The valves are held in place by metal guides installed in the cylinder head and on some C90s (including the c90 z2) there is a small synthetic rubber seal on top of exhaust valve guide. This can be seen below (note that Honda use the term ‘stem seal’ rather than ‘valve seal’ in the CT90 service manual that this diagram was taken from):
Honda tried a number of different valve seal arrangements over the years and, on later bikes, settled on using them on both the intake and exhaust side.
The purpose of the seals are to reduce the amount of oil that seeps past the guides into the combustion chamber. In addition to consuming oil, the burned oil creates a smokey exhaust and contributes to the build up of hard black carbon deposits on the piston head, combustion chamber and back of the valves:
Here is the exhaust valve seal used in the c90 z2: the rubber seal fits inside a “top hat” shaped cap that covers the top of the valve guide. A small washer fits over the top-hat and acts as the bearing for the inner valve spring:
I can’t find a definitive explanation as to why Honda chose only to put a seal on the exhaust side for older bikes. The opposite arrangement – where only the intake valve has a seal – is generally explained because when the intake valve is open it is subjected to a vacuum that tends to suck the oil past the guide, whereas the exhaust valve is under pressure.
A possible explanation for Honda’s decision is that there is something about the design of the oil delivery/return in the cylinder head that tends to cause oil to puddle on top of the exhaust valve guide when the engine is off and that it is this that required an additional measure to stop excessive amounts of oil being lost through this route.
It is worth noting that many 1960s Hondas came without any valve seals at all, as was common on bikes of this era. Honda’s valve seals experiments began as they started to compete with other makers on warranty periods: in the early 1960s it was unusual to get a car or mortorcyle manufacture warranty at all, but over the next decade or so many makers introduced one, initially with a 3 months guarantee then 6 months and, eventually, offering 1 and 2 years. Honda were at the forefront of these improvements in quality and guarantees, having realised it was a good way to differentiate themselves from the makers of British and American bikes which were unreliable in comparison. Given that excessive oil consumption is closely associated with maintenance issues in many people’s minds this helps explains Honda’s ongoing tinkering with valve seal arrangements.
Anyhow, when you have the valves out for inspection and cleaning you may as well replace any valve seals at the same time. The valve guides can also wear in high mileage engines and, according to the manual, they can be replaced but the guides are made over sized and you need a special tool to grind them to the right profile.