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The IH gas start diesel has a unique cylinder head and valve arrangement. It's got an auxiliary "starting valve" in each cylinder that connects a starting chamber (containing the spark plug) for each cylinder. When the starting valve is open, the compression ratio is significantly reduced, making the engine easier to crank. When you pull the starting (or compression release) lever a bunch of things happen: It opens the starting valve in each cylinder; it opens a diverter valve inside the intake manifold to divert inlet air thru the starting carburetor; it releases the float in the starting carburetor so the carb fills with gas; it closes the primary circuit to the ignition coil. Now when the motor is cranked it'll start and run as a low compression gasoline engine. After the motor warms for a minute or two on gas, the starting lever is moved to the run (or diesel) position. This closes the starting valves, effectively raising the compression ratio to the diesel range (about 14:1), moves the intake manifold diverter valves so only air is fed to the engine, closes the float valve in the carb, and shuts off the ignition. At the same time the injection pump starts feeding fuel to the injectors. The motor is now running as a diesel. The starting system is almost fiendishly complex, but properly adjusted it DOES work, and permits a relatively small battery and starter (or even a hand crank!) to start a fair-sized diesel engine.
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