Closed
They are closed during the power stage.
Closed
Both the intake and the exhaust valve are closed.
-intake: exhaust closed, intake open -compression: exhaust closed, intake closed -combustion: exhaust closed, intake closed -exhaust: exhaust open, intake closed
Closed
The engine may backfire excessively. You may also smell a strong exhaust odor in the riding position. In worst cases, it can cause the engine to run lean.
During the power or combustion stage of a piston cycle, both valves are necessarily closed, because the expansion of gases is driving the piston down.
Possible exhaust leak
inboard gas engine exhaust
fire wall side of engine block under the exhaust manifold.
exhaust pipe connects to the exhaust manifold, which connects to the main engine.