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The Impco - Garretson systems of gaseous fuel carburetion are designed to run internal combustion engines with gaseous fuel in the simplest, most economical manner for top performance and lowest emissions.

The Garretson systems operate using the venturi principle. As air is drawn into the engine by the piston, it must pass through a venturi which creates a vacuum proportional to the amount of air being used. Whereas the vacuum created by the venturi in a liquid fuel system acts on a float and needle valve to draw in fuel, the vacuum in a gaseous system acts upon a dia phragm in a fuel controller (zero governor). As the diaphragm is drawn toward the source of the vacuum, an attached lever opens a valve and all ows the gaseous fuel to flow through to the carburetor. The proper fuel - air mixture is then obtained by a mixture adjustment in the fuel line between the fuel controller and the venturi.

This sounds very simple, and it is. However, the key to satisfactory performance in the ven turi system is the sensitivity of the fuel controller throughout its operating range. A properly set fuel controller (zero governor) using a rubber seat and spring tension against an orifice will shut - off the flow of fuel when the engine is not running. Yet the slightest amount of vacuum should cause the seat to move and allow gas to flow. If the spring force is too great and requires greater vacuum to open the device, a flat spot or sluggish progression off idle will develop with a possibility of lean mixtures and power losses at full load. Also, if the fuel controller allows fuel to leak through at shut - down, hard starting and the danger of fire will result. In either case, the fuel controller should be re - adjusted per the specific instructions for the unit.

Besides a sensitive fuel controller, an accu rate and constant fuel pressure leading into the fuel controller is vital for top performance. In remote tank supplied LP - Gas systems a primary regulator feeds the fuel controller a constant pressure regardless of tank pressure or flow. Most natural gas and domestic LP - Gas installa tions have a constant pressure regulated at the storage equipment, eliminating the need for a primary regulator.

Although there are other methods or designs of gaseous carbure tion, we feel that the venturi principle provides a simple, easy to service conversion with no need to sacrifice performance or exhaust emission levels. A properly adjusted venturi system can produce results comparable to other more complicated systems.

There are three ways to convert a venturi system from liquid to gaseous carburetion, either the spud - in method, adaptor method or replacement carburetor method. The spud - in kits use the existing gasoline venturi with a fuel delivery tube installed in the area of greatest vacuum. The adaptor method uses a special venturi and is placed between the carburetor and the air cleaner. The replacement method uses a specially designed carburetor to replace the existing gasoline carburetor.

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