Hi Bill Eastman!!
On Thu, 12 Jun 1997, Bill Eastman wrote:
> It has been my experience (5 SU's, 1ZS) that LBC's run rich at idle. It
> seems that if I get the idle just right, then the car runs lean under
> idle. I don't know what characteristic of the constant depression carb (an
> apt name at times given the amount of emotional distress they can cause)
Bill, if you're running lean at speed, you've got the wrong NEEDLE.
Just as with a Holley, or a Carter, you CAN adjust the idle, mid-range,
and high speed circuit. You adjust the Holley by changing the JETS,
while in the Carter, and the Rochester you change the metering rods.
The SU uses different NEEDLES to change the fuel delivery curve, and
uses the internal spring (inside the piston housing) to act as the
Holley's "power valve". The SU's accellerator pump is the oil inside
the dashpot assy.
> I don't know what causes it but I do have some theories. I don't like to
> drive with the choke out. The A idles too fast and it just doesn't feel
If it idles too fast, your tuning is wrong... there IS a high speed
(choke) idle adjustment on the carb linkage.
> right. Because of this, I tune the car for a slightly richer mixture so
> that I can push in the choke faster on a cold start.
Not only would you fail our AirCare test, you're diluting your oil with
unburnt fuel. Very hard on the top piston ring as the fuel tends to
wash down the cylinder walls. Remember that when you hit the throttle,
the mixture richens automatically while you're accelerating.
> Second theory- Lots of us run 160 F thermostats. These are really too
> cold as the car need to run richer at that temperature than when fully
When you start your engine up cold, it has a certain amount of fuel, and
water condensate that dilutes the oil. The oil in these engines tends
to run about 30 degrees hotter than the water temp in the block, so
you're trying to boil off the water contaminant at only (160 + 30) about
190 degrees... it doesn't happen. Note that when using a 180
thermostat, you end up with a oil sump temp of about 210 degrees, on a
STOCK MKI "A". My MkII came standard with an oil cooler.
> Theory three. Most of us tune our cars in the Spring when it is colder.
> Either that or we tune them as soon as the water temperature reaches 160.
> This doesn't allow the heat to soak the intake manifold and engine
> compartment so, even though the combustion chambers are near temperature,
> the air charge is still relatively cool and dense so more fuel is needes.
Yup, you're right. You can't do a proper tune-up unless the engine is
thoroughly warmed up to running temp. You'll never get the carbs right.
TTUL8r, Kirk Cowen (who hasn't seen a proper thermostat for the "A"
since the early '70's... that's the one with the
blanking ring on it)
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