This tutorial describes the general way of adjusting the valve tappet clearance of a Honda engine. The mentioned clearances are applicable to Honda PF50. But just to be sure, double check the clearances for your model.

Alright, first of all, you need a so-called feeler gauge, which you can see here: http://en.wikipedia.org/wiki/Feeler_gauge
They can be found sometimes cheap in the dollar bin at the gas station.

Settings to remember:

Intake valve tappet clearance : 0.05mm (0.002in)
Outlet valve tappet clearance : 0.08mm (0.003in) (note that this is not the factory setting, but a setting "from experience"

The picture of the cylinder head you see is from a PC50, but that is the book I have at hand now. PF and PC do not differ in the way of adjusting the tappet clearance.

Here we go:
The tappet clearance will have a great deal of effect on the valve timing. Further, if the clearance is too small, it may prevent the valve from fully closing and result in pressure leak at the valve or even a burnt valve seat. On the other hand, an excessive tappet clearance will produce tappet noise, causing noisy engine operation. The tappet clearance will also have a varying degree of effect on the engine power output and engine operation.

1. Remove the dynamo cover and align the timing mark "T" on the flywheel with the timing index mark on the crank case. To be absolutely sure that your piston is at the top-dead-center, remove the spark plug and carefully feel the position of the piston with a small screwdriver. If you do not want to do this, then you can check it by feeling both rocker arms. They should be loose at the "T" mark. The screwdriver way is better, because it also gives you the opportunity to clean and check the spark plug.

Fig 1.8

2. Remove the tappet adjusting cap on the cylinder head and check the clearance between the adjusting screw and the valve. If the valve is actuated by the screw, rotate the flywheel one complete revolution and realign the timing marks to set the piston at top-dead-center (TDC) of the compression stroke. Check the tappet clearance with the gauge to see if it is of standard clearance as described in the list above. If adjustment is necessary, loosen the adjusting screw. Lock the adjusting screw after adjustment has been completed.

Fig 1.9

How to FEEL the thickness: It's always a debate what is the best way to determine the correct clearance, but my way of feeling is if you have to wedge the gauge in and the gauge is really difficult to move back and forth, then the clearance is too tight. If the gauge easily slips through, the clearance is too loose. When you move the gauge back and forth and it's moving "sticky", then you are on the right way and the setting is probably right.

Of course, there is no such thing as a 0.08mm valve gauge, that would make tings too easy. So, of you have set the exhaust tappet clearance to 0.05, loosen up the screw a very tiny little bit. That should cover it.

NOTE:
1. The adjustment must be made with a cold engine.
2. Hold on to the screw while tightening the cock nut. Sometimes the screw is dragged along a bit while locking.