The Ubiquitous Schebler Model D Carburetor
(Page 2 of 3)
November/December 1999
Richard
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.
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The typical small work-boat marine engine up until the 1930s
turned between 400-700 RPM running from 1? horsepower in single
cylinder engines and up to 40 horsepower 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 two 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',
1?', 1?, and 2' 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). 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 for director
connection to two-port two 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 after that date.
One of the most common questions regarding the 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.