Comet timing


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Non-VOC Member
I’ve had to start a new thread on this subject 'cos I 'aint been a VOC member for donkeys years but I have owned the same Comet for 45.

The linear movement of the piston near TDC is very small so accuracy cannot be guaranteed, far better to use a longer probe through the plug hole of sufficient length to reach down the bore to approximately half stroke where the piston movement per degree is at it greatest. Used in conjunction with a timing disc and by rotating the engine clockwise and anticlockwise until the piston contacts the probe an accurate determination of TDC is easily arrived at by simply halving the difference on the timing disc. This point is then your datum from which all other angles are determined or set.

Also the half time pinion, (ET49), because of the mismatch between the number of teeth and the number of splines, if rotated on the mainshaft results in small angular changes in tooth position relative to the mainshaft. This then advances or retards the rest of the train and thus the position of the cam relative to TDC may be refined. This procedure will probably require moving the cam pinion a tooth plus or minus also.

This is where the single has an advantage over the twin, for the single it matters little where the cam is located in it’s pinion whereas the twin’s cams are effectively tied together by the large idler and must thus be correctly located relative to a tooth on their respective pinions. Optimising the cams on a twin in this way is still possible although some compromises may be required along with some mental gymnastics.


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VOC Member
I did a full mapping of the mark 3 cams in my nephews Comet today as I felt that it lacked a bit of lustre in the performance department.
I found that the lift timing and static timing,inlet relative to exhaust,did not match the stated figures.
I found that the inlet opening was near enough correct and the exhaust closing was right,but inlet closing was 10deg late and the exhaust closing correct.
The lift at TDC was inlet opening 0.80in(late) exhaust closing 0.56(early)
BDC was inlet closing 0.210(late) exhaust opening 0.225(ok-ish)

I know that it is better to have the inlet opening early rather than the exhaust close late,but any ideas as to the best compromise.
My gut feeling is to leave it alone as I am comparing it to my Comet that has standard compression ratio with mark2 cams,which also do not match the stated figures but I compromised on the timing marks and the performance is fine.:D