[38 coupe]

I chased an identical sounding problem recently on a 1953 Mercury. In that situation I rebuilt the carburetor. The car idled great, but ran like crap and would die at anything past fast idle. Our problem turned out to be the ignition timing way too far advanced, something like 30 degrees at idle.

The main jets and main wells are the only parts of the supply common to both the idle and main fuel circuits in the carburetor. There are fuel to air mixture parts in the main fuel circuits that have to be clean or nothing works properly (the "main well tube" and the "aspirating nozzle"). Also, the carburetor top has air metering holes that line up over all this stuff. These holes have to be clean. These can be seen in the "main metering system" diagram in the manual.

One other thing to note: the carburetor is a Holley 1901. The 9510 is Ford's part number, not the true carburetor identification from Holley.