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Valve Timing

brian gains

Well Known and Active Website User
VOC Member
It has been pointed out that my Comet , it not being a complete nail, could have a bit more get up and go so I have been reading around the subject and have a couple of initial questions.
was it common with all British m/c manufacturers to use vague timing marks on their gears sufficient for starting and road riding, why not use accurate timing marks at point of assembly or was this a compromise allowing for use of different cam profiles.
My next query may possibly be due to introducing confusion by comparing notes on parallel twin valve timing with Vincent valve timing. Bruce Metcalf on Vincent.Com noted that when considering valve timing ;
" split the overlap at TDC at .050" lift. This means if intake opening is at 0 deg and exhaust closing is at X deg, make the intake opening at 0 + X/2 deg. Then you can retard that for more peak power or advance it for more low down torque".
Is this saying the same thing as when considering parallel twin valve timing and trying to achieve manufacturers specified amount of lift at X deg and where this can't be achieved to the degree, better to err on the inlet opening early than the exhaust closing late.
 

bmetcalf

Well Known and Active Website User
VOC Member
Those was the instructions I got with my Andrews/Megacycle cams. There is an extensive thread here with a lot more information. It is my understanding that any engine can have its cams timed per the method, regardless of cylinder configuration.
 

davidd

Well Known and Active Website User
VOC Member
Andrews did recommend splitting the overlap. I usually think of this in terms of the "cross over point", that is, the point where the exhaust valve and the intake valve are precisely the same height. Vincent put this point at 4 degrees BTD. Andrews recommended 0 degrees, or 4 degrees different. The bikes will run at either position and it may be difficult to note the performance difference. I am not certain that Andrews offered that advice after dyno testing a Vincent. I suspect not. Vincent did dyno testing, but their 4 degree spot was based on running engines that were built to a very standard spec.

I know you can advance or retard that "cross over point" and get different performance as you say. I am not certain that the window of movement is very large. That window will depend on the height of the piston and the lift of the cam. Often, the cross over point is shifted simply to keep the valves from hitting the piston.

David
 

vibrac

Well Known and Active Website User
VOC Member
There is always a compromise on a twin on a single its far easier especially if you work backwards :start by setting the cam and valves,lock that in position then in free rotation set the 4 degrees and finally seeing which keyway on the HT pinion is best match (an easily removed HT key is the best option)
 

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