As many of you know our big banger engines were not designed for aircraft. Although they have been adapted, with some modifications, many have been successful to the point where we accept them for what they are today. The Quadra was certainly not designed to use on aircraft, however over the past five or six years a certain part of the factory production run has been modified and aimed toward use in miniature aircraft because of a new found market. Others have followed.

Most of all engines used by us utilize a diaphragm type carburetor. Namely the Tillitson HU series and the Walbro WA series. Both of these carbs look and function very much alike. They are an absolute marvel at pumping, metering fuel and do so in any position at the attitudes we fly. However, some carb locations with respect to cowls, airstream, and yes, even negative pressures, affect the diaphragm in the fuel metering section which is vented to atmosphere. An example: if the vent hole in the diaphragm cover happens to be facing the airstream, any change of speed will affect it. Remember, it was designed to run in a static atmosphere. We are subjecting it to all kinds of transit pressure forces. The end result? Inconsistant engine runs.

To remedy this condition we look for a source of steady positive pressure. Then apply it to the vented side of the diaphragm. This can be done simply by closing all vent holes and in the diaphragm cover, solder in a short length of 3/32" ID copper tubing so that it will end up right in front of the venturi. The size of the tube is not important except the larger the hole the faster the response in pressure change. Once in place, the tube will lead to a more consistent engine with good throttle response. With the carb properlyadjusted you will see and hear a difference in performance.

If you are satisfied with your engine performance, by all means don't modify your carb. However, if you like to experiment try it. It's a simple modification that may help your engine performance a great deal.

1. Metering diaphragm cover before modification. Cover all holes venting to the diaphragm. Solder in a short piece of tubing in cover as vent.

2. Modified carb showing location of atmosphere vent tube picking up consistent pressure signal to metering diaphragm. Shutter shown in venturi in choke.