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Fuel pumps: how do they work?

Discussion in 'Technical and Troubleshooting Torque' started by mattb, May 7, 2011.

  1. G'day All.

    My 1992 Virago has a fuel pump taking fuel from the sub-tank to the carbs. How does it meter the fuel properly? I'm assuming the pump is designed to meter a certain amount which they calculate is about what's needed to keep fuel in the carb (despite different throttle positions on this otherwise old-fashioned design)? Or is it more sensitive than that?


  2. Couple of possibilities.

    On most of the older cars that I've played with, the fuel pump pushes fuel to the carb float bowl(s) until the float shuts the needle valve at the carb. The pump then is confronted with a significant amount of back pressure and either stops (reciprocating electric pump) or is designed to be unable to push fuel against that back pressure (engine driven mechanical).

    On more modern/sophisticated (if such a term can apply to anything with a carburettor nowadays :D) systems, the fuel pump pumps fuel continuously and any that is not required by the engine is returned to the tank via an overflow line in a similar manner to modern efi systems where fuel is constantly recirculated through the system.

    Nothing sensitive about either at all. The pump is sized to provide more than the engine's maximum demand and the carb is designed to dump or reject what it doesn't need.
  3. How do fuel pumps work? Its obviously magic.
    The fuel is mixed with electricity in a small container and it doesn't explode. If you do the same thing in a cylinder it goes 'bang', its one of life's little mysteries.