Build a Pair of Dual-fuel Carburetors

Enlarge Image Image Gallery [1] The modified Ford Autolite/Motorcraft 5200 carburetor mounts to our 6-cylinder Chevy sawmill powerplant with a home made adapter box. [2] 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. [3] Here's the dual fuel Quadrajet after doing 15,000 miles on our Chevy's V-8. [4] A twist-grip throttle and two bicycle shift levers serve as inside-cab wood-gas controls. Staff Photos

The ability to back up your wood-gas power with gasoline can be an inexpensive convenience.

When we reported on the wood-gas-powered sawmill at MOTHER'S Eco-Village (see issue 78, page 158), 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 (gasoline or wood-gas) carburetion setup that feeds our six-cylinder sawmill engine, and then explain how we converted our Chevy V-8 truck's fourbarrel 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) . . . and so, after a bit of preliminary searching to locate common production Garbs 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 . . . since 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.

Put very simply, what we did (with each carburetor) was to disengage the secondary circuit from the primary and adapt the auxiliary system to enable it to utilize the gaseous wood fuel . . . by doing little more than sealing several orifices with epoxy, modifying the throttle and choke assemblies, and running a large gas inlet port through the carb body's rear wall.

Then, to allow both stages of the device to function independently, we used separate throttle and choke plate controls for each one. This permits the gasoline, or primary, side of the carburetor to deliver liquid fuel to the engine as it's always done . . . but when the throttle plates are closed and airflow through that section is effectively shut down, the secondary — or wood-gas — half takes over and feeds the powerplant instead. In this mode, the secondary throttle plates work to control the speed of the engine (as usual), and the choke flap can be adjusted to regulate the air/fuel ratio. (A small tension spring — placed between the choke control cable and the flap lever — allows the pivoting valve to fluctuate slightly and thus seek the ideal position automatically.)