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MG MGB Technical - John Twist on ZS mixture adjustment

In the newer Moss catalogs, there is an article on the page that shows parts for the Zenith Stromberg carb.. The article states: Check the ZS for lean or rich mixture, by lifting the air valve ...... if too lean, RPM drops and almost stalls; and if too rich, the RPM should increase.

My question is: If the air cleaner is removed from the carb., the carb. will get more air, so it will always be leaner, (when compared to when the sucking air through the air filter), correct? So how can this check be accomplished correctly?

I have a 77B and am in the process of correcting some air leaks around manifold, (base of EGR valve, etc., etc., etc..) I also found the check valve for the air rails manifold was bad. When I finsih this, I want to use John Twist's method to determine if my mixture is in the ballpark, before hooking up the smog tester. But I am baffled as to how this mixture check will work, when the air intake has to be altered in order to perform the test.

Any ideas?

Thanks,
Greg
Greg Knodel

If you have a gunson's gastester in your
garage, or you're just a short drive from the smog station,
just adjust the needle so that the bottom of the needle
holder is flush with the bottom of the air piston.
This will get you close enough to drive a little ways.

I found that the lift the piston test doesn't work well -- the
Zenith is normally set up lean anyway, and raising the
air piston a sixteenth of an inch or so always causes the
idle to drop a bit.

Ronald

This thread was discussed on 17/07/2002

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