Build Dual Fuel Carburetor

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Here's the dual fuel Quadrajet after doing 15,000 miles on our Chevy's V-8.
Here's the dual fuel Quadrajet after doing 15,000 miles on our Chevy's V-8.
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The modified Ford Autolite/Motorcraft 5200 dual fuel carburetor mounts to our 6-cylinder Chevy sawmill powerplant with a home made adapter box.
The modified Ford Autolite/Motorcraft 5200 dual fuel carburetor mounts to our 6-cylinder Chevy sawmill powerplant with a home made adapter box.
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[1] The parts removed (after separating the air horn from the main body) from the Pinto carburetor include ?clockwise from the top ? the choke thermostat assembly, two casting bosses (cut from the main body's upper left side, next to the primary bore), the secondary booster venturi, the brass vent fitting, the fuel mixture stop pin, the distributor vacuum advance fitting, the secondary main metering jet, main well tube, idle jet, high speed bleed, and operating lever. The center group includes unneeded choke parts. [2] Place the body in a vise and ? using a 1 1/4
[1] The parts removed (after separating the air horn from the main body) from the Pinto carburetor include ?clockwise from the top ? the choke thermostat assembly, two casting bosses (cut from the main body's upper left side, next to the primary bore), the secondary booster venturi, the brass vent fitting, the fuel mixture stop pin, the distributor vacuum advance fitting, the secondary main metering jet, main well tube, idle jet, high speed bleed, and operating lever. The center group includes unneeded choke parts. [2] Place the body in a vise and ? using a 1 1/4" hole saw ? bore an opening into the rear wall of the secondary barrel, centered and 1 7/16 inches up from the base. [3] Cut a 3 1/2" length of 1 1/4" thinwall chromed sink drainpipe and slide it into the opening so it butts against the shoulder of the booster venturi slot. Mark the cutting line with a felt-tip pen, using the barrel's wall as a guide, then cut along the mark with aviator's snips.
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[7] Remove the choke shaft and cut it in half, midway between the two center plate-mounting holes. [8] Cut two pieces of 1/8
[7] Remove the choke shaft and cut it in half, midway between the two center plate-mounting holes. [8] Cut two pieces of 1/8" welding rod to 1 7/8" in length, flatten one end of each, drill a 1/16" hole through both flat parts, bend each rod to a 90? equilateral, then silver-solder or braze the rods to each choke plate where shown. [9] The independent choke shafts and plates are reassembled within the air horn, and the control cables are fastened as indicated (note the tension spring on the wood-gas choke flap lever, and the homemade clamp that holds the cables tightly to the air cleaner housing.
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The air/fuel ratio on our truck's V-8 engine can be regulated, in the wood-gas mode, by a control cable connected to the secondary choke flap.
The air/fuel ratio on our truck's V-8 engine can be regulated, in the wood-gas mode, by a control cable connected to the secondary choke flap.
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A twist-grip throttle and two bicycle shift levers serve as inside-cab wood-gas controls.
A twist-grip throttle and two bicycle shift levers serve as inside-cab wood-gas controls.
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[4] These matchsticks indicate the holes that'll need to be filled with epoxy (we used a two-part pliable type that could be blended and formed like modeling clay). The pen tip points to a channel that feeds the power valve, and therefore should not be plugged. [5] The ported vacuum orifice for the distributor advance (at pen point) and the vacuum feed for the choke get sealed as well. [6] The wood-gas inlet pipe should be carefully fitted and fastened to the carb body with epoxy. When that's done, check again to make sure the power valve feed channel isn't plugged.
[4] These matchsticks indicate the holes that'll need to be filled with epoxy (we used a two-part pliable type that could be blended and formed like modeling clay). The pen tip points to a channel that feeds the power valve, and therefore should not be plugged. [5] The ported vacuum orifice for the distributor advance (at pen point) and the vacuum feed for the choke get sealed as well. [6] The wood-gas inlet pipe should be carefully fitted and fastened to the carb body with epoxy. When that's done, check again to make sure the power valve feed channel isn't plugged.
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[10] The choke flap cables pass through 3/16
[10] The choke flap cables pass through 3/16" holes drilled in the filter box and element. [11] The modified carburetor and all the parts necessary to mount it to the engine of your choice. [12] Here's the assembled package, ready to go. A standard radiator hose can be used to connect the Garb's smoke inlet pipe to the wood-gas generator's outlet tube. Note that the gasoline throttle lever is offset from that of the wood gas. We've installed stop arms on the latter control to allow idle position adjustment and prevent the throttle blade from opening beyond a 90? angle. (The apparatus should be compatible with any size engine up to 300 cubic inches.)
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[17] remove the secondary linkage assembly from the throttle body. [18] Using a 1 1/2
[17] remove the secondary linkage assembly from the throttle body. [18] Using a 1 1/2" hole saw with a 1/4" pilot bit, carefully drill into the rear wall of the carburetor's main body until the saw penetrates the divider between the two rear barrels to a depth of one inch. (The pilot bit should be set to bore no farther than the mounting slot for the secondary baffle, and the cut should be centered both horizontally and vertically.)
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[13] To make the modification process simpler, the carburetor should first be cleaned in solvent, then disassembled into its three main parts (air horn, main body, throttle body). The parts that get removed in the course of the operation are, from left to right, the entire secondary linkage unit (and choke rod) ... both secondary main discharge nozzles, main well air bleed tubes, and accelerator tubes (center: top and bottom) ... the secondary metering rods, their two-part lever, and a mounting screw and hinge pin (middle center) . . . the secondary bore baffle . . . and a 1 1/2
[13] To make the modification process simpler, the carburetor should first be cleaned in solvent, then disassembled into its three main parts (air horn, main body, throttle body). The parts that get removed in the course of the operation are, from left to right, the entire secondary linkage unit (and choke rod) ... both secondary main discharge nozzles, main well air bleed tubes, and accelerator tubes (center: top and bottom) ... the secondary metering rods, their two-part lever, and a mounting screw and hinge pin (middle center) . . . the secondary bore baffle . . . and a 1 1/2"-diameter section of the main body's rear wall. [14] With the air horn inverted, remove the brass air bleed and accelerator tubes from both sides of the secondary circuit.
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[15] The larger main discharge nozzles should be pulled out as well. Once these brass parts have been taken out, you can seal the six holes with pliable epoxy. [16] Slip the secondary bore baffle out of its slots.
[15] The larger main discharge nozzles should be pulled out as well. Once these brass parts have been taken out, you can seal the six holes with pliable epoxy. [16] Slip the secondary bore baffle out of its slots.
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[19]Remove the hole saw, then using a No. 12 bit, intersect the main cut, making sure to drill only deep enough to ... [20] separate the newly formed plug from the rest of the body so it can be pulled free. With that done, dress the opening with a file to smooth any rough edges.
[19]Remove the hole saw, then using a No. 12 bit, intersect the main cut, making sure to drill only deep enough to ... [20] separate the newly formed plug from the rest of the body so it can be pulled free. With that done, dress the opening with a file to smooth any rough edges.
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[21] Cut a section of 1 1/2
[21] Cut a section of 1 1/2" thinwall sink drain to about 3 1/2" in length (you can use either a straight or an elbow fitting, depending on where your wood-gas feed hose enters the engine compartment), then slip it into the opening and mark a cutting line on both its sides, using the carburetor's walls as a guide.
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[22] Clip the drainpipe to form a contoured
[22] Clip the drainpipe to form a contoured "fishmouth." [23] Place the pipe in position, then drill a 1/8" hole through the carb body's upper gasket surface (between the two oblong holes as shown) and the wall of the pipe beneath. Countersink the bore 3/32 inch with a 5/16" bit in preparation for installing a 1/8" x 3/8" countersunk pop rivet. [24] Fasten the pipe to the carb body and check to make sure the rivet's head is flush with (or below) the gasket surface, then knockout the small nailhead left in the rivet with a 1/16"" pin punch. [25] Seal the joint between the pipe and the carburetor body with silicone compound
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[26] Cut apiece of 1/8
[26] Cut apiece of 1/8" x 3/4" flat stock to 3" or so in length, drill a 3/16" hole in each end and another that's offset a bit from the center, and tack-weld this lever to the end of the secondary throttle shaft to replace the small arm you removed previously. If you want to use a motorcycle twist grip control as we did, crimp a ring tongue terminal to the end of the cable and mount it to the lever. If not, standard linkage hardware can be used. [27] Drill a 5/32" hole through the lower arm of the secondary choke shaft lever and fasten a cable-and-ring control (as shown) or a ball-and-socket terminal. A bracket ? made from a piece of 1/8"" X 5/8"" X 4"" flat stock bent to a 90? angle ? can hold the cable in place. [28] The modified carburetor, assembled and ready for installation. A 1-1/2"" radiator hose completes the joint between the smoke supply tube (from the wood-gas generator's cooler/filter) and the sink drain inlet pipe fitted to the rear wall of the altered Quadrajet.

