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.

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Joel Sanderson uses this 1898 15 HP Reid, serial number 578, to drive 90 feet of line shaft in his Michigan iron working shop. The demands on his engine for steady running required him to make a versatile carburetor that would respond quickly to varying loads, as well as allow him to easily run the engine on either gasoline or propane without making any permanent changes to the engine itself.

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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.

Carburetor specifics
My carburetor has two jets, with both jets and the butterfly placed in the venturi. Each jet has its own needle valve for independent fuel regulation, and the main jet screws in and out for the option of different sizes. For further control, I made the fuel bowl’s height adjustable, affecting the ease with which the fuel may be drawn into the air stream; the lower the bowl, the greater the vacuum required.

My venturi consists of a 2-inch pipe with an hourglass shaped restriction of 1.100 inches diameter. At the bottom of this is the interchangeable main jet. In order to get it to function only when the engine slows and the butterfly opens, this jet stands up 0.150 inch above the floor of the venturi. When the engine is running at idle (faster) the butterfly closes behind this standing main jet and no air passes over it, drawing no fuel through it. The idle jet is positioned immediately behind the butterfly on the floor of the venturi (this is a tiny hole from a no. 56 drill). With no load, the butterfly runs nearly closed, and the fuel comes only through the idle jet; as load is applied to the engine and its speed drops, the butterfly opens more, pulling air across the main jet and allowing more fuel to mix with the air. The height of the main jet determines when it begins to function.

Plumbing it for propane
In order to be able to run it on either propane or gasoline, I needed a configuration that would allow me to go back and forth between the two fuels without changing either system. This meant leaving the metering system in place for the propane and having the carburetor’s restriction not affect the propane’s slide valves. To do this, I made the air for running on propane enter downstream of the carburetor. I then mounted a ball valve in this line, which, when closed, forces the engine to draw through the carburetor. Opening this valve immediately stops the air from being drawn through the carburetor and likewise stops the intake of gasoline. Both intakes for gasoline and propane are plumbed to the same intake pipe, which enters the engine room through the attic.

To connect to the butterfly, I extended a fuel valve cross bar beyond the propane mixer links and ran 3/16-inch rod to the butterfly’s lever. This allows both metering systems to respond simultaneously to the governor’s demands.

In the line leading to the carburetor, I placed a simple choke for starting. This is sealed from the room with an O-ring to prevent unwanted gas leaks during propane runs.

How it works
I start the engine on propane with the hot tube. This allows me to start the engine with the usual rocking back and forth motion, and without having to spin it fast enough to draw air through the carburetor. As soon as the engine is running, I turn the hot tube off and the spark on (the spark plug is mounted in the release valve hole over the main valve). I let the engine heat up to 100 degrees F before switching to gasoline. If I do not do this, the fuel tends to condense in the cold charging cylinder, and the engine will flood. This is especially problematic in the colder months when the engine’s jacket temperature might be as cold as 25 degrees F when I start it in the morning. During this warm up period, the hot tube cools off. This is important, because propane and gasoline have different combustion temperatures, and the gasoline will pre-ignite with the hot tube timed for propane, causing the engine to knock. Once the jacket reaches 100 degrees F, I turn the propane off, close the ball valve (which forces the intake through the carburetor) and turn the gas on. Usually, it requires only two or three revolutions with the choke on before it runs up to speed on gasoline.

With this carburetor, the Reid runs very steadily, with an impulse every revolution and with very little pulse in the drive belt. With only the line shaft running, the butterfly remains nearly closed, drawing fuel only from the idle jet. As machines are added to the load, the engine’s speed begins to fall, the butterfly opens, and the engine begins to draw fuel from both the idle and the main jets. Even the lighter machines, like the planer and the pedestal grinder, will cause the main jet to function.

I had some blacksmith friends visit my shop last summer, and this allowed me to monitor my engine while it was under load. One person ran my 8 HP 250- pound power hammer, another ran my 5 HP 100-pound hammer, and my 5 HP 60-inch metal planer was also running. With a draw of about 5 HP to run the line shaft itself, this totaled 23 HP, which the Reid and this carburetor handled with a 15 RPM drop. It ran smoothly with even carburetion. It seemed to me the engine had plenty more to give.

One interesting thing to note about this engine is that once it is up to temperature (160 degrees F) and running on gasoline, I can turn off the spark, and the engine will keep running with no ignition other than its compression temperature. It will not do this on propane. Whenever I have pulled the piston and cleaned the ports, I have found no residue in the cylinder which might be forming glowing embers to cause this.

Fuel efficiency
The economy of this carburetor was very surprising to me. Under light load this engine uses about 2 gallons of propane an hour; under heavy load its consumption shoots up to around 5 gallons of propane an hour. With this carburetor, I use between 0.70 and 1.08 gallons of gasoline an hour, depending on the load and the grade of fuel I am burning. This is comparable to an automobile getting between 60 and 85 miles per gallon driving 60 miles an hour. With the prices of propane and gasoline being very similar, this is significant savings.

Contact Joel Sanderson at 425 Maple St., Quincy, MI 49082 or visit his website at where you can view more images of his shop.