The cam to crank timing is initially set when you asssemble the engine. Timing refers to the regulation of the parts of a mechanism to ensure it works as designed. In this context it a reference to the mechanism by which the crank turns the camshaft and the valves in the engine are made to open and close at the right time.
The c90 is a very simple engine with a single cylinder and two valves (an exhaust valve and an intake valve). You can see the parts below:
As you can see above, a chain is used to connect the crankshaft sprocket and the sprocket on the camshaft (called the timing sprocket). There are twice as many teeth on the camshaft sprocket than timing sprocket which means the camshaft turns once for every two rotations of the engine.
As the engine turns, the two lobes on the camshaft lift the rocker arms mounted in the cylinder head causing them to push down on their respective valves so that they open in sequence.
As noted above, the camshaft sprocket turns once for every two revolutions of the crankshaft. During the first revolution of the crankshaft 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 crankshaft. 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:
Setting cam to crank timing
The cam to crank timing is set when you assemble the engine. You must align the timing mark on the cam sprocket (a small “o”) with an index notched on the cylinder head. This picture shows the timing marks in the 89cc engine from a late 1970s 6v C90 (the same engine was used on many models including the CT90):
This must be done when the piston is at the top of its stroke. In this position the woodruff key on the crankshaft will be on the cylinder centreline and pointing in the direction of the cylinder head:
Honda made the holes on the cam and sprocket slightly off centre to prevent the cam being fitted incorrectly. In the timed position the hole in the cam shaft for the dowel that locates the spark advancer will be pointing towards the notch in the cylinder head.
When you are all done the engine is at top dead centre on the compression stroke (ie both valves are shut).
Out of time
That’s all there is to it, but this is still important to know since the timing between cam and crank determines the position of the open valves in relation to the piston and – if the timing is badly wrong – they might collide.
If you aren’t paying attention it is not difficult to get the timing out by one tooth on the cam sprocket: if the error results in the sprocket being rotated clockwise relative to the crank then the the valve timing is retarded (which is to say, the intake valve and exhaust valves will open later), and vice versa when the sprocket is rotated anti-clockwise.
For reasons I failed to understand, retarding the cam to crank timing tends to improve power at higher rpm’s and advancing the timing tends to improve low-end power and throttle response.
Doing so also changes the clearance between the piston and the valves: In the case of this engine there are 30 teeth on the cam sprocket and 15 on the crank
so one tooth out at the cam equates to 1/2 a tooth at the crank, or an error of 48 degrees (360/15*2). ooops, sorry! edit: The crankshaft turns twice for each rotation of the camshaft, moving the piston through four strokes ("suck, squeeze, bang, blow") as it does so. The c90 89cc engine has a 45.6mm stroke and therefore the piston moves a total of 182 mm (45.6 x 4) during this cycle. Therefore, when we are out by one tooth at the camshaft sprocket - which has 30 teeth - the piston is out of position by about 12 mm 6mm (182/30), or 24° (1/15*360) of crankshaft rotation. Despite this, these engines will apparently still run when the cam to crank timing is 'out by a tooth', presumably resulting in them being accidentally ‘tuned’ for either low or top end power.
The next component to consider in cam chain which, if incorrectly maintained, can also cause problems with ignition timing in these bikes: