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 larger camshaft sprocket than on the crankshaft, which 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 engine 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. During the next revolution – when both valves are closed – the mixture is ignited forcing the piston downwards and 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”.
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):
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: