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Is this an original production Marvel-Schebler or one of the more recent reproduction carbs? There is, in my opinion, a design defect in the reproduction carbs. If original Marvel, is it a TSX-33 or a TSX 241A, 241B, or 241C? You must get the RPM slow enough that the main metering jet is no longer contributing most of the fuel. Otherwise the idle screw won't do anything. Are you tuning it at below 800 RPM or thereabouts? What is the slowest RPM you can get the engine to run? Does the engine run OK at speed? If the engine won't run smoothly at low RPM, and the idle screw has no effect, the usual reason is that the engine is too lean already, even with no additional air introduced via the idle air bleed. As the main metering system drops out due to the closed throttle blade, there is either too much air (coming from somewhere) or not enough fuel coming from the idle system, and the engine runs roughly. You can compensate for this somewhat, at mid-RPM ranges, by enrichening the main jet.
You can confirm this by applying the choke part way, or by waving an unlit propane or acetlene torch at the air cleaner inlet (or at the mouth of the carb if the air cleaner is disconnected).
If the engine smooths out, it is running too lean. Common causes for it being lean are vaccuum leaks at the carb to manifold junction, manifold to inlet port junction, or a crack within the manifold. You can check this out by waving that same torch around each of these places, or by spraying WD40. If the idle improves, there is your air leak. Another common cause is a poor gasket seal at the idle fuel passage between the case halves of the carb. See Zane's frequent post on lapping the case halves. If it is a reproduction carb, or TSX-241 carb with economizer, or has a plastic venturi, try opening the carb drain plug as the engine is running slow and rough (be careful about where the spilling gas goes). If the idle improves, there is a problem with the carb bowl venting inside the carb. See if the flow of gas noticeably improves when you shut off the engine -- a partial vaccuum in the float bowl at idle will starve the idle system for fuel. Poor low speed idle appears to be a common complaint with this family of carbs. One of Zane's past posts comments to the effect "This doesn't really affect the operation of the tractor in any way..." But it is still, to me, somewhat aggravating that some guys can get their carbs to idle, and I can't.
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