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IMPCO carburetors use a flexible diaphragm made of fiberglass and synthetic or silicone rubber, except the Model 50 which uses a piston and O-ring to reduce size for small engines. Both work in the same manner however. Cranking or running the engine produces a vacuum in the throttle body. Through passages in the air valve (P), the pressure on the top side of the diaphragm (D) is lowered; atmospheric pressure on the lower side of the diaphragm pushes upward against the metering spring (S).
At approximately 0.2PSI (6" WC), the air valve lifts off its seat. At approximately 0.5PSI (13.8" WC) the air valve is in the fully-opened position.
The lowered pressure on the top of the diahragm varies with engine speed and throttle position (T). The air valve therefore measures the air flow through the engine by responding exactly to engine speed and throttle position.
The controlled pressure drop of 0.2 to 0.5PSI setup by the metering spring provides the signal necessary to drawn fuel into the air stream within the mixer. The gas metering valve (V) is mounted on the bottom of the diaphragm, and is shaped to proportion the fuel/air mixture for the air valve position.
IMPCO carburators have two mixture adjustments:
1. Idle mixture bypass (I). Due to the air vavle design, the total volume of air/fuel passing through the throttle is constant; increased idle bleed (turn screw out) introduces air into the mixer, and the air valve closes slightly, thereby lowering the amount of fuel in the fuel/air mixture.
2. Power mixture adjustment (A). This effect air/fuel mixture only when the air valve is nearly or fully open; it then limits air/fuel ratio by effectively restricting inward fuel flow. It can be set only under near-full-load conditions.
Mixtures between idle and full-load are determined by the shape of the gas metering valve. It is shaped to produce lean mixtures at low loads and richer mixtures at high speeds and hevy load.
The shape of the gas valve is designed for optimum mixture for the middle of the carburetor's design range.
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