Many dual fuel propane systems used the Impco CA300A mixer, which was specially designed for dual fuel operation.  This mixer was designed with the capability to lift the mixer's gas valve to the fully raised position during gasoline operation so as to minimize the flow restriction to the gasoline carburetor.  The gas valve was lifted with a "Boden" cable operated by the driver.  The control panel for a CA300A-based dual fuel system included the Boden cable (PN C2-32) beside the fuel control switch and it was pulled out for gasoline operation and pushed in for propane operation.  It was also possible to use a vacuum lift for the gas valve:

The Model CA300A mixer is still available in 2 sizes (flow ratings) and for each size, there a a few options. Previously, each of the two sizes was also available with a setup for automatic switch over between the 2 fuels with the operation of the Boden cable.  This automatic setup added a microswitch and a vacuum valve.  However, the automatic version of the Model CA300A is now obsolete but repair kits are still available.

Flow Basic Automatic (Obsolete)
348 CFM @ 1.5” Hg CA300A-1 CA300A-20
 432 CFM @1.5” Hg CA300A-50 CA300A-70

In Impco's part numbering scheme, the CA300-1 is appears without the "1".

 

Gas Valve Diaphragm Boden Cable
Included
Cam
Included
CA300-1
348 CFM
CA300-50
432 CFM
Standard Silicone Yes Yes CA300A-M-2 CA300AM-50-2
Standard Hydrin No Yes CA300A-M-3 (N/A)
Standard Silicone No Yes CA300A-M-3-2 CA300AM-50-3
Standard Silicone No No CA300A-M-3-2 CA300AM-50-4
Feedback Silicone Yes Yes CA300AM-50-4 FB300AM-50-2
Feedback Silicone No No FB300A-M-4-2 FB300AM-50-4

The CA300A-1 mixer is suitable for engines from 231 to 337 CID, while the CA300A-50 is suitable engines from 287 to 418 CID. The CA300A mixer without a cam (PN AC2-2) would be used in mono-fuel applications when mated to a throttle body such as the Holley 4bbl square body throttle body with adapter PN A3-96.

The CA300A mixer has an overall height of 4-1/8".  Adapters that mount the CA300A mixer on top of a gasoline carburetor add more height.  The A1-16-2 adapter (300A to 5-1/8" air horns) is 2-7/8" high while the A1-50-2 offset adapter for 5-1/8" air horns adds 1½".  For low-headroom applications, Impco has the UNIDAPT inverted mixer adapters that would add 1¾" to the overall height of the 4bbl gasoline carburetor.

A smaller version of the Impco CA300 mixer was the CA175 mixer, which is now obsolete. It was suitable for engines in the range of 193 to 203 CID. It had a vacuum-operated lift for gas valve that eliminated the need for a Boden cable.  CA175A used the same adapters as the CA300A.

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