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Cold weather starting -- The Drill

To: Lawrie Alexander <>
Subject: Cold weather starting -- The Drill
From: Robert Allen <>
Date: Wed, 30 Dec 1998 21:36:30 -0600
Geez, Lawrie, your New Year's resolution should be to get a little more and
worry a little less. Of course pumping the pedal does some good as anyone
capable of properly deciphering an SU carburetor schematic could tell you.

You will notice that pulling the choke cable rotates the outboard shaft between
the two SUs. The shaft has a lever arm that pulls down on the jet tube and
results in a larger fuel pool to be exposed to the venutri. Also on the shaft is
a cam that rotates around to engage an adjustment screw on the throttle shaft.
This has the inconsequential effect of cracking open the throttle plate.

Now (and please pay attention) pumping the throttle effectively provides a
jack-hammer action of the adjusting screw of the throttle shaft against the cam
on the choke shaft. Following Newton's 4th law (hitting stuff with a hammer
makes things fall off), this pounding action is transferred via the lever arm to
the jet tube and, naturally, knocks off any junks of varnish/ ice crystals that
may have formed.

Furthermore, as these varnish/ice junks fall into the pooled fuel in the jet
tube, the resulting splash releases a fuel-rich vapor. The pumping of the
throttle causes the throttle plates to act as small fans wafting the splash
residue vapor into the intake manifold.

Quite elegant, I might add, compared to the mechanical complexity of an
accelerator pump.

Glad to be of service,
Bob Allen, Kansas City

Lawrie Alexander wrote:

> One more "For Pete's Sake" and I'm done for 1998, I promise.................
> WHY, oh why do so many of you keep referring to starting procedures that
> either call for "giving the pedal a few pumps" or (in another post)
> "DO NOT PUMP THE PEDAL"? It takes but a moment to look at the SU carburetor
> design, or the Zenith Stromberg design, and see that THERE IS NO ACCELERATOR
> PUMP on either of these carbs!
> Any "pumping of the pedal" is doing nothing but exercise your ankle and
> shorten the life of the cable and associated pivot mechanisms!

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