Build a Pair of Dual-fuel Carburetors

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The air cleaner housing and filter assembly (Fram Kit No. FA189PL2) was purchased as a replacement unit for the standard Vega air box. To fasten it to the carburetor, we simply slipped the assembly over four posts that we'd made by brazing 10-32 nuts to one end of 1-1/2"-long sections of 3/8" brake line, and 5/16" X 1" studs to the other. Wing nuts hold the filter/cleaner unit tightly in place, and a piece of duct tape — spread over the round opening within the center chamber — prevents dirty air from bypassing the element.

The photographs accompanying this article illustrate the modifications that must be made to the carburetor body itself in order to complete the conversion. These, together with their captions, should be all you'll need to build your own dual-fuel carburetor based on our design . . . which can be adapted to any gasoline-powered engine of 300 cubic inches or less.

A FOUR-BARREL CONVERSION FOR BIG-BLOCK ENGINES

Those of you who are working with larger powerplants will be interested to know that the dual-fuel system on our Chevy truck's 454-cubic-inch V-8 was fabricated from a two-stage, four-barrel Rochester Quadrajet carburetor. Since this piece of equipment was, at one time or another, installed on almost every big-block GM engine, it should be no problem to find one in good condition at an auto salvage yard. (Be aware that the series designation on our unit is 4MV, which simply means that it's a four-venturi design with a manifold-mounted thermostatic choke coil. Other Quadrajet series Garbs may work as well, but we'd suggest finding a pre-1973 model, as pictured.)

Since the Quadrajet is such a popular carburetor, there's no need to make an adapter box to fit it to a non-GM engine, because most auto parts stores sell adapter plates — for about $10 — which will allow you to do just that.

In our conversion, the primary — or gasoline — throttle linkage remained stock . . . and the secondary, or wood-gas, setup demanded little more than the addition of a 1/8" X 3/4" X 3" arm to the throttle shaft, which we connected to the throttle cable of a motorcycle twist-grip equipped with a manual cruise control (refer to Photo 26).

The secondary choke flap position, which determines the amount of air entering the wood-gas section of the carburetor, can be adjusted with a bicycle gearshift lever and cable. (This setup, like that on the smoke throttle, is secured to the controlled component by way of a crimped eyelet at the cable's tip.) A second bike shifter — this one connected to the center rivet of the distributor's vacuum advance diaphragm — governs the engine's ignition timing under load.

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