Build a Pair of Dual-fuel Carburetors

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A TWO-BARREL CONVERSION FOR SMALL ENGINES

The fuel delivery system used on our 250-cubic-inch Chevy-powered sawmill is based on a Ford Autolite/Motorcraft 5200 (used in Pintos with 122- and 140-cubic-inch four-cylinder or 171-cubic-inch V-6 engines), or a Holley 5210 (found in Vegas equipped with 140-cubic-inch fours) two-stage, two-venturi carburetor.

Before making modifications to the Garb body, we fabricated an adapter box to mate the two-barrel fuel mixer to the six-cylinder's single-hole manifold. To do this, we simply cut a 4-1/2" length of 1/8" X 2" X 4" tubular steel, and then trimmed out two 1/8"-thick flat plates to cover the openings at each end of the tube.

Then, prior to welding these caps in place, we used the base gaskets from the original and the new carburetor as templates to determine the position of the mounting and venturi holes in the upper and lower surfaces of the chamber. (Note that the bottom opening will be directly below the port for the secondary venturi, since this powerplant is run almost exclusively on wood gas.)

Once the 1-3/8" and 1-1/2" holes had been drilled in the top of the chamber, and a 1-3/4" opening in its bottom, we went on to drill a pair of 13/32" bores to use when securing the box to the Chevy manifold, and four 11/32" holes to accommodate the 5/16" X 1-1/4" carburetor mounting bolts . . . which were inserted from the inside and welded in place.

Finally, we drilled and tapped a 27/64" hole in one of the end plates, to accept a 1/4" pipe to 3/8" hose elbow fitting (for the engine's PCV line, since these wasn't an appropriate fitting on the manifold) . . . then welded those caps in place to complete the chamber.

With the adapter box finished, it was a simple matter to make up some hardware to support the carburetor's air cleaner and to hold the control cables in place. The latter piece of equipment is nothing more than a 12" length of 1/2" steel rod with 3 inches at one end hammered flat (to a thickness of about 3/16") under heat. The support was then bent into an offset so approximately 5 inches — opposite the flattened end — was left straight.

Because we used an outboard motor throttle control on the sawmill, we fabricated two slotted collars — from 1" sections of 1/4" pipe — to accommodate the grooved cable housings which came with that piece of hardware. By tacking the collars to the support arm, then welding the arm to the side of the adapter box (after first positioning the carburetor on top of the chamber to assure proper alignment with the throttle shafts), we were able to provide a sturdy mount for both gasoline and wood-gas control cables. (Of course, if you should choose to use a different kind of throttle arrangement, it'd be a simple matter to weld whatever hardware was required to that support arm instead.)

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