How to Build a Metering Valve to Run Engines on Propane

Fuel your engine with propane by building these simple metering valves


| October/November 2012



Mixer Body Closeup

Close-up of the main mixer body howing the body drilled and tapped to 1/8-inch pipe thread halfway down the body.

Photo By Wayne Summers

I saw a letter in a recent issue of GEM asking about propane carbs. I sent a brief email reply to the writer on how I build my propane carbs, but I thought other readers might be interested in my very simple and easy way to build a metering valve.

Start with a 5/8-inch piece of brass rod 2-1/2 inches long. Drill a hole all the way through that is 10 percent of the bore of the engine it is going to be used on. For an example, an engine with a 2-1/2-inch bore would need a 0.250-inch or 1/4-inch hole drilled all the way through. On one end, drill a 1/2-inch hole 1/2-inch deep. Thread the other end to fit the engine it will be used on.

Halfway down the outside, with a No. R drill bit (0.339-inch), drill a hole about three-quarters of the way through and tap it with a 1/8-inch pipe thread tap. I use an old 1/8-inch pipe tap, grind the first three or four threads off and run it in a little deeper to get two full turns on the fitting.

For the metering valve, use a 1/8-inch threaded gas line elbow and a needle valve out of an old four-barrel carb. The longer the point on the needle valve the better it will work. On the threaded end of the elbow, press in a piece of brass turned down to fit the hole 1/4-inch deep, or far enough that the needle valve will reach it with two or three full turns of adjustment. If you do not have a lathe, you can put a drop of solder in the hole, but you will have to have a longer needle valve.

After the hole is plugged, use a No. 50 drill bit (0.07-inch) to drill the center of the plug all the way through the fitting. On the other end, use the hole you’ve just drilled as a pilot to drill and tap the threads for the needle valve. The size will depend on the needle valve you use. Screw the needle valve with its spring fitted all the way into the bottom, then back it out two to three turns; there should still be some spring pressure on the valve to keep it tight. Set the valve at 1-1/2 turns open. The metering valve is done and ready to install on the engine.

Before you put the LP gas line on the fitting, turn on the gas valve and put the end of the hose in a cup of water to see if there are any bubbles. If there are, you have a gas leak in your regulator. LP gas engines will flood and not start if there are any gas leaks in the demand regulator. Check the regulator needle valve also; gas may be leaking around the threads. If there are no bubbles, the hose is now ready to be attached to the metering valve. Use a 15-pound pressure gauge to fine-tune the fuel system to make the engine run better. The gauge will show how much fuel the engine is pulling and if the tank is empty.