Comet Valve Timing

[Hugo Myatt]

Whilst researching every possible reason for Phil's starting difficulties with his Comet I discovered the following. (It was not relevant to Phil's bike.) I believe it was once known in the club but seems to have got lost in the mists of time. The Comet valve timing diagram in Richardson page 120 and the BMS book is incorrect in so far as the relationship of datum slot in the camshaft to the double dot on the pinion is concerned. The Instructions sheets state "On the 500 c.c. machines the toothspace marked by two dots should be 197degrees 27' in advance of the centre line of the camshaft slot in a clockwise direction, viewed as indicated above." i.e. from the pinion end. This is clearly not the case in the Comet diagram. It appears the draughtsman erroneously copied the cam pinion from the front camshaft of the twin diagram on page 115 in Richardson. But as we all know the Comet uses the rear camshaft of the twin. When the double dot marks on the Comet pinion are transposed to the rear camshaft pinion on the twin diagram the relationship with the datum slot becomes correct. This is, of course academic, unless the timing marks have been lost or the cam has been removed and replaced in the pinion. Or am I talking twaddle?

 

[chankly bore]

You are correct. With an unknown pinion it is best to set the crankshaft at 4 degrees BTDC on the exhaust stroke and have the pushrods "rocking" at equal lift. The slot will be pointing up at about one o'clock, sort of between the pushrod tubes if you see what I mean. This works with all known cams as far as I know. If you have Mark 3 cams, you can set them at lift at TDC as per Richardson.