Carburetor and Float Assembly

Note that the carb pictured here is not the carburetor that ultimately gave me the best running characteristics... this carb is for a big .90 CI 2-stroke O.S. engine, and ultimately it was too large. The float chamber, ultimately, has been a huge success. It permits almost any fuel tank placement so long as it is above the carburetor. Fuel head pressure remains constant, and the mixture is unaltered throughout the entire engine run. This is one option that I can highly recommend, as setting a correct mixture for gasoline, when using a glow-plug carb, can be tricky. Gasoline is far more volatile than glow-fuel, and the needle valve is very sensitive indeed.

The carburetor shown was ordered from Tower Hobbies. It is an O.S. Engines rear crankcase carb for their .60 to .90 CI ducted fan engines. I chose this carb not knowing the throat diameter, but hoping it would be close to the .30" I had in mind. The throat measures roughly .460", way too big, but otherwise is a gorgeous little carb, albeit a bit pricey. The throat is easy to sleeve... I made a pair of venturi-shaped throats which are a very light hand press fit into the barrel. The adapter is machined from 1/4" aluminum plate.
Shown together, and held in my hand for some idea as to the size; the lower-right portion of the adaptor plate is left flat and level for the float chamber attachment. A float chamber is a very desireable feature, often ignored in engine models, which provides the carburetor a constant fuel head pressure so that the mixture will not change as the fuel tank level decreases. Further, if the fuel tank is above the carb (as mine is), the fuel will simply leak out in a continuous stream through the needle valve and into the engine or simply splatter onto the ground. The float chamber prevents this.
The float chamber consists of a tiny reservoir which functions quite a bit along the lines of a toilet. The reservoir on the back of the toilet will fill with water, and as the float rises it eventually shuts off the supply. This is a miniature version of the same principle, using gasoline, of course.

I first soldered some smaller floats out of brass sheet, extremely tedious and tricky. When I finally had one that would float on water, as soon as I tried it in gasoline it sank like a rock! Of course gas is very much less dense than water. In disgust, I ripped apart a Briggs & Stratton float assembly to see what "made 'er tick".

The plastic float is cannibalized, then, from an old Briggs and Stratton carb, as is the valve cone and seat, which are internal, below the brass fuel inlet. As the fuel level in the float chamber rises, the float pushes the cone against the seat and prevents more fuel from entering the chamber. As the engine draws fuel, the float of course lowers and allows more fuel to be admitted into the chamber.

The carb, adaptor plate, and float chamber shown together. The fuel line of the float chamber (where the fuel rises to before activating the shutoff valve) is roughly 1/4" below the black gasket, visible on the body of the chamer. The carb will have to draw by suction about 1" of fuel.
And finally mounted in the very busy rear crankcase area. Next stop will be a final ignition system mounting and proper control levers!

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