It is very important that the gap between the rocker arm (nr.5) and the valve stem has the correct distance of 0.05mm (0.0019685") when both the valves are closed. The piston has reached then the highest position in the cylinder during the compression stroke. The easiest way to do this is to adjust the mark T on the flywheel with the mark on the housing. Just make sure that you do this during the compression stroke.
This gap is always measured with a cold engine! Especially the gap for the exhaust valve must be correct, otherwise this valve won't close anymore and the valve will burn because of the hot exhaust gasses. This valve is cooled during the contact with the cylinderhead. If a valve is burned, the engine's power is dramatically reduced.
From camshaft to valves
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To adjust the gap, unscrew the little nut (nr.17) and then the big one (nr.6). You can measure the gap with a feeler gauge. Place the gauge between the valve rocker and the valve stem and tighten the big nut until the gauge is just moveable. Tighten the small nut and check the gap again, it is very likely that the gap is changed because of tightening the nut. Adjusting this gap gets quite on your nerves.
When should you replace the valves and valve springs?
| standard size | replacement size | |
| Valve stem diameter | 5.5 mm (0.2165") | 5.4 mm (0.212598") max |
| Total length valve | 47 mm (1.85039") | 46.1 mm (1.81496") max |
| Thickness valve head | 0.5 mm (0.019685") | 0.2 mm (0.007874") max |
| Free length valve springs | 24.5 mm (0.964567") | 23 mm (0.90551") max |
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Kris Maeswith the co-operation of Hans Hartman