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Setting valve timing

alscomet

Well Known and Active Forum Website User
VOC Member
I read with interest the article from the Dorset section notes in MPH No741 about Cyril
Malem and his approach to setting his valve timing.Can anyone expand on this and what other implications are involved? Does the Ignition timing have to be altered. If Cyril uses the forum can he comment on how the bike runs after the timing mods ,will it still tick over etc,is it easy to start ,fuel consumption?
I was speaking to Dick Craven recently who runs a Comet sprinter and he also said that he gets a sparkling performance from his comet without any other aids other than a Mark 2 amal carb and a valve timing sequence that defies the normal set up
but he also has to adjust the ignition timing to get the results on his rolling road.He intimated that he may alter the valve timing over a hundered times in a sprint season.
Also will this approach work for any motorcycle engine. Alan
 

John Appleton

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VOC Member
Does he say the valve is opening at .050" lift or some other figure?

How does it end up comparing to the hallowed 4 deg BTDC?
He is saying that the valve opens to whatever clearance exists between valve head and piston pocket, minus 0.050" Obviously this is liable to vary with other factors that differ from engine to engine, and without trying the theory out in practice it difficult to predict the outcome. My first thought is that seems a bit hit and miss (pun intentional) but I will check it out to satisfy my own curiosity.

John
 

timetraveller

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VOC Member
HHHMMMMMM!!!!! I am very suspicious about this. At the end of the day what counts is what happens on a brake, a rolling road or at a sprint meeting where the same rider and conditions prevail. All else is either subjective or due to the rider of the day at the racetrack and the competition etc. Some facts and figures might help. I have looked back through some of my graphs for Mk II and Mk III cams and what I consider to be the relevant figure in this context is the lift in the middle of the overlap, typically at about 4 to 6 degree BTDC. For the Mk IIIs the figure is 0.050" and for the Mk IIs the figure is 0.180". This means that if one were to advance the cam to the max allowed, less 50 thou, then one would have very different timings for the two cams. The situation is a little bit complicated because Cyril seems to put the engine at exactly TDC. Now with the Mk III cam the valve is lifting at about one thou per degree of engine rotation at this stage while with the Mk II it is lifting at slightly more than double that figure. So by doing the adjustment at TDC different cams are going to have yet a further variation in their timing. Therefore John is quite correct to say that this method of timing the cams is going to be variable. The rate of lift, the timing of the cam and the depth of the piston cut out are just the first three things that I can think of which will cause some variation. It might be fair to say that what Cyril is doing is to advance the whole of the cam timing, inlet and exhaust, as much as possible consistent with the inlet valve not hitting the piston. How does this compare with what is regarded as the optimum for valve timing in modern books on the subject? If one looks at the book by John Robinson, (Motorcycle Tuning, Four Strokes, ISBN 0-7506-1805-1) then on page 55 you will see a series of graphs do do with the duration of the inlet valve opening and the engine timing of the inlet valve closing versus engine revs. The latter graph shows power increasing with revs as the closing of the inlet valve becomes later. That is the higher the revs the later the valve should close. If I have understood this correctly, what Cyril is doing is advancing the cam to the maximum amount and thus making the inlet closing point earlier. For a 500cc pot the graphs suggest that a 30º retardation in the time of inlet valve closure will result in maximum power dropping from about 6,000 rpm to about 5,000 rpm. If that is where the maximum power is required then fine. However, looking at one of the other graphs on the same page shows that if one wants a 500cc pot to produce maximum power at 5,000 rpm then the inlet valve opening duration should be 240º. The figure for Vincent cams is more like 280º for Mk IIIs and 303º for Mk IIs. None of this looks like a good way to optimise ones valve timing but if Cyril can actually demonstrate from either rolling road or sprint times that his system produces better results then I could be persuaded that there is much still to be learnt.
 

John Appleton

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VOC Member
Norman, I think that this a badge of honour awarded to skeptics, know-it-alls, dissidents and people secure in the knowledge that we know far better than the lesser mortals.
icon7.gif
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Welcome to the club. I am now going to change my one and fade ointo obscurity.

John
 

Tