The pump is actually to add additional pressure in the fuel system. Some of the 4 stroke engines do not have a pressure tap in the exhaust to run a pressure line to the tank and require the use of a pump such as the OS Gemni 160 twin. Some Scale planes have to have the fuel tank installed further away or lower than recommended, and require the extra pressure to supply sufficient fuel pressure whether it be 2 or 4 cycle, even with a pressure line in the system (from muffler to the tank) I am not the best at explaining things but I hope this helps.
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