Dual fuel (aka bi-fuel) propane carburetion conversions were commonly done for carbureted and throttle body injection (TBI) engines.  All propane (liquefied petroleum gas, LP Gas, LPG) injection conversions are dual fuel because these systems incorporate the original gasoline injection system in their operation.

A mono-fuel propane carburetor consists of a propane mixer mated to a throttle body, which the part of a gasoline carburetor that contains the throttle (butterfly) valves.  In a dual fuel system, a dual fuel propane mixer was attached to the airhorn of a gasoline carburetor.  The fuel used by the engine was controlled by a pair of solenoid valves for each fuel.  When gasoline operation was desired, the gasoline solenoid valve opened and the propane solenoid valve closed.  Conversely, when propane operation was desired, the gasoline solenoid valve closed and the propane solenoid valve opened.

The control of these solenoid valves was controlled by the driver with a 3-position switch (gasoline / OFF / propane).  The off-position was necessary to allow the gasoline fuel bowls to empty.  Once the engine started to stumble from a lack of gasoline, the switch would then be moved to the propane position.  When switching from propane to gasoline, the switch would be immediately put in the gasoline position so as to fill the fuel bowls while the propane system bled off pressure.

A key part of the dual fuel system is the adapter that mates the dual fuel mixer to the gasoline carburetor. As this conversion has become increasingly uncommon since the introduction of multiport fuel injection, dual fuel mixer adapters have become increasing unavailable through their obsolescence.  If not available through used sources, suitable adapters would have to be fabricated, thereby making such dual fuel conversions impractical and uneconomic. Adapters are still available for a limited number of applications through Raso Enterprises.

Another important aspect of a dual fuel conversion is fuel-specific ignition timing.  Autotronic Controls Corporation (maker of MSD ignition systems) used to have its Dual Curve division, which manufactured ignition and fuel control controllers for alternative fuel systems.  Autotronic Controls shut down their Dual Curve division at the engine of 2008, which made Impco mixer-based dual fuel systems for fuel-injected engines obsolete.  Timing advance and feedback processors are still available for non-computer controlled engines through Raso Enterprises.

Impco still makes some dual fuel mixers but the selection has become much smaller.  While a many mixers can be made to work on a particular engine, the main consideration is starting.  Gas-valve mixers need a minimum air flow to lift the gas valve off of its seat in the mixer and approximately 6" wc (water column, ~0.2 psi) of internal mixer vaccum is required to lift the air valve off its seat. The cranking RPM of an engine can be as low as 200-300 RPM, which is about half of idle RPM.  An oversized mixer may be very difficult to start and a primer or start assist valve may be required to provide enough fuel to start.

Undersizing the mixer improves starting and low-speed operation.  However the gas valve has a limited amount of travel and reaches its full-open position when internal vacuum reaches approximately 13.8" wc (~0.5 psi) and this corresponds to the rated flow of the mixer.  Since the gas valve cannot lift any higher, higher than rated flows mean that the fuel mixtures become increasingly leaner with a corresponding gradual reduction in power.

Impco only publishes the rated flows of their mixers at a specified manifold depression.  The manifold depression is the resulting manifold vacuum from the pressure drop across the mixer at full throttle, which automotive applications using 1.5" Hg and industrial applications using 2" Hg.  A variety of mixers can often be used on a particular engine size, it is the maximum RPM seen by the engine in normal operation that should be used in sizing the mixer.  Although some engines can have reline of 5000+ RPM, this speed is likely rarely attained in normal operation.  For automatic transmission vehicles, a reasonable maximum engine speed is the transmission's full throttle shift point.

The recommendations in this article are based on a maximum speed of 4200 RPM and a minimum idle speed of 425 RPM.

The Model 425 mixer is typically used on larger V8 engines in the range of 305 to 445 CID.  Engines larger than 445 RPM would have lower maximum engine speeds.

Since the Model 425 mixer is 6.25" tall, it is normally used in dual fuel applications turned 90° sideways.  Commonly used adapters for carburetors with 5-1/8" air horns are the AA2-40 (90º x 4¾" tall) and the low-profile AA2-46 Air Horn Adapter (90º x 3½" tall).

Using the Model 425 mixer in this orientation leads to premature wear on the gas valve.  This mixer was designed for vertical use and the gas valve's orifice will tend to wear egg-shaped over time when used on its side.

This is dual fuel system using a Model 425 mixer and a AA2-40 adapter on a TBI engine.  This type of system requires an propane system system controller to manage timing advance, fuel mixture control, and gasoline injector simulators.  Without an integrated controller, the engine management system would generate check engine faults. A better alternative is to use an propane injection conversion with the propane injectors controlled by the vehicle's PCM.



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
348 CFM
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.

The next smallest dual fuel mixer for automotive applications is the CA125M-10, which was suitable for engine in the range of 134 to 196 CID.

The CA125M-10 similar to the CA125M mono-fuel mixer except that it has been modified with a vacuum lift for the gas valve.  A CA125M-10 mixer can be fitted to the air horn of a carburetor with the now obsolete AA3-4 adapter, which mated to CA300A adapters.  However, a machinist could easily build an adapter like the AA3-4. The CA125M-10 mixer can also be fitted to a gasoline carburetor's air horn with the low-profile Unidapt system using the A1-53 adapter.





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