Trevor Boicey wrote:
>
> Charlie Brown wrote:
> > twin stock AUD 2140 bowl configuration, no matter how carefully you
> > adjust the individual float levels, the front fuel level will always be
> > higher than the rear. Therefore, set equally, the rear carb will
> > always run leaner than the front, and the front will be more prone to
> > flooding.
>
> I don't agree with this assesment, unless the spitfire is
> vastly different than any other twin SU car I have known. I
> don't suspect it is.
>
> Pressure is supplied to both float valves at the same time
> from the fuel pump. If one float is at the level it is set
> to close the valve at, that bowl will not accept any fuel. This
> doesn't close the line to the other carb though, it is
> free to take fuel until it is as full as it chooses to be.
>
> There is no "linkage" between the two floats other than
> they are fed from the same line. One could be sat on top
> of the other, they would still work. (by your definition
> the top would not receive any fuel at all, and the
> bottom would be flooding).
Trevor,
You're absolutely right. If BOTH carbs were configured identically, that
is, with the float bowls either forward or aft of the venturi, and
with both carbs on a fluid level with one another, what you propose would
be correct. That would make both carbs functionally equivalent.
But, because the throttle linkage must go between the carbs, the front
float bowl is uphill of the front venturi (higher fluid level), and the
rear bowl is below the rear venturi (lower fluid level). Not by much,
mind you, but there is a difference. With the 3-5 degree down/rear angle
of a Spitfire engine, and gravity being an everyday norm, the rear fuel
bowl level will be lower.
I'm not trying to start a battle here. Not every car is the same. All I'm
saying is, with your Spitfire on level ground, remove both dampers and
pistons. Look down into the jets. If your fuel levels are
identical--typically about 1/8"-1/4" below the top of the jet--go about
your life, being content to know your are as one with your carbs. If the
levels are very different, you'll never get the carbs to pull the same
amount of fuel at the same time--the rear carb will not draw fuel
as early as the front carb. Either bend your tabs or shim you needle
seats to achieve similar levels. Aim for matching fuel levels in the jet,
not some arbitrary measurement of the float. Only if you can't get your
two AUD 2140s to level, do you need to replace the rear bowl with a
higher offset.
Like I said, this is only a suggestion.
Charlie B.
Spitfire Pilot
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