swnf-The exhaust cam and the intake cam are fixed in position relative to each other by being Loctited to the camshaft. I can not change much in relationship to the intake cam because anything affecting the intake cam also affects the exhaust cam. The cam timing on this engine is given by the original designer, and there is a definite overlap of valves both being open at the same time as the piston approaches top dead center on the exhaust stroke. When an intake valve first begins to open, there is very little flow through it because inertia tends to keep the column of air in the carburetor intake "at rest" until the piston begins it's descent on the intake stroke. That is why the intake valve actually begins to open before the piston reaches top dead center on the exhaust stroke. As well, in order to fully remove the exhaust from the cylinder, the exhaust valve stays open for a few degrees after top dead center. The theory is that the column of exhaust in the exhaust pipe is already in motion, and inertia will keep it flowing until the valve closes, and this flow will actually help to start pull a fresh charge of unburned fuel through the intake valve which is beginning to open at the same time.. The same effect takes place when the piston approaches bottom dead center on the exhaust stroke. Most of the power has been extracted from the burning charge of fuel by the time the piston has traveled roughly 2/3 of the way down the bore, so the exhaust valve actually begins to open before bottom dead center on the power stroke so that any expansion left in the still burning charge of fuel will start the gasses flowing out the exhaust system. The intake valve, which began to open before top dead center on the intake stroke of the piston stays open all the way through the intake stroke, and stays open after the piston passes bottom dead center on the intake stroke and begins to ascend on the compression stroke.
