Both the gas valve and the main valve reside on the same stem. The main
valve is actually part of the stem and the gas valve rides along the main valve
stem. The drawing is shown with both vavles in the closed position. This would be the
position when the piston is coming up on compression to fire. After the mixture is ignited
and the piston travels to BDC the exhaust gases go out the exhaust port towards the
rear of the cylinder. This is called a ported exhaust.
The engine is now up on the governor and waiting for the speed to slow down to become unlatched and start the cycle over again.
Now to start the next cycle as the piston comes up towards TDC the cam, through a push rod and rocker arm move the stem and main valve in about .4375 and the old stale air is moved out the check valve. Just as the piston comes to TDC the next cam lob advances the push rod and rocker another .625 and the bushing and brass collar bottom out and open the gas valve. As the piston heads towards BDC it is sucking a fresh charge of air gas mixture. Now the piston comes up on compression and starts the cycle all over again.
I am not real good with words but hope I have explained the operation of the single valve B & S so you might be able with the drawing to see how it operates. I hope the people reading this have a good understanding of the 4 cycle engine operation and the different strokes. It will help in understanding my poor discription of the single valve operation. Thank you.
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