When we reported on the wood-gas-powered sawmill at our Eco-Village, we detailed the mechanical aspects of the unit’s drive train and described several changes we’d made in the generator, cooler/filter, and circulation systems which together produce and prepare the wood-derived fuel.

With this article, then, we’ll go on to tell you about the dual-fuel carburetor (gasoline or wood-gas) setup that feeds our six-cylinder sawmill engine, and then explain how we converted our Chevy V-8 truck’s four-barrel Rochester to give that vehicle twin-fuel capability.

In both cases, our goal was to simplify our earlier fuel metering designs by eliminating the need for two individual carburetors (one for each fuel used). So after a bit of preliminary searching to locate common production carbs that had [1] separate primary and secondary circuits and [2] large enough bores in the latter half to accommodate the necessary flow of wood gas, we went ahead and modified the devices to allow them to handle either liquid or gaseous fuel.

Generally, two-stage carburetors — whether they’re of the two- or four-barrel type — are designed to operate, under most conditions, on their primary circuits because the small throats, booster venturis, and lean jetting found in these main circuits assure that a minimum of fuel is used.

However, should there be a demand for additional “punch” — as might be required for passing — the large secondary barrels come into play when the accelerator is depressed beyond a certain point, and deliver a supplemental mixture of fuel and air to the engine’s cylinders. This backup system is generally activated by a mechanical linkage that’s connected to the primary throttle shaft.

  • Published on Jan 1, 1983
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