The Ubiquitous Schebler Model D Carburetor
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Of course the adoption of the automotive engine to marine propulsion brought with it its own set of problems. Not the least of which was the need for a reduction gear to match the high RPM of the engine to the low RPM of a propeller suitable for the heavy-displacement hull of the period.
The typical small, work-boat marine engine up until the 1930s turned between 400-700 RPM running from 1-1/2 HP in single-cylinder engines and up to 40 HP in four cylinder engines. This speed range was an excellent match for a propeller for hulls of the period. The typical small pleasure launch engine was one or two cylinders turning up to about 1000 RPM in the 1-6 horsepower range with a slightly smaller propeller. In any case, in either application the 2-speed operational limits would apply, therefore their carburetor demands were similar, air/fuel volume being the principal difference. The Schebler Model D met this requirement by making six different sizes. These were 1/2-, 3/4-, 1-, 1-1/4-, 1-1/2-, and 2-inch National Pipe Thread.
In addition to different size pipe, Schebler offered a butterfly valve that went in place of the simple throttle plate (see K on the diagram of the Model D above). The butterfly valve was used in typical governed operation such as on cord wood saw rigs, water pumps and similar stationary engine applications.
Schebler offered a check valve built into "R" (in the diagram) for director connection to 2-port 2-cycle engines. This eliminated the need for an external intake check between the Model D and the engine intake.
The intake choke (see I) was added to the Model D in 1912, but it did not appear on all production carburetors after that date.
One of the most common questions regarding the Schebler Model D is what is the correct setting for the springs regulating the air valve in the intake throat? (See A, O, Y, W, and M.) The answer is, there is no correct setting, as the air valve only comes into play when the engine is working under load. Under load conditions one has to adjust the air valve, needle valve and throttle plate along with spark for best engine operation with lowest fuel settings. In other words, every load application has to be tailored.
There is a critical requirement that gasket "N" be in place if one attempts to run the engine at idle. Either a cork gasket or neoprene O ring works, but either must seal the gap between the top of the throat and the cover of the bowl. If this gasket isn't sealing the gap the engine may run at a fairly low speed, but it won't idle down as it should and the needle valve setting may need to be changed at different speeds.