Here are a few posts on the topic of valve timing and valve adjustment. Happily, since the c90 is such a simple engine, it is a good place to learn about these automotive fundamentals. Here goes!
The c90 has one cylinder and it has two valves: an exhaust valve and an intake valve. The valves move in time with the piston.
You can see the parts involved below (note only one valve is shown in this diagram):
The crankshaft drives the camshaft via a chain. Note that there are twice as many teeth on the camshaft sprocket than on the sprocket mounted on the crankshaft. This means the camshaft turns once for every two rotations of the engine (as explained later, this is important to know when trying to understand how the valve timing works).
The two lobes on the camshaft lift rocker arms mounted in the cylinder head as the engine turns, causing them to push down on their respective valves so that they open in sequence:
As noted above, camshaft sprocket is geared so that it turns once for every two revolutions of the crank shaft.
During the first revolution of the crank shaft the intake valve opens and the descending piston draws in air and fuel into the combustion chamber, then – as the piston ascends – the valve closes and the mixture is compressed. As the piston reaches the top of the cylinder the mixture is ignited, forcing the piston downwards and starting the second revolution of the crank shaft. Finally as the piston returns upwards, the exhaust valve opens so the burnt fuel can be expelled. This sequence is easy to recall with the mnemonic “suck, squeeze, bang, blow”.
This excellent video shows these parts in action
Here are the cylinder head parts for the c90 z2. As you can see the z2 has contact breaker points and a spark advancer installed on the end of the camshaft (in later models this was changed and these parts were moved to the end of the crankshaft):
It is easier to remove the tappet caps when the engine is installed in the bike, as they can seize tight and you might need to apply a bit of force to get them off. They are made of a soft aluminium alloy and can easily be damaged during removal.
My covers were no exception and were already rounded off, so I had to use a metal file to get two vaguely flat edges on the caps before I could get a good purchase with a pair of monkey grips. Once I had a tight hold it was just a matter of tapping the grips with a hammer to free the covers. New caps they are still readily available.
If you still have the original toolkit supplied with the bike then the 6 sided 23mm ring spanner included is a good fit when you refit the covers (don’t over tighten them!). Honda recommend applying a little grease to threads to stop them binding in the future.
The detailed steps to remove the cylinder head are described in this c90 manual from the late 1970s, and there is a helpful pictorial guide in this CT90 manual (The CT90 and C90 models made in the late 70s shared a nearly identical engine, although the CT90 had additional gears on account of its intended off-road use).
To remove the cylinder head you first remove the contact breaker assembly and the spark advance mechanism that sits behind it so that you can get at the camshaft sprocket. This is held on by two bolts and once it is is removed you can extract the camshaft. Finally you remove the cylinder head cover and can then lift the head off the engine:
The cylinder head can now be removed, here is mine, pre-clean up: