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
power. If I get the plugs to read correctly under power, the car lopes at
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)
that make this happen. Maybe they are better when new but I remember a
friend of mine many years back who complained that he still couldn't get
his car to idle hot after spending a lot of money on rebuilt H4's. (when he
told me that his A had wood floorboards I told him that we had a machine
shop and that I would be happy to help him weld a new set in to fix that)
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
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.
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
warm. Since these cars heat up at idle, I expect that they exceed the rich
limit for the air density at higher temperatures when adjusted to run
slightly rich (see theory 1) at a lower temp.
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.
Theory four. The people at SU, etc. thought that a lumpy idle made the
cars sound tougher so they left them rich.
I inspected the A's rear end last night. The noise is definately coming
from the wheel bearing / brake area. The bearings are fairly tight still
and turn very smoothly so I think I will be OK for a while. My Spit**re
made these noises for two years before I sold it. I probably won't
autocross it again until it is fixed although you never know.
Regards,
Bill Eastman
61 MGA that runs rich at idle and sounds like it has a big cam. It hasn't
died yet though. (the other night my daughter asked- Dad, is did it run
like this when it was new?)
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