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.

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