lean mixture will run your engine HOT, in fact, so hot that your valves and pistons will literally burn up! Whatever you do, especially with a shovelhead motor, avoid a lean condition. Follow your carburetor’s instructions to the letter, and carefully dial in your engine by starting with a rich mixture and working toward a mixture that is slightly rich, but never lean. A telltale sign of lean running is when the motor backfires through the pipes and sputters through the carb. The motor is trying to tell you something, and it pays to listen. A well set-up carb will give good solid acceleration without backfires or sputtering.

Things that can go wrong:

Your carb has gaskets and O-ring seals. Be very careful to install all the seals when re-assembling the carburetor! For example, if you fail to install the O-ring seal that sits on top of the accelerator pump nozzle in an S&S Super E carburetor the float bowl will be at atmospheric pressure and leak fuel constantly around the nozzle. As air is drawn through the Venturi on its way to being mixed with fuel, the air will cause a vacuum over the opening left by the absent O-ring and pull fuel right up out of the float bowl. This appears as a constantly rich condition, with little control offered by changing jets.

As you drop the float bowl make sure you pull straight down, and avoid tearing the float bowl gasket. If you tear the gasket, the float may not be able to rise high enough in the bowl to close the gas needle valve. If this happens, flooding of the engine will result. You might get “washdown”, where raw gasoline washes down over the pistons and into your oil system. Once this happens you should change the oil immediately because gas is a solvent and will destroy your oil’s lubricating qualities.

An air/fuel mixture sensor is a great tool to help you diagnose carburetor problems. The device is based on a sensor that is installed in your bike’s tailpipe. You have a welder install a threaded “boss” in the pipe and you thread the sensor into it so that the sensing element is exposed to exhaust gases. The sensor hooks up to an electric meter that measures from 0 to 1 volt. Such meters are readily available on the market. A good source is “Split Second” who can be found on the Web and sell racing engine tuning devices. Here’s a link to Split Second

An air/fuel meter will detect the fuel mixture you are running and enable you to select the right jets to optimize your mixture. It is by far the best way to give yourself some peace of mind on this important issue. It will also detect when your carburetor is leaking, or when you have not installed that one tiny O-ring that makes all the difference in the world. Most important: It will warn you when your fuel system goes south and your motor starts to run lean – BEFORE IT BURNS UP!

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  1. Rob Fleming - August 30, 2011

    New blog post: Harley Davidson Carburetor Tips http://t.co/Rv5YU9N

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