ThunderJet is the trademarked name of an aftermarket addition to motorcycle carburetors. ThunderJet-enhanced carbs are not legal for street use in California. The purpose of the ThunderJet is to deliver additional fuel when the engine is running above 4500 rpm.
The Mid-Range Problem
The typical unmodified motorcycle carburetor has two fuel circuits. Assuming the carb is normally tuned, the low-speed jet is used from idling speed up to approximately 2500 RPM, when the main jet kicks in. Motorcycle carbs are like this because jets designed to feed fuel to a running engine are too small to provide the fuel needed at idling speed. The problem is that, while using only two jets ensures smoothness at low speeds without compromising the top end, it leaves a mushy mid-range.
How ThunderJet Works
Thunderjet is a third fuel circuit that kicks in above 4500 RPM. It is mounted externally and machining of the carburetor bowl is necessary, so it requires professional installation. Once your mechanic installs ThunderJet, the entire carburetor must be tuned again, of course, to optimize the main jet for mid-range rpms. Properly installed and tuned, the ThunderJet smooths out the power curve.
The original ThunderJet system is complemented by the ThunderPro ThunderJet system for engines with large displacement. According to Zipper's Cycles, a Maryland-based company specializing in engine design for racing motorcycles, the ThunderPro is a product of the Research and Development done during the development of the ThunderJet SuperPro carburetor, a carburetor with integrated ThunderJet technology.
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