Seeking Validation On Jetting Change

[Guest Anonymous]

I think the confusion lies with your use of the term adjustment.

Adjustment can be read as a turn of the screw, a twist of some linkage, a bend of a float tang or even a change of jet. Since we can agree that an idle jet change can effect much of the engine operation, it stands to reason that further fine tuning of the mixture screws after changing the jet also effects a broad range of engine operation.

A turn of the idle screws, by themselves (mit out jetting change) will not do much except for idle or very low speed operation (barely opened throttle).

[Chris_B]

I think the confusion lies with your use of the term adjustment.

Adjustment can be read as a turn of the screw, a twist of some linkage, a bend of a float tang or even a change of jet. Since we can agree that an idle jet change can effect much of the engine operation, it stands to reason that further fine tuning of the mixture screws after changing the jet also effects a broad range of engine operation.

A turn of the idle screws, by themselves (mit out jetting change) will not do much except for idle or very low speed operation (barely opened throttle).

Except that the original poster didn't say that he had twisted linkage, bent float tangs or changed his idle jets.

You seem to be saying two conflicting things: (i) "...it stands to reason that further fine tuning of the mixture screws after changing the jet also effects a broad range of engine operation," and (ii) "A turn of the idle screws, by themselves (mit out jetting change) will not do much except for idle or very low speed operation (barely opened throttle)."

I agree with your second statement, but not the first. "Fine tuning of the mixture screws," whether before or after changing the idle jets, has no measurable effect on mid- to high speed air fuel ratio (or engine operation). If you look at the anatomy of the idle/transition circuit, the idle mixture screws/holes are up stream from the transition holes. Very small throttle openings will move the throttle plates into the transition area and well past the idle mixture holes, reducing the vacuum in that area (and flow from those holes) to almost nothing.

Going back to the original poster's question: it is possible to introduce (or remove) a stumble in the off-idle throttle range by positioning the throttle plates (at idle) closer or farther from the first transition holes with the idle speed (throttle plate positioning) screws. Adjusting the idle speed screws certainly is part of the "adjusting the idle" process.