Dave, After some thought, now I agree with you. After I replied to your
message I saw Randall's. I then gave more thought to the issue (better late
than never). I realized that I did not KNOW the effect of any potential idle
air passages and whatnot on the emissions ZS carbs. I also dd not KNOW what
the CO reading is when set the old way, and I did not KNOW how this compared to
the setting when done the "new" way. I am not positive that the needle
profile was not changed in the idle position to alter the idle characteristics
which was not represented further along the profile. (I.E. the profile might
be set to be lean at idle, and if set to "correct the old way" at idle then the
rest would be too rich). (I still would have thought that setting the
emissions carbs the old fashioned way would be no worse for CO AT IDLE than the
same technique for the old carbs. I guess it is just not what the rest of the
profile was designed for, and the rest of the mixture will not be ideal)
I still think that I deserve the beers because I did give the correct ultimate
solution. Dump the crappy old emissions ZS carbs!!!!!!!
-Tony
-------------- Original message --------------
> 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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