Build Dual Fuel Carburetor

The ability to back up your wood-gas power with gasoline can be an inexpensive convenience. Our dual fuel carburetor designs will allow you to do it.

| January/February 1983

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.

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.

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