Firefly Slide Show

Positive Pressure Tank Prime

I believe it was Herb from the Kolb List, who mentioned that he did not use a squeeze bulb but forced the fuel up to prime the fuel pump and carburetor with positive pressure. I thought this was a good idea for system simplification and weight reduction. Since I carry a tiny bicycle tire air pump in the FireFly, I had a good pressure source. I purchased a short tubeless tire stem and drilled the tank and popped it into place. The tire stem valve was removed. To boost fuel to the engine one puts the tire pump and valve extension, as described below, on the stem and increases the pressure sufficient to prime the float bowl. When the pump and extension is removed, the stem acts as the tank vent.

The problem with the bicycle pump alone, is that it depends on the stem valve to keep the air from returning to the pump on the return or fill stroke. The minimal weight way to get around this is to make a valve stem extender that includes a valve. This was done by purchasing a short valve stem and removing all of the rubber vulcanized to the brass stem. Then the gasket was removed from a brass cap and a hole drilled to accommodate the stem OD and it was soldered into place. Then the gasket was returned to its original position.

Here is the completed extension. This change gets rid of the squeeze bulb and reduces FireFly dry weight by two ounces.

Then as I came up on the Tillotson carburetor learning curve, I discovered, one cannot force the fuel up to the carburetor, and so, no fuel line squeeze bulb or positive tank pressure is required. All one needs is a fuel line, line shut off, and in-line filter. The carburetor push button primer will lift fuel from the tank.

Update April 4, 2011 I erred in that I did not test the valve stem by soaking it in gasoline. The rubber like material adsorbed gasoline and swelled to the point that it cracked and the lower portion of the stem fell into the bottom of the tank.

To get around this problem one must strip off all the soft rubber material from the brass stem and make two brass shim stock collars that will slide over the stem. One is soldered the tank end of the stem. A string is dropped through the hole and fished out to the fuel fill opening. The string is threaded through the stem and knotted so that the knot will not pull through the stem bore. JB Weld is mixed and used to coat the stem and the collar and then the assembly is pulled into the bore hole from inside the tank. After the JB Weld has set up, the bottom side of the second collar is coated with JB Weld and slid into place over the stem and down to and on the tank top. Additional JB Weld is placed on top of the collar and around the base of the stem. Then the string can be removed from the vent bore.