Experiments have been carried on for quite some time pertaining to the development of a more successful carburetor which will burn the heavier fuels. The chief difficulty encountered is to find a more suitable way to vaporize these low grade fuels.

Kerosene can be used, only with an application of heat to the manifold to aid in the evaporation of the heavier parts of this fuel. The exhaust pipes are available for this source of heat, but as there is no heat from this source until the engine is running, it is necessary to start the engine on gasoline and switch over to the heavier fuels after the warming-up process.

Fig. 43 shows the Holley kerosene carburetor which is adaptable to any type of engine by making simple changes in the exhaust manifold to include the heating coil tube. This carburetor will operate successfully on any hydro-carbon fuel with a boiling point below 600° F. Two float chambers are provided to take care of the starting and running fuels. The engine is started on the gasoline part of the carburetor and after a short warming-up period the feed is switched to the kerosene part of the device.

The principle upon which this device operates is to provide a primary mixture by means of a needle valve and a very small aspirating jet which gives a mixture that is too rich for combustion. This rich mixture of atomized fuel is carried through a coil tube of very thin wall thickness, which is exposed to the exhaust gases, directly in the exhaust manifold.

The temperature in this coil tube reaches as high as 500 degrees F. The globules of the over rich mixture are broken up here and flow directly into the mixing chamber, where additional air enters, diluting the mixture to make it combustible. The opening of the air valve is controlled by the suction of the engine and by the throttle valve. The shifter valve for changing the operation from gasoline to kerosene is conveniently arranged for dash control, when the engine becomes warm. A primer is arranged in the manifold just above the carburetor and aids in cold weather starting.

Fig. 44 shows the installation of the Holley kerosene carburetor. In this case it was necessary to add a compartment on the exhaust manifold to contain and heat the coil tube. There are some details that must be taken care of on installation. A small auxiliary tank must be provided to hold the gasoline for starting, while a larger tank must be provided to carry the main supply of kerosene.

The adjustments of this type of carburetor is through a needle valve located in each fuel chamber, and as it is impossible to give any set adjustment that would apply to the many different types of motors, the proper adjustment must be worked out. This is done by shifting to the gasoline and turning the needle valve to the right and left and noting the point at which the engine runs the smoothest. The needle valve is then set at this point. The fuel shifter valve is turned to feed the kerosene, and this adjustment is made in the same manner.