> Joseph Van Meter writes:
>
> > I have a problem and don't know where (else) to turn for technical advice.
> >
> > Here goes.
> >
> > I bought a 78 Spitfire back in October 93. Seemed to run fine. About
> > three weeks later I took it to work. Still running fine. Same day I
> > started it up and seemed to race at idle at about 2300 RPM range, and
> > would not kick down, even after heating up.
> >
> > ...
> >
> > The original scenario has now reoccurred.
>
> There is a congenital problem with the auto-choke sealing against the
> carb body. It is held on by three screws that shake loose. Don't lose
> them; they're rare as hen's teeth. Take off the auto-choke, maybe paint
> a THIN layer of blue Permatex on both sides of the gasket, and re-install
> ...
> Note location of shorter screw. Install the screws to recommended torque
> (I think about 40--45 inch-lbs) with something that gives you a bit of
> leverage, like an offset screwdriver or screwdriver bit in a socket wrench.
>
> (hushed gasp from the audience)
>
A not so hushed gasp from over here. Don't ever use a silicone sealer on
any part that might come in contact with gasoline. I don't care how little
the amount of gas or silicone. The fuel attacks the sealer, and makes this
really icky runny glob of goo. This stuff then finds the nearest small fuel
passage and plugs it slam up. I've had to throw more carbs in the trash
due to hopeless silicone plugging than I care to count.
The Care and Feeding of a Z-S autochoke:
The autochoke we're talking about here was used on late Spit's, 1500
Midgets, MGBs, and mid S2 XJ-6s. You TR7 owners have your own can of worms.
The choke is very simple in operation. It consists mainly of a rotary
arm that lifts a fuel metering needle, and via a cam opens the throttle
further. The potition of the device is determined, in theory, by a
coied bi-metalic spring. This spring is mounted to a water heated housing.
So, the position of the spring is governed by engine temperature.
Now for the fun part. This bi-metalic spring is not strong enough to
fully set the choke at normal cold start temps. To set the choke fully,
there is a spring-loaded plunger that grabs an ear on the cam and drags
it all the way over. Once the engine is started, the plunger is retracted
by carb vacuum, allowing the bi-metal spring to position the choke about
half way (cold). And herein lies the problem.
The plunger is supplied vacuum through a metered orfice (restrictor) in
the carb body. This orfice plugs up with carbon. When it plugs up (this is
not an "if"), the plunger can no longer retract. It insists on holding the
choke in the fully set position. Worse, as things heat up, the air in
the (supposed) vacuum chamber heats up, expanding, and forces the plunger
down harder against fully set; offsetting any whimpish efforts the bi-metal
spring might apply against the choke/plunger. The result is full choke, full
time.
To fix this, simply remove the choke housing, locate the restrictor (it's
flush in the carb body at the top of the choke mount area), and using carb
cleaner and fine wire, clear it out. Make sure you do not remove any brass
doing this. The size of the restrictor, the shape of the fast-idle cam, and
the tension of the plunger and bi-metal springs are all calibrated for
your particular engine to give the correct temp response curve.
The major faults I find with these chokes, in order, are:
Plugged vacuum port.
Melted plastic caused by overheating cat converter due to overly rich
mixture because of plugged vacuum port. (MGB mainly)
Mangled bi-metal spring caused by P.O. attempt to make choke work without
clearing plugged port.
Missing/loose mounting screws causing fuel or vacuum leaks.
Gummy, sticking plunger.
Wear notches on the cam or metering needle slot causing sticking/bind.
The original poster asked a few other questions. Availability: None, nada,
go fish. No longer made. Not in stock. Not even used parts. They *all* got
destroyed. Manual chokes: The manual choke pieces listed for pre 76 (?) Spits.
will not fit the autochoke carb body. There are many differences in the
body machining and throttle shafts. Some subtle, some not. If you want to go
this route, get a complete carb from a early 1500. As an alternative, there
are several manual conversions for the autochoke available. These replace
the bi-metal spring and heater housing with a mechanism to attach a choke
cable. They do work sorta. Every one I've seen was quite flimsy, and didn't
do a good job of supplying a linear control at the cable. Translated, this
means they don't feel right, and are touchy about getting the setting just
right for the engine temp of the moment. however, since parts for the
autochoke are NLA, you sometimes do what you have to do. If the bi-metal
is shot, you go for a manual conversion. But if the fast idle plunger is a
glob of melted plastic, the conversion won't help.
Randy
randy@taylor.wyvern.com
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