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SU lean/rich confussion

Subject: SU lean/rich confussion
Date: Thu, 21 Dec 1995 12:00:54 -0500
My recently aquired '57 MGA is powered by an early (18G, 3 main bearing) B
engine.  I've been trying, so far unsuccessfuly, to get a decent idle at
somewhere under 1000RPM (workshop manual says 500).  There is a very
irregular misfire at any idle speed, but it is worse as I reduce the idle.
 It shakes violently and threatens to die at anything less than about 900RPM
and 500RPM is out of the question.

I posted a question about this recently and many opinions were in favor of
leaky throttle spindles.  I have overhauled the SU's and replaced the worn
throttle spindles.  I haven't had the bodies rebushed yet and I'm sure there
is some leakage as the engine stumbles when I spray carb cleaner around the

Since the spidles are leaking, I reasoned (a risky move) that an excessively
lean mixture must be the problem.  Rather than get the carb bodies rebushed
right away I bought a Colortune Kit in order to see for myself what the fire
looked like in there.

Using the colortune I find that I have an excessively rich (yellow combustion
color) mixture in all cylinders which hardly changes at all with jet
adjustment.  I can even have the jets cranked all the way up flush with the
base and the colortune still shows very rich combustion.  I've checked and
doublechecked the usual things:  needle shoulders are flush with the piston
surface, jets are centered, pistions don't stick, float chambers are not
flooding, float needles are new, and I'm pretty sure I have the float levels
set correctly at about 1/8th inch.

Other things like timing, point gap/dwell, valve lash, are adjusted and seem
to be fine.

How can I have a funky idle with air leaks and an uncontrolably rich mixture
at the same time!?!  The DPO seems to have been satisfied with a very erratic
1400-1800RPM idle, since it runs great as long as you're accelerating.  I'm
beggining to wish I was satisfied with it also, as it's ruining my holiday
spirit!  Any help would be greatly appreciated.

Keith (all I want for xmas is a decent idle) Baer

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