My SU's had a similar problem last week or three ago
when the cold weather set in. I bet you even have to
tug on the choke a good deal on chilly mornings, don't
you?
There is a bi-metal metering plate built into the
integral fuel reservoir of your ZS's (just like my SU
HIF4's). The bi-metal plate is connected to the jet
and flexes when hot (in summer) to lean out the jets
when air temps are warm. If you're running headers
under the carbs, it's even more exhaserbated (sp).
My hypothesis is that you've actually been running
rich all summer (if you've checked your plugs, they
were probably just a hint of black), but with the
onset of colder temps the bi-metal plates have now
done you the favor of making your carbs run super rich
so you don't have to tug on the choke on chilly
mornings. But evidently we either get a whole lot
colder than the UK (doubtful) or when tuning, you did
it on a hot day, and the bi-metal plates pushed the
jet all the way up against the piston. Now that it's
cold, the jet is now recessed. That's what has
happened to me.
But why does it start when you pull the choke?
When you pull the choke to get it started, even though
you're letting in even more fuel, you're also opening
up the butterflies so that enough air is now getting
in to start the car. Without the choke or pressing on
the accelerator, you've probably found that you just
flood the engine.
I turned my SU's back two flats each, and it's running
well without black smoke on start-up, and with only a
hint of a tug on the choke for about 30 seconds at
start-up first thing in the morning. But I'm guessing
it's still a tad on the rich side.
To adjust, you need an SU adjustment tool. DO NOT USE
A N ALLEN WRENCH. You need the collar to keep the
diaphram from spinning and tearing. And I suggest that
instead of just "playing" with the needle setting, you
follow the instructions on balancing and tuning as
it's outlined in several on-line sources or your
Haynes.
Unless you're very experienced with "playing" with
your carbs, you will do more damage than if you just
took the time to start from base and adjust your carbs
correctly.
-Terry
'76 Spit 1500
http://www.firespitter.com
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