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.
This fuel system is now complete and ready to run. Turn on the tank regulator; the pressure gauge should read around 5 pounds of pressure. Roll the flywheels over compression and the pressure should drop 1-1/2 to 2 pounds. If the pressure drops more than 2 pounds, the hole in the 5/8-inch brass rod is too small. Enlarge the hole one drill bit size at a time until the pressure drops 1-1/2 to 2 pounds. If the pressure does not drop and reads 0, then the hole is too large. To fix this, tape a washer with a 1/8-inch hole over the 1/2-inch hole on the end of the 5/8-inch brass rod so air can go through it. Turn the flywheels over and check if there is now a pressure drop. The hole will now need to be enlarged one drill bit size at a time until you get it to drop 1-1/2 to 2 pounds. Replace the washer with one made to fit the 1/2-inch hole with the new, correct center hole size and press or glue it in the 1/2-inch hole all the way to the bottom.
The engine should now be ready to start. To start the engine, put a finger over the 1/2-inch hole in the metering valve — the intake — to work as a choke; turn the flywheels over one or two times to fill the line with gas. Take your finger off, start the engine and then adjust the needle valve to where the engine runs the best. Most engines run the best at three-quarter to one turn open, but not all engines are alike. I use this metering valve on 2- to 4-inch bore engines. It also works great on model engines; you’ll just need to make the metering valve smaller.
When I built this fuel system I used the following parts:
• ProStar PRS20100 pressure gauge; you can get it at most welding supply stores.
• Mr. Heater F273607 Regulator; you can get this online at Mr. Heater.
• I used a homemade demand regulator for hit-and-miss engines.
• I put a tee in the line between the bottle regulator and the demand regulator, and then installed the pressure gauge in the tee.
• I used a clear hose between the demand regulator and the metering valve. The shorter the hose, the better it will work.
Good luck with your new fuel system. If you have any questions, please don’t hesitate to contact me after 8 p.m. CST.
Contact Wayne Summers at (660) 258-7990 • wayne@shighway.com
