Thought I'd share the benefit of some recent experience:
One day the engine on my 1977 MGB decided to start idling at 3000 rpm.
Made me feel kind of conspicuous at stop lights. When started it would
run at 1500-2000 rpm and gradually increase to 3000+ rpm as it warmed.
The carb is a Zenith Stromberg CD175 on which both idle adjustment screws
were all the way out and the air idle screw had no effect (it never has).
I'd rebuilt the carb twice and fixed all manner of problems with the
rest of the engine, to the point where I was certain that the rest
of the engine was in reasonable shape. I was sure it was the carb
causing the trouble so I bought a book and learned how the thing
works. From this I learned that, how well an engine idles is a good
indicator of the condition of the carb. I started at one end of the
carbs throat and worked my way to the other end, figuring out what
each port and orifice did and how it affected performance and inspecting
each carefully. I finally got to the engine side of the carb and
discovered that when looking down the throat I could see light around
the edges of the butterfly valve when it was fully closed. The throat
and edges of the butterfly valve had worn away enough to let some gas/air
through. Still, the gaps were fairly small. Then I noticed the
spring loaded valve set in the butterfly valve. I decided to try
permenantly closing this valve to see what effect it had. I tried
soldering it but found that solder wouldn't stick to it. So I wrapped
many turns of copper wire around the spring and soldered them together,
thus preventing the valve from opening. I then proceeded to reassemble
the carb. In the midst of reassembly I happened to look at the
butterfly valve again. It was now open a crack! I fiddled with it
a bit trying to get it to close and then discovered that when I'd
put the choke back on it had moved an idle control lever out a little,,
thus opening the butterfly valve. There was no way to adjust for
this so what I did was bend the control lever back a little so that
the butterfly valve would close all the way. I put the carb back
on the engine, started it and viola! it idled normally! Later when
working on the exhaust system I had to remove the carb again so I
decided to try and isolate the problem further. I removed the wire
I had soldered on the valve inset in the butterfly valve and tried
running it. Once again the engine idled at 3000 rpm. I wired the
valve closed again and the idle returned to normal. So if you ever
experience high idle problems, this valve is a likely suspect.
Apparently the spring gets weak after years of use (15 years in this
case---most of the car was original when I bought it; much to my later
regret!)
So even though I have the engine running reasonable well, I think
the carb is essentially shot (the idle screws are both almost maxed
out and the car backfires a lot.) Anyone have a good deal on a
Zenith Stromberg CD175 in good condition for a '77 MGB? If so,
please contact me. I've checked the local junkyards and haven't come
up with one yet; perhaps you could make a few dollars off that carb
sitting on your local yards shelf.
ron@vicorp.com or uunet!vicorp!ron
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