Making a Carburetor for an 1898 15 HP Reid Type A Gas Engine

Joel Sanderson crafts a custom carburetor for the engine that runs his home shop.

| June 2009

Editor's note: Iron sculptor and blacksmith Joel Sanderson first wrote about the 15 HP Reid Type A gas engine he uses to run the line shaft in his blacksmith shop back with a two-part article in spring 2006. Read about the Reid's acquisition in Part 1 and then read how Joel tunes the Reid for line shaft duty in Part 2.  

My shop and my livelihood are powered by an 1898 15 HP Reid, driving 90 feet of line shaft to more than a dozen machines demanding varying loads. I first ran this engine on propane, but as fuel prices increased over the last few years, I decided to use gasoline as a fuel, hoping for better economy. I also switched from hot tube ignition to a spark plug in order to save fuel.

Because of the demands on my engine for steady running, I needed a carburetor that would respond quickly to varying loads, providing both idle and full power. I also wanted my engine to be able to run on either gasoline or propane without making any changes to the engine, and without either affecting the other’s performance. After trying different commercial carburetors, I decided the best solution would be to make my own.

Carburetor basics
In a nutshell, a carburetor’s function is as follows: the intake air is drawn through a restriction, called a venturi, which increases the velocity of the air as it passes through. As the air speed increases, the pressure decreases. In the vacuum of the venturi, gasoline is sucked into the air flow through a fuel jet, ideally forming a fine, atomized mist which is then easily and completely burned in the engine.

To control the amount of air passing through the venturi (and therefore the amount of vacuum and fuel drawn in), many carburetors employ a simple butterfly, just past the venturi. As this butterfly closes, the air through the venturi slows down, less fuel is drawn in (because there is less vacuum), and the engine’s power is curtailed. If the butterfly closes too far, however, there is not enough vacuum in the venturi to draw in the fuel, and the engine will stall. To allow fuel to be entered into the air flow at this low intake volume, a second fuel jet, called an idle jet, is usually placed just under the butterfly where the air is still passing at a high enough velocity to create the necessary vacuum to atomize the fuel.

This type of carburetor, with the butterfly and idle jet located past the venturi, works well in an engine that idles at a low RPM, and runs many times faster under load. A car might idle at 900 RPM and work at a few thousand RPM. This higher draw at several times the idle speed creates the necessary vacuum in the venturi to effectively atomize the fuel from the main jet. With a Reid, however, or any other single-speed engine, the maximum load occurs at a lower speed than idle. In order to get smooth operation with even carburetion for varying loads, I needed something other than the conventional carburetor.


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