> Marcus writes about his '74 Midget --
>
> > I see there is meant to be an
> > air pump and so on. I DO have tubes and things which go
> > to an evap. canister and around the valve cover and carbs
> > but I DO NOT have an air pump.
>
> The early '70's Midgets (at least) had an air pump that
> indeed was driven by the fan belt. It bolted to the
> thermostat housing/cover at the top and to a tensioning
> bracket below. It was on the opposite side of the engine
> (assuming you have a 1275, anyway) from the carbs.
So far, so good. Yes, all 1275s had air pumps of this basic
configuration, or rather, all 1275s after 1968; the '67 Spridgets
came without the pump.
BTW, the air pump uses/loses all of about 2 bhp. I seemed to feel
that removing it from my Midget made a difference in performance,
but that was a long time ago. On Bs, I've never noticed a difference
with it on or off the car, so I generally run the air pump on the
assumption that it keeps the car greener than it already is.
> I don't remember where the intake came from, but it pumped
> the "bad stuff" back into the combustion chambers via
> metal tubes which went into the head just below the spark
> plugs.
Not quite. It took filtered air and injected it into the
exhaust ports. The purpose of this was not, as some people
have suggested, to dilute the exhaust and fool the sniffer;
it was to continue some combustion in the exhaust system,
or rather to produce a chemical reaction that required the
extra oxygen. So no "bad stuff" and no combustion chambers.
That would have been managed by an EGR (exhaust gas recirculation)
system, which the A-series powered Midgets at least did not have
(don't know about the Spitfidgets).
One additional function on the '74, at least, was performed by a
"gulp valve" which was vacuum-operated and dumped air from the
air pump into the intake manifold. This is why the air pump
has an air filter on it; the air pump's intake is at the outboard
side of the pump and draws air from under the hood. The purpose
of this was to lean out the intake charge when you suddenly
dropped throttle, such as on a shift or when you hit the brakes.
In a carbureted engine, sudden throttle closure causes a very
rich (and very polluting) condition for a few fractions of a
second, while the high-velocity air continues to run over the
jet at an even higher pressure, due to the closed throttle. The
throttle butterflies on post-68 M.G.s also have that little
spring-loaded butterfly for the same reason.
The air pump is fairly expensive, because so many people yanked
them from their cars in the largely mistaken impression that
doing so would make the cars faster, so there are few to choose
from.
For Marcus' benefit, the 68-74 Midget and the MGB used the same air
pump, but different air injection rails of course as the port spacing
is different. If you need to replace the air pump on your Midget to
pass smog, you might be able to save money by pulling one from a B in a
Pick-N-Pull lot. I have installed a Midget air pump on a B, so I
presume it'll back-translate as easily. Use the Midget hoses and
other gadgets, of course. Since they made so many Bs, you might have
better luck finding one that way.
The Haynes manual should show you a picture of the air pump so you
get the right thing.
--Scott
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