So---we have touched on materials, ignition, valve timing and operation and a small bit about cooling.
A brief word about carburation. In general, conventional four cycle engines require a carburetor, which serves to add the correct amount of fuel to the stream of air flowing into the cylinder to be burned. This mixture of fuel and air is generally controlled by a needle valve. A fuel heavy mixture is said to be “rich” and a mixture with only a small amount of fuel is said to be “lean”. A rich mixture is desirable for starting a cold engine. After the engine warms up for about a minute, the fuel mixture can be “leaned out” by screwing the needle valve in “about” ¼ to ½ turn. If the mixture is left “rich”, the engine may flood out and stall, and the high amount of carbon produced by an overly rich mixture can foul the sparkplug and prevent it from firing. The needle valve is not intended as a device to control the rpm of an engine. For that, you need a carburetor which has a “throttle” built into it. The throttle can be in the form of a butterfly valve plate, or a portion of the air intake that revolves to partially close the main air passage through the carburetor. The placement of this throttle is very particular, as it needs to not only control the air passing through the carburetor, but must not interfere with the flow of fuel through the needle valve and “spray nozzle”. If the throttle plate is placed too far upstream from the “spray nozzle”, it will act as a very effective “choke”, but very poorly as a throttle. Small 4 cycle engines don’t need a throttle, but if you have any desire to make major adjustments to the rpm of the engine, then yes, you will need a carburetor with a throttle built into it.—Now---this is very important!!! The carburetors designed for use with these engines create a slight amount of vacuum right at the point where the “spray nozzle” enters into the main intake airstream. This vacuum is what pulls the droplets of fuel from the spray nozzle into the airstream and mixes it with the air to form a true “gas” as opposed to a stream of liquid fuel being pulled into the carburetor. This same vacuum has the ability to pull fuel up from a tank mounted in such a position that when full to the top the fuel level is 1/2 to 3/4” BELOW the point where the spray nozzle empties into the carburetor air stream. These carburetors do not have a float and shut-off valve like the older automobile engines to stop the flow of fuel. Consequently, having the tank higher than what I have specified leads to uncontrolled gravity flow of fuel into the carburetor, which in turn causes flooding, poor running, and in a “worst case” scenario a total hydro-lock of the engine or even worse, fuel pouring out through the inlet of the carburetor and creating a fire hazard. Now, if I haven’t confused you already with this depth of wisdom about carburetors, Hit and miss engines do NOT require a carburetor with a throttle. On hit and miss engines, the carburetor is adjusted by means of the needle valve for “optimal performance”. The governor, which operates from centrifugal force holds the exhaust valve open when desired rpm is reached, and this in turn prevents the engine from firing until the engine slows down enough that the governor return spring returns the governor to it’s previous position, allowing the exhaust valve to close and the engine to fire once more. The strength of the governor return spring is what controls the speed of the engine, not the carburetor.