I have been following along with your idle dilemma. As mentioned previously high idle can be a result of several conditions signally or in concert such as vacuum leak conditions, sticking carburetor parts, air/fuel adjustments, and incorrect timing. It seems as though you have investigated mechanical and vacuum plumbing problems listed above, except timing controls.

Back to square one; checking everything:
As Reed instructed, base or static timing must be set before other adjustments are attempted, but before base timing is set one has to determine if timing mark on vibration damper (pulley hanging off of crank shaft) has not spun out of position from failed rubber innards by using a piston stop to find TDC and making sure mark on pulley lines up with zero on timing tab.


One easily checked condition involving distributor adding additional timing even with weights freely moving is addition of vacuum advance because of vacuum can’s incorrect vacuum source at carburetor. Check to see if its vacuum line is correctly plugged into a tap on carburetor that is located at an elevation above throttle plate that only sees vacuum when throttle plate is open, and none when closed. If its vacuum line is attached to a vacuum tap at base of carburetor, vacuum can is seeing high vacuum as soon as engine lights off activating vacuum advance can which runs up timing 6 to 15 or so degrees depending on unit over base setting.


Next check freedom of movement of centripetal advance mechanism or mechanical advance weights housed within distributor. If these weights won’t fully retract to resting position, that is, back to base timing setting, either from stuck weights or broken retractor springs, engine idle will never settle down because now you have a condition of base timing plus any advance degrees mechanical advance dialed in. To correct problem disassemble distributer clean and lubricate weight’s pivots.


Once you are sure all timing devices and reference marks are in order, move back to carburetor. Remove carburetor, make sure base of carburetor is dead flat with a known straight edge; if cupped use a #10 milbasterd flat file to dress base flat and remove filings.


While carburetor is off engine closely inspect it for throttle plate sticking in bore, and lose throttle shaft. If throttle plate is sticking, post again to us about it before reinstalling carburetor. If carburetor checks out, install it using a new carb to manifold gasket. Don’t over torque carburetor mounting nuts, or base will be cupped again. Check operation of carburetor’s linkages for binding, and correct any found.

with all parts back in place, rough set timing, crank up engine, and go about making adjustments; timing first, idle speed, air/fuel. Once static timing is set, it is a round robin back and forth dance tweaking A/F mix, idle speed. Remember idle speed is also controlled by timing advance, and you may have to revisit base timing setting, as a degree or two change can add or subtract up to several hundred rpm.