Friday, September 18, 2009

Should you pump the throttle on a carbureted engine?

I'm a CFI and have always instructed my students not to pump the throttle when starting a carbureted engine. A recent discussion between one of my students and another CFI has me wondering if I telling my students bad info concerning the accelerator pump on carburetors.
Can you confirm any reasons why it's either bad or OK to pump the throttle during start?

Sounds like one of those questions that generates lots of differing views, so here is mine: There is nothing worse than sitting in a burning airplane way out on the ramp without a fire extinguisher.

Our carburetors in flat Continental and Lycoming engines are up-side-down meaning that the fuel has to go up hill to get to the cylinders. Fuel squirted into the intake system just flows back down into the air box unless it's being sucked up into the cylinders. Too much fuel dripping out of the airbox can catch fire as my two personal experiences attests.

So just pumping the throttle with the propeller stopped is only useful for washing out the air box with fuel. In my C-182 it might be necessary to pump the throttle on a cold morning to keep the engine running - but only a little bit as too much pumping and the fuel starts draining back into the airbox - better to use the primer, if equipped.

So to sum up my recommendation:
  • Don't pump the throttle on a stopped engine
  • Avoid pumping or pump as little as possible

With that said, that is based on my limited experience. Some might have better recommendations that I would be interested in hearing.

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