Hit - n - Miss Engine Governor


I have often been asked why Antique Hit - n - miss engines fire the way they do. Many have noted that when a hit - n -miss engine is runnning at idle speed, it will coast for several revolutions and then fire. While not all Antique Engines fire this way (Volume or throttle governed engine do not), most of the earlier ones do.
The answer as to why some engines run this way is actually quite simple and is due the way the engine is governed (how the speed is maintained). To get a visual idea of what I'm referring to, see the sketch below:


As one can see from the sketch, there is a small pair of weights that are hinged near the hub of the flywheel (locations and configurations may vary). As the engine spins, the weights in the flywheel begin to fly out due to centrifugal force. When the weights fly out, they cause a guide that is free move back and forth on the crankshaft to be pulled inward. Also located in the guide on the crank of the engine is a small catch lever. The catch lever sits parallel to the pushrod of the engine and is able to pivot like a seesaw. It's fulcrum is a stationary point sticking out of the base of the engine. When the guide on the crankshaft of the engine is pulled inward, it in turn, pulls in one end of the catch lever. The catch lever then pivots about it's fulcrum and the opposite end of it move in toward the pushrod of the engine. The catch lever then latches the pushrod of the engine and holds it in a forward position via a small "catch block" that is fastened to the pushrod. Since the pushrod of the engine is stuck in a forward position it causes the exhaust valve on the end of the head to remain open. As a result, the all compression is vented out of the exhaust port and the engine beings to slow down.
As the engine decelerates, the reverse of the process occurs and the engine will unlatch the pushrod and allow the engine to fire again. If the engine were put under a load, it would cause the flywheels to spin slowly and make the engine fire more often.
This type of system is not by any means the exact system used on all makes and models of hit - n - miss engines. It is however, probably one of the most common setups you'll see (if you see) on an antique stationary engine.

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