Why add or modify engine carburetor inlets? Perhaps we like to tinker. However, most chain saw engines aren't completely suitable for our airplanes as they are available to us. They usually need a little special attention when operating in an aerodynamic environment. An aerodynamic pressure difference can develop across critical carburetor parts in some installations, resulting in poor carburetion in the flight envelope. A little attention and work during aircraft building will usually eliminate this problem. The pressure referenced fuel flow metering carburetors work fine in a chain saw where the atmospheric pressure is the same for the carburetor inlet and that little hole on the plate that covers the fuel metering diaphragm.

There are three basic carburetor situations in our aircraft. The engine carburetor is completely inside the engine cowling, the carb is inside the cowling with the inlet outside or the carb and the inlet are both outside the engine cowling. Pogo III is an example of the first situation. Flood Bean Special and the planned use of the Tartan engine illustrates the second situation. Ole Tiger and Pogo IV are examples of the third case.

Carburetion is generally acceptable in an airplane installation such as Pogo III (Q-42) where the engine cowling provides fairly equal pressurization of both carburetor inlet and the diaphragm pressure reference hole. Baffling in this cowling assures no direct impingement of air upon the carburetor or velocity tube. Accordingly, the entire interior of the cowling sees total or ram air pressure. The velocity tube is completely inside of the cowling. It is an extension to minimize fuel mixture back flow at low engine speeds. This keeps the inside of the cowling clean. The only other addition to the carburetor is a Vortex choke that also serves as a mounting for the velocity tube.

Floyd Bean Special (Q-40) uses a ram inlet with ram pressure on the reference diaphragm. It is especially important in this cowling configuration to connect the pressure reference hole to a ram pressure source, since the pressure reference hole sees the pressure inside the cowling. Brass tubing and silicone fuel line are used to obtain the same pressure the carburetor inlet sees. This installation is acceptably clean but does "spit back" some fuel on the wing and windshield. A plastic plumbing elbow is used for the carburetor inlet with an aluminum mounting plate.

The Ole Tiger installation (Q-40) required a different approach since the carburetor could not be buried in the cowling. A combination of the velocity tube with ram pressure is used on the inlet. The same ram pressure is applied to the reference diaphragm with metal tubing and a short piece of silicone tubing. Choking is done by covering only the carburetor inlet. This is a very clean running arrangement and works well.

Pogo IV (Zenoah 23) uses a copper elbow ram with an aluminum mounting plate. A brass tube is soldered into the pressure reference hole and faces forward to provide the same ram reference pressure. The brass tube is just below the carburetor inlet. This is a clean running arrangement for the Zenoah 23 engine.

Two configurations have been tried on the Super Tartan Twin 44 engine on the test stand. The first used a short direct ram inlet. The second used a much longer double curved ram inlet. This configuration is clean running and does not appear to affect the engine power. It consists of an aluminum mounting plate, a copper elbow, with a brazed aluminum extension tube. The following is a note of caution for Tartan engine owners using the inverted carburetor. I witnessed a friend's Giant model fire just a few days before this was written. The engine was a Super Tartan Twin using alcohol fuel with the supplied choke and air filter installed in a new Star Duster. Some of the priming fuel ran out of the carb into the cowling where either a backfire through the carburetor or the exhaust flame ignited the raw fuel in the cowling.

I don't think my friends cup of coffee put out the flames because he said in no uncertain terms " I need a fire extinguisher!" I made a 20 yard dash in five seconds and used my carbon dioxide extinguisher for the first time. Luckily, the damage was confined to scorched fiberglass under the carburetor, melted throttle control rod, melted wiring insulation, fuel lines and no doubt some melted pride.

In view of this fire I believe the double curved ram inlet may be safer since the carburetor inlet is sealed, possibly eliminating raw fuel from entering the cowling. However, flight testing this configuration remains to be done.

Although this is not a construction article, a few things that have to be worked out in my airplanes may be helpful in your designs. Aluminum and fiber mounting plates have been used with aluminum, copper and plastic ram inlets. These plates were carefully reamed undersize and a small chamfer was done on the carburetor side. Plastic ram inlets were secured with a good tight fit and CA glue was applied. Aluminum and copper inlets were secured with a good tight fit, peening in the chamfered area and CA glue was applied. None of these parts have come loose in several years ... even with soft engine vibration mounts.

The "ram" air pressures for typical Giant models is only about 0.1 pound per square inch. However, any differential pressure between the carburetor inlet and the fuel metering pressure reference hole will cause undesirable change in fuel/air mixture going to the engine. It is important to observe this, otherwise variations in model speed or angle of attack and sideslip angle may change the fuel mixture adversely.

Obviously, the ram configurations don't supercharge the engine but every little bit helps. They do eliminate a lot of guess work and make carburetor tuning easier.