Help!
I've run into something with my 1977 MGB that has me baffled. I fixed
a short that was grounding the positive lead to the ignition coil (was
shorting in two different places intermitantly---really fun to track
down! The tow truck man made a tidy little sum from this.) After fixing
the first short, the engine started idling very fast (3000 rpm).
The car has a single Zenith-Stromberg carb which has never worked
properly. I turned the idle screws all the way out and still it idles
at 3000 rpm. It starts at 1500-2000 rpm when the engine is cold and
slowly increases to 3000 rpm as the engine warms. It tends to wander
between 2500 and 3100 rpm while driving around when the engine has
reached tempurature. The air idle screw has no effect (it never has).
All internal passages in the carb are unblocked. I've completely
rebuilt it more than once: new gaskets, new diaphragm, new needle
valve, new needle. I'm sure the float floats. The vacuum and oil
pressure haven't changed from what they were before the idle went
crazy (I have gauges on both.) Running it with the air cleaner off
I observed that lifting the piston caused the speed to decrease and
the engine to run roughly. Pushing the piston down caused the speed
to increase. One question I have is, what makes this piston move up
and down? Does air rushing past the holes in the bottom of the piston
create a vacuum above the diaphragm, thus raising the piston so that the
more air the engine sucks the higher the piston is raised?
When idling the butterfly valve is completely closed and the choke is in
the fully off position (manual choke). I don't know what the little
spring loaded door in the butterfly valve is doing at idle since
I can't see it. Disconnecting the line to the EGR valve has no effect
on idle speed. Disconnecting the hose just above the gas line to the
carb has no effect on idle speed. Changing the setting of the needle
has no effect on idle speed (tried both raising it and lowering it.) The
timing is advanced pretty far beyond the recommended setting (it's
set to about 25 degrees) as this seemed to be the only way to get
some power out of the engine. What could have suddenly changed to
make the idle jump up in this way? It seems unlikely that fixing the
short to the coil would affect idle speed.
The ideal solution would be to replace the &$^*! thing but I can't
afford the $500+ for a replacement or the $600+ for a Weber plus new
manifold and exhaust header.
If nothing else, are there any good books that someone could recommend
on Zenith-Stromberg carbs? The book that Moss Motors sells seems to
only cover up to 1976.
And still it idles at 3000 rpm.
ron@vicorp.com or uunet!vicorp!ron
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