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Valve adjustment
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All four-cycle engines have a camshaft geared (or chained or belted) to rotate at half the speed of the crankshaft. Lobes on the camshaft raise and lower a tappet that rides on the lobe. A push rod leads from the tappet to one side of a pivoting rocker arm. The rocker’s opposite side pushes on the valve’s stem, which raises or lowers the valve face inside the combustion chamber. Overhead-cam engines eliminate the push rods, but the concept remains the same.
A small air gap between the valve stem and rocker arm tip allows the valves and other parts to expand slightly while the engine is running. Over time, as things wear, the gap gets smaller. If wear eliminates the gap, the valve face can be prevented from fully contacting the cylinder head/valve seat. This causes combustion chamber leaks. Wear also can increase the air gap, resulting in valves that are out of time and don’t open far enough. Either condition can cause hard starting, low power, backfires and other troubles.
Some engines have self-adjusting hydraulic tappets. Some with hydraulic tappets require initial adjustment. You’ll need to check the service manual.
Adjustment usually is simple. Remove the valve cover and rotate the crankshaft to move the valves into a specific position. The book may require that you adjust one valve while its mate is fully open, or that both valves be fully closed. It may specify doing this on a warm engine or a cold engine. The valve clearance gap will also be specified in the book. I prefer the “Go-No Go” system, where you make sure you can slide the right-size feeler gauge into the gap, but cannot insert the next-larger gauge. The smaller the engine, the more delicate the touch required.
Adjusting the clearance won’t help if there’s a pit worn into the rocker-arm tip that touches the valve stem or if there’s a bent push rod or sticking tappet. Going through the valve clearance check and adjustment will help you spot problems and give you a starting point for trouble-shooting. If valve adjustment is called for, it’s a good idea to check it out before blaming the ignition, carburetor, fuel injection or other systems for performance problems.
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