Honda C90 – Cam to crank timing

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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:

timing mark and index notch in cylinder head (I’ve turned the cam shaft out of the way so the timing mark is visible)

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:

cam to crank timing

Or, If the rotor and stator are already installed, when the the “T” (TDC) mark on the rotor is aligned with the pointer on the stator

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 of the compression stroke (ie both valves are shut). That’s all there is to it, but this is still important stuff 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 2 teeth at the crank, or an error of 48 degrees (350/15*2). The piston is therefore out of position in relation to the valves to the same degree, which is to say the piston will be advanced or retarded by 48 out of the 180 degrees the crank must rotate to move the piston one stroke. In the case of the c90 89cc engine, which has a 45.6mm stroke, this equals about 12 mm. Despite this, these engines will apparently still run when timed incorrectly like this, presumably resulting in them being accidentally ‘tuned’ for either low or top end power.

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