Park the bus on a level surface and turn off the engine.
Open the engine compartment.
Locate the valve covers on the left and right sides of the engine. Remove them.
Check the valve cover gasket and spark plug boot O-rings for damage or cracks. If necessary, replace them.
Place a thin blade feeler gauge between the rocker arm and the valve stem of cylinder #1.
The feeler gauge should slide in and out smoothly with slight drag.
If the feeler gauge is too tight (won’t slide in), loosen the locknut and turn the adjustment screw inward.
If the feeler gauge is too loose (slides in too easily), tighten the locknut and turn the adjustment screw outward.
Repeat steps 5-7 for the remaining cylinders in the following order: 1, 4, 3, 2.
Install the valve covers.
