Message text written by "Anthony Rhodes"
>Dave, I respectfully disagree with you about the lift-the-piston technique
for ZS carbs. I believe that the technique WILL work. The carbs do not
know what sort of profile is on the needle. Additionally, as far as the
needle goes, lifting the piston should enrich the mixture, but the huge
"air leak" caused by lifting the piston will create significant leaning of
the mixture.
<
As Randall said, you can (theoretically) adjust the Z-S carbs to giv the
desired respond to piston lift but it will put the mixture over the rest of
the range way rich. And that is the important part. You are right about
the carb does not know what sort of profile the needle has but the point is
the older SU needles are profiled to act this way (lift-the-piston) when
properly adjusted for running conditions. This is the result of some very
clever carburettor designers setting things up so that one could adjust the
carbs with nothing more than a few spanners and screwdrivers. Due to
emissions considerations this technique could not be employed after 1968
and carb adjustment should be done with the aid of a CO meter.
It is easy to tell on which carbs this technique can be used. The ones
with the piston lifting pins can be adjusted this way. BTW, this technique
only works if you lift the piston the prescribed amount, as you pointed
out, and the pin does just this. If there is no pin the method can yield
misleading results due to too much piston lift.
Cheers
Dave
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