John,
A 1401 is a direct replacement for a Qaudra-Jet/Spreadbore.
The 1405 & 1406 are 600cfm square bore type.
If the idle mixture screws aren't working it could be an issue with the idle speed being to high or border-line. Once your throttle plates open to a certian point the idle circuit becomes inactive. Then the main jets take over. It could also be an issue with the main metering rods opening to soon or sticking, which would be allowing fuel to bleed over while at idle. At idle both metering rods should be down all the way, as the carb loses vacuum to metering rod piston it should start lifting the metering rods to allow fuel to flow through the main jets.
You can see the metering rods on top of the carb airhorn if you take off your aircleaner.

Mark D.

1968 W200 Sweptline
1969 D100 Utiline
1971 D100 SE Sweptline
1993 D250 Club Cab Sweptline
1986 W250 Cab & Chassis
1978 D300 Sweptline

E. Brookfield, Massachusetts