Re: IML: Carb fix

[David Whitney <david.whitney@xxxxxxxxxxxx>]

Generally, the accelerator pump will not cause a "twitchy" response.  The
purpose of the pump is to smooth the transition off idle to medium throttle
and prevent bogging.  The much-ballyhooed "double pumper" has a second pump
that performs the same function as the secondary throttle plate opens.  If
you feather the pedal off idle, there is a passage called the transfer slot
that performs much the same function.

The accelerator pump shot does not go into the carb all at once.  It is a
measured dose, the size and timing of which vary depending on pedal
pressure -- the harder you push, the more of a shot you get in less time.
That creates a temporary rich condition while waiting for manifold vacuum
to draw sufficient fuel through the primary circuit (the big nozzles you
can see from the top).  You can observe this when the engine is off by
pulling back on the throttle lever and watching gas squirt from the
accelerator pump nozzles.  It does not squirt when you give it a little tug
and let go -- you really have to lean back on it to get some pump action.

While changing the lever hole will give you more or less gas in the pump
shot, it is not designed to increase performance unless you like to floor
it off the line.  Want to know for sure?  Disconnect the lever and feather
the pedal off idle.  If it's still "twitchy" then you have ruled out the
pump shot.

I would look at aggressive ignition timing as the culprit.  Racers want as
much advance as soon as they can get it because they like it twitchy.
Check initial advance, vacuum advance (assuming you have the adjustable
type) and mechanical advance.  You'll need a timing tape for the damper or
a light with built-in delay to do this properly.

-- David


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