Steve wrote:
>
> Here's my setup: I have a '77 MGB with a Lumenition kit installed.
> I replaced the stock ZS carb with a Weber 32/36 DGV with manual choke.
> I'm still running with vacuum advance (at least I think since I connected
> the vacuum line to the Weber via the *switch* located near the
> master cylinder). It connects to the Weber on the engine side of
> the carb by way of a small 1/4'' tube which protrudes from the
> carb body. Is this indeed the correct connection? I ask because
> I don't see much difference while driving whether I have the vacuum
> line connected or not. I know the line from the distributor to the
> switch is clear, and the line from switch to carb is clear. I
> believe the switch is ok also.
Okay, a few things here. First off, make sure you have the vacuum lines
on the switch in the correct order. The switch is not symetrical. The
nipple that points up goes to the intake manifold. The one the points
foward goes to the distributor. Very non-intuitive. Second, do not use
the vacuum port on the Weber. This port is controlled by the throttle
plate, and is designed to work with Ford's version of advance/retard
cans. The MGB distributor is designed to work off of intake manifold
vacuum. You need to drill and tap a hole in the manifold, and install
the vacuum fitting from the original manifold. If this is not possible,
leave the vaccum advance can unplugged. If it is possible, you will
want to bypass the vacuum switch above. It's been "laid off" due to
the carb change anyway.
>
> The original poster mentioned setting the timing very different
> from stock. I'd like to know what that setting is, and if my
> configuration is ok. The car runs much better now with the Weber,
Assuming stock everything else (cam, pistons, distributor advance curve),
start with 15 degrees before, vacuum off, at 800 rpm idle. You may have
to retard it a bit if your Weber has a particularly nasty lean spot in
it's fuel curve.
> but I have some concerns since the idle seems a bit rough (especially
> when cold), and initial pickup - when first stepping on the accelerator
> with some authority - leads to a bit of hestitation. I realize some of
> these symptoms may be due to the Weber itself, but I'd like to be
> able to rule out any timing problems.
Welcome to the world of MG's with DGV. The idle can be cleaned up with some
jetting work. The flat spot when you bang the throttle is tougher. There
are several problems, with both the carb and the manifold, that combine
to make this bog happen.
First off, the manifold's water heating is a joke. This is made worse on
most installations by being hooked up wrong, or not at all. The token
heating lug under the manifold needs to be plumbed into the lines that
used to go to the ZS' auto choke. You do not want it plumbed into the
heater return hose as water flows through this only when the heater is
on. The installation requires the original metal heater pipe, fabricating
a support bracket for it, and some adapters to hook the 3/8 choke lines
to the 1/2 manifold nipples. It's a class A kludge, but is far better than
doing without, especially when the engine is cold.
The other problem that's fairly easy to help is the accel pump. The carb
as supplied is typicly jetted for a German Ford V6 (see dizzy vacuum above),
which wants it's pump boost in a slow, *long* stream. The BMC engines want
the pump boost all at once.. right now... just dump it, thankyou. I pull
the pump squirter, and bore the .35mm jet out to .70mm. Note that fuel flow
is proportional to the area of the jet hole, not the diameter. So this doesn't
double the flow, it quadrupals it.
If you bought the Weber from an honest, reputable, British parts supplier,
it may have had this mod done. Since I've never installed on of these things,
I don't know who, if anyone, rejets the carb before shipping it out. I've
never installed one (it's illegal for a pro mechanic to do such things in
this state), but I've had to sort out plenty of them. :<
You can do a quick check to see if this accel pump mod has been done. Take
the air filter off, and with the choke open (and engine *off* :), jab the
throttle wide open while watching the pump squirter. If you see a small
stream come out, and it keeps pee'ing for several seconds after you hit
full throttle, you need to bore the jet out. It should just go "goosh" and
be over.
Randy
randy@taylor.wyvern.com
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