In my opinion the Schebler Model D carburetor was probably the most widely used of all the old time carburetors on marine engines. While Henry Ford made more than 15,000,000 carburetors for his Model T, not many of his carburetors seem to have been used on marine engines. A December 1921 article in Motor Boat it showed that 85 percent of the marine engine makers in the USA supplied Schebler as their standard offering.
It should be noted that there were a number of 'knock-offs' that were essentially Model D's. Michigan Wheel, Monarch Valve Company, Generator Valve Company, Acorn, etc., offered almost identical units to the Model D. Whether they had any agreements with Schebler regarding licenses, etc. is unknown. In any case none of them ever achieved the widespread use and production life of the Schebler Model D. It is my understanding that Schebler stopped making the Model D in the late 1930s, however, Canadian marine engine-makers such as Lunenburgh Foundry went on making them up until recent years. The highly polished finish on the Schebler Model D regrettably seems to have ended around the early 1930s. Currently available New Old Stock Model Ds from that period typically have a rough sand-cast finish.
There were a number of marine carburetor makers in the first third of the twentieth century. Most did not remain as marine carburetor makers, as the rapidly developing automobile industry was a much more attractive market. Kingston and Krice were two companies that made excellent marine carburetors, and most marine engine makers would supply a customer with the carburetor they preferred. On the other hand, large marine engine makers such as Ferro made their own carburetors. Other companies such as Belle Isle used a unique carburetor which was more like a float feed mixing valve than a carburetor. This unit was possibly of their own design. Belle Isle only lasted from 1908 until 1912, yet a few of their engines still survive, though most don't have their original carburetor.
The Schebler Model D
One point that needs to be made is that the typical small-boat marine engine that was in use up until the practice of converting automobile engines began to dominate the marine engine market really had very simple carburetor demands. For example, small-boat marine engines were operated basically at two speeds. Dead slow for docking and fishing, wide open throttle with the engine speed controlled by the size of the propeller for moving the boat at or close to its hull speed. With the widespread conversion of automotive engines to marine application, often the carburetor that applied to the automotive application became the carburetor of choice.
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.
The float valve cover "U" on some Schebler Model D carburetors may have a gasket, and on others it is simply a ground joint.
The hex fitting holding gasoline in let to the carburetor body will be found in two thread sizes. One 16 threads/inch, the other 18.
Modern teflon round valve packing works much better than the old time felt or cotton packing in the needle valve nut "X".
I do not know the best setting for the float. I have read in Automobile Engineering, published in 1920, that the fuel level with the carburetor level should be 1 mm below the tip of the jet (see "D" in the diagram). I also know that there is a special float level gauge for setting the float, but I have only seen pictures of it and the picture didn't give any dimensions. Perhaps someone reading this may have an answer. I must say it's not clear to me that a waterman in a boat thrashing around in disturbed water is going to worry that his float is set 1 mm below the jet. He obviously doesn't want the float setting so high that the fuel level is above the jet, as serious leakage into the throat would occur. It is my opinion that the float setting in a non-throttling engine application would not be particularly important, as setting the needle valve for any given speed would tend to overcome a fairly wide range of fuel level settings at the jet. Furthermore, it is probably true that most of the engines running at shows with Schebler Model D carburetors don't have their floats set precisely.
In conclusion, it is my opinion the Schebler Model D was a remarkable development at the beginning of the widespread use of the gasoline engine that outlasted all its peers essentially unchanged and unsurpassed in its limited-use field.