> 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.
Tony, I disagree with your disagreement. Lifting the piston slightly leans
the mixture, due to lowering the air velocity through the venturi (which
weakens the Bernoulli effect and so draws less fuel through the jet). On
pre-emissions carbs, the idle mixture is very slightly rich (of optimum
power/max idle rpm), so lifting the pistons causes the idle rpm to first
rise (as the leaner mixture is mixed with the richer mixture already in the
manifold, hitting optimum mixture) and then fall back as the leaner mixture
reaches the cylinders.
However, 'emissions' carbs are tuned with the idle mixture already well on
the lean side (down several hundred rpm from 'ideal'; this lowers CO
emissions), and so raising the piston always causes the idle rpm to fall
further.
This has nothing to do with ZS vs SU carbs, except that Triumph chose to use
ZS carbs for their 'emissions' models.
Randall
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