Spark advance control mechanism for dual fuel engine

A spark advance control mechanism for an engine capable of being selectively operated on a first fuel and on a second fuel, said mechanism comprising an air induction passage having therein a movable throttle plate and communicating with the engine, a member electrically connected to the engine for timing of the sparking thereof and movable between a first position providing retarded sparking and a second position providing advanced sparking, means biasing said member for movement from said second position toward said first position, a vacuum actuated motor operable to displace said member from said first position toward said second position in response to application to said vacuum motor of an increasing amount of vacuum, a vacuum chamber, first conduit means communicating with said air induction passage downstream of such throttle plate for providing a vacuum in said vacuum chamber in response to engine operation and including check valve means affording fluid flow from said vacuum chamber to said manifold and preventing fluid flow from said manifold to said vacuum chamber, second conduit means communicating between said vacuum chamber and said vacuum motor and including first control valve means movable between a normally closed position and an open position, third conduit means communicating between said air induction passage and said vacuum motor and including second control valve means movable between a normally open position and a closed position, and operator controlled means connected to said first and second control valve means for opening said first control valve means and closing said second control valve means when the engine is operating on gaseous fuel, and for permitting closing of said first valve means and opening of said second valve means when the engine is operating on liquid fuel.