In the 60s, many cars had idle stops that dropped the idle below a setting that would allow an engine to run.

When I tuned my brother-in law's Oldsmobile Cutlass, the idle stop was a solenoid that was about 1 inch in diameter and about 1-1/2 inches long with what looked like a 1/2 inch bolt protruding out the end,

With the power on, you adjusted the idle, and when the power was turned off, the bolt retracted at least 1/4 of an inch, helping the engine to die.

If you cannot find a happy medium, you should check some 60s cars for their throttle stop set-ups.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks