Franklin Engine Modification

Adding a carburetor when “city gas” isn’t an option.

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Ronald McClellan
McClellan’s engine with a carburetor fabricated from an old soda-acid fire extinguisher.

I bought an engine at the end of April 2019. Cast on the base is “Franklin Gas Engine Patd.” Everyone knows these engines as Parsell & Weed engines. I have the vintage book Gas Engine Construction by Henry V. A. Parsell, Jr. and Arthur J. Weed, dated 1900. It gives detailed instructions for building a 1/2hp horizontal or vertical engine.

The name of an engine cast on a metal base.

The design and castings were supplied by The Franklin Model Shop owned by Parsell & Weed. These engines were Franklin engines and were not called Parsell & Weed until modern times. It weighs 92 pounds. It has 14-inch flywheels with a 2-1/2-inch bore and a 4-inch stroke. It would be interesting to know who, why and when the name change happened.

Close-up of two metal check valves on a gas engine

When I got this engine there were things that didn’t look like the other engines that I have seen. At first glance, it looks like it was made into a compressor. It had two large check valves mounted on the head and a poppet valve in the head. The pushrod for the valve was normally run off the timing gear as an eccentric. On this engine, there is a cam to operate the poppet valve. That made it necessary to make an arm to support the pushrod.

A close-up of a poppet valve on a gas engine.

The original engines were made to run on lighting gas, or city gas. The head on this one is nowhere close to what it shows in the book. However, if you didn’t have city gas, there is a chapter in the book that gives the instructions for building an evaporation carburetor.

I spent some time checking all the parts. There is a camshaft, the original plan used an eccentric. It is a 4-cycle engine.

Starting with the exhaust cycle, the cam opens the poppet valve and the exhaust exits the upper check valve. The cam holds the poppet valve open and fresh gas is taken in by way of the lower check valve. The poppet valve then closes for the compression and power strokes.

Building the carburetor

A piece of rough burlap held in place by a brass-colored ring.

I did some repair on the electrical contacts for the ignition. Then, I built the carb, trying to follow the instructions as closely as possible. The main body is from an old soda-acid fire extinguisher 7-inch diameter by 15-3/4 inches long with 1/16-inch thick flange and end plates. On the inside, there are many curtains of burlap, held in place with a 1/2-inch-wide spring-ring. There are large ball-type valves on each end for filling with gas — an air inlet and outlet to the engine.

To use, fill the tank about half way with gas. The gas will wick up the burlap and the air going in one end will evaporate the gas and send only the fumes to run the engine. When I tried to run the engine, I could only get it to pop once in a while. But if I disconnected the carb, it would fire several times. I found that the fumes were too heavy to fire, like having a flooded engine. Talking to another collector I was told that I needed a fresh air intake. So, I added a second ball valve and modified it with a hole in the back side to allow air intake. As the valve moves from 100% fuel, it opens the side air intake and closes the fuel opening. I now have a continuous adjustment of fuel to air.

A wooden box behind a paper.

The engine ran, but after only a few minutes the head got very hot. It is a tank-cooled engine, making the next project creating a cooling tank. Looking through my collection of stuff I might need someday, I found a large artillery shell. It is 5-1/2 inches by 24 inches long — perfect for the tank. I added hose connections and mounted it to a wooden base for stability.

The engine does not have a governor. As I see it, a governor as shown in the book would not work because of the check valve setup. The speed is controlled using the first ball valve from the carb as a throttle to close down the intake.

A newspaper clipping

I have seen only two other horizontal engines and neither had the Franklin name cast on the base. I added the skids to make it easier to handle. It would be interesting to know if there are any more. I also have the vertical engine as shown in the book. I am able to run it on propane. I also have a Franklin Dynamo with the original box. It is 4-3/4 inches tall. Unfortunately, the armature is missing.

Gasoline is a liquid which possesses the property of rapid evaporation when placed in intimate contact with air, and the more thoroughly the two can be brought into relation with each other the richer will be the gas produced. In this carburetor the desired result is obtained by passing the incoming air through a great number of thicknesses of burlap or other coarse cloth.Source: Gas Engine Construction by Henry V.A. Parsell Jr. and Arthur J. Weed.

A technical diagram of a carbonator.

Photos by and contact Ronald McClellan at:

Gas Engine Magazine
Gas Engine Magazine
Preserving the History of Internal Combustion Engines