Dave Ambrose scribed:
Subject: My SU fuel pump is doing the long goodbye.
>
> Rather than going to the hassle of repairing a flakey design,
A time honourned, traditional design. I certainly wouldn't have anything
else on my PA! The SU pump does a good job with SU carbs, and has that
added audiable "fuel low" indicator feature. (Tic.tic.tic.tic.tic...)
An important part of the character of an LBC!
> is there something else I could fit which will work better?
As Dave (and others) have now found, modern pumps can do a fine job, and
can deliver more flow. Just make sure you still use a low pressure
pump, or add a regulator. The float valves in SU's (and Z-S's) can't
take higher pressures, and will blow by and flood or overflow. Gross
Jets (ball valves) can be *more* sensitive to overpressure than the
original cone shaped needle valves.
The "Diodes" which may appear on new pumps will be Zener diodes.
They are speced with a particular "reverse breakdown voltage",
which basicly means they act as a shunt to bleed off excess
voltage spikes generated by the coil when you open the points.
They are used on most relay/soleniod designs today, in order to
protect any electronics which is turning on the coil.
Adding a zener will not only keep down the arcing on the points,
but will reduce noise spikes sent back into the wiring, which you
may hear on your radio.
You may be able to extend the life of your points by adding a zener
to your existing SU pump. The zener voltage must be higher than the
cars normal operating range, as it will try to shunt off any over
voltage. With the car's voltage range around 12-16 Volts, a 20-28
volt zener will be a good, safe choice. Connect the zener as close
to the coil as possible- right on the terminals is best.
Adding a Zener diode: (Negative ground car)
o-------------------------o / o-------- +
| | + points
% ___|___/
coil % / /\
% /__\ 24 V Zener Diode
| | -
ground - o-----------|
On a pos ground car, the shown polarities and Zener must be reversed.
If the points are on the ground side of the coil instead of the hot side,
the zener is still hooked up the same- accross the coil contacts.
Theory of operation: Most diodes conduct current in one direction, and
block flow in the other. Current flows through a diode in the direction
of it's "arrow-head" as drawn on schematic diagrams.
| + | +
___|___ ____
/\ \ /
/__\ \/
| ----
| - | -
Current will not flow through the left diode, this is known as
"reversed biased". Current will flow through the right diode,
shown "forward biased". If these were "ideal diodes", the
reversed biased direction whuld be seen as infinite resistance, and
the forward dirction would be seen as no resistance (zero Ohms).
The zener is a special diode- it works normaly when forward biased
(no resistance), but only blocks reverse current until voltage reaches
it's breakdown or "zener" voltage, at which point it begins to conduct.
Thus, for the reversed biased zener hooked to the coil above, nothing
will happen until positive voltage rises above 24 volts (spike created
by opening points). But, the zener will shunt everything above 24 V
back to ground. There is an added benifit, in that any negative spikes
generated by the coil will be shunted back to the coil by the now forward
biased diode. Both of these actions will prevent high voltage spikes
from reaching the points, arcing across them, and resultant pitting.
________
/___ _ \ Roger Garnett (Roger_Garnett@cornell.edu)
/| || \ \ Agricultural Economics | "The South Lansing Centre
| |___|| _ | 3 Warren Hall | For Wayward Sports Cars"
| | \ | | | Cornell University | (Lansing, NY)
\| \ |__/ / Ithaca, N.Y. 14853 | (607) 533-7735
\________/ (607) 255-2522 | Safety Fast!
|