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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Comet Valve Timing
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<blockquote data-quote="timetraveller" data-source="post: 23599" data-attributes="member: 456"><p>I do not want to discourage anyone but some of the above remarks should be noted. If the cams have ever been replaced then the marks are not necessarily any use at all. Never trust the marks unless you know the to be correct. I spent seven and a half hours last weekend doing the valve timing on Norman Lord's twin which had had everything removed from the timing side etc. The 'rocking' method is certainly a way to get things about right but as far as I am concerned the only correct way is to measure the valve lift every ten degrees of engine rotation an all four valves (or both valves if it is a Comet) and then to plot graphs which is now easily done with Excel or any other spread sheet. When the graphs are plotted you should mark off the curves where there is five thou lift at the start and end of each lift profile and then see what timing the cams can give. Not all cams time up as they should and you will have to compromise. While you are at it you should also check the base circles. Two of Norman's base circles were not good to a couple of thou and if you set the tappet clearance to zero in the wrong place then the valves concerned could be lifted off their seats for much of the time. I have also seen new cams with a twenty thou flat in the middle of the base circle. Not good for valve, seat or power. With a Comet you should be able to find a happy compromise. Using Norman's twin as an example we could get the rear cylinder just about spot on but with the front cylinder it would have meant pressing the cam out of it's pinion and then reinserting it again a few degrees rotated. It is a touring engine and we left it alone and it should be good enough. When we finished both cam pinions, the large idler and the half time pinion were all engraved so that in future the marks can be used to time this particular set of cams. Remember, seven and a half hours! It is not a job to be rushed but rather one to be done correctly.</p></blockquote><p></p>
[QUOTE="timetraveller, post: 23599, member: 456"] I do not want to discourage anyone but some of the above remarks should be noted. If the cams have ever been replaced then the marks are not necessarily any use at all. Never trust the marks unless you know the to be correct. I spent seven and a half hours last weekend doing the valve timing on Norman Lord's twin which had had everything removed from the timing side etc. The 'rocking' method is certainly a way to get things about right but as far as I am concerned the only correct way is to measure the valve lift every ten degrees of engine rotation an all four valves (or both valves if it is a Comet) and then to plot graphs which is now easily done with Excel or any other spread sheet. When the graphs are plotted you should mark off the curves where there is five thou lift at the start and end of each lift profile and then see what timing the cams can give. Not all cams time up as they should and you will have to compromise. While you are at it you should also check the base circles. Two of Norman's base circles were not good to a couple of thou and if you set the tappet clearance to zero in the wrong place then the valves concerned could be lifted off their seats for much of the time. I have also seen new cams with a twenty thou flat in the middle of the base circle. Not good for valve, seat or power. With a Comet you should be able to find a happy compromise. Using Norman's twin as an example we could get the rear cylinder just about spot on but with the front cylinder it would have meant pressing the cam out of it's pinion and then reinserting it again a few degrees rotated. It is a touring engine and we left it alone and it should be good enough. When we finished both cam pinions, the large idler and the half time pinion were all engraved so that in future the marks can be used to time this particular set of cams. Remember, seven and a half hours! It is not a job to be rushed but rather one to be done correctly. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Comet Valve Timing
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