Here are the valve adjustment instructions taken from a CT90 workshop manual (the late 1970s CT90 and c90 z2 shared many engine components):

The idea is to set the gap so that the feeler gauge just slides between the tappet adjustment screw and the top of the valve stem.

As explained here the adjustment must be done when the engine is cold and when the piston is on top-dead-centre on the compression stroke

Finding TDC on the compression stroke

As explained in this extract from a Honda manual, valve adjustment measurements must be taken at top dead centre (TDC) on the compression stroke. Honda outlines a convenient way to check the engine is in the right spot:

valve adjustment instructions from the 1978 C90 Z2 owners manual

this way is not foolproof, however, and a reliable alternative method is to do the following:

1. Remove both tappet covers
2. rotate the generator rotor anticlockwise until the intake valve opens
3. continue to rotate the rotor until the next TDC

To understand these instructions, recall the basic operation of a 4 stroke engine: the piston rises and falls twice in order to complete a full cycle where fuel and air is drawn into the cylinder, compressed, ignited and the resulting gases expelled from the engine:

The second illustration shows the point in the engine cycle when the piston is ascending on the compression stroke

On the first stroke the intake valve opens and the piston descends creating an area of low pressure that draws in fuel/air mixture from the carburettor into the cylinder. As the piston ascends the intake valve closes, sealing the cylinder and causing the mixture to be compressed. The mixture is then ignited by a spark from the spark plug and the expanding gases force the piston downwards.
Finally, on the fourth stroke, as the piston ascends the exhaust valve opens and the gases are expelled.

This excellent film of an engine converted to use a perspex cylinder lets you see this process in action:

It is now easier to understand Honda’s instructions: as discussed in an earlier post the point of this adjustment is to create a small amount of play between the tappets and the rocker arms when the valves are closed. We now know that both valves are closed when the piston is at the top of the cylinder on the compression stroke so we want to make sure the engine is in this position before making any adjustment. We can’t see the position of the piston in the cylinder of course, but the ‘T’ marked on the alternator rotor by the factory, when aligned with the pointer on the crankcase, shows that the piston is at the top of its stroke (the “T” stands for top dead centre).

But as we have seen above the piston is at the top of the cylinder twice for each cycle of the engine, so another step is needed to confirm that we are on the right stroke. Honda’s suggestion is to wiggle both the tappets and this works because, when the valves are closed, the pressure on the tappet is relieved so you can feel a tiny amount of play between the rocker arm and the valve stem.

It is just an indication you are at the right spot though. What do you do if someone has already over tightened or loosed the tappets so that this test is no longer works? A reliable alternative method is - recalling the suck/squeeze/bang/blow cycle - to rotate the engine (anticlockwise as per the directional arrow on the rotor) while watching the intake valve until it opens. At this point the engine has just sucked in the fuel/air mixture from the carburettor and is about to begin the compression stroke. This means that continuing to rotate the engine until the next top dead centre (TDC) will get you to the right spot.

So the rule is quite simple: before adjusting the tappet valve gap put the engine on the next TDC after the inlet valve closes.

One benefit of doing it this way is that, if you forget the rule, you can still work it out for yourself so long as you can remember the suck/squeeze/bang/blow mnemonic.

Now we know how to set up the engine before adjusting the valves – keeping in mind this must be done when the engine is cold

Setting the gap

the 1971 C90 manual specifies the gap should be set to 0.05mm, as does just about every other manual for small Hondas of the era, but for unexplained reasons the CT90 manual mentioned above specifies the gap can be within a range of 0.05mm +/-0.02mm.

The adjustment screw has a 3mm square head and you can get a specially designed tappet adjusting spanner, like the one in the illustration below, which makes it easier to keep the screw in the right location as you tighten the 9mm lock nut.

Some people argue that this method for measuring the gap is not completely reliable because over time one or other of the top of the valve stem or the end of the adjustment screw tends to develop a concave depression resulting from the hammering it gets when the engine is running.

small concave dent worn into the end of a tappet adjusting screw

Because the feeler gauge does not conform to the little dent the the actual gap is bigger than the clearance measured and, although the difference this makes must be tiny, so is the gap you are trying to measure.

An alternative method

An alternative approach is to use the pitch of the thread on the adjuster screw as a vernier scale. This method is not advertised by Honda, but was described in old Triumph owners manuals like this one:

Triumph 1970s workshop manual

The thread pitch is the distance between the threads (measured along the length of the fastener) and the pitch of the Honda adjuster screw is 0.5mm:

In other words one complete turn moves the screw 0.5 mm. Therefore if you tighten the adjuster screw until is lightly seated and then turn it out ⅛ of a turn the resulting gap will be 0.06mm. This is close to the expected 0.05mm setting and in the range allowed in the CT90 manual. Of course this method is only as reliable as your ability to correctly judge the degree the screw turns and will also be impacted by any lash in the screw threads, which is presumably why Triumph describe it as “approximate”.

Factory Manuals

You don't need to rely on random internet posts like this one to maintain your vintage Honda motorcycle, if you would like definitive instructions it is always best to refer directly to the original Honda workshop manuals. I have uploaded my scanned manuals for reference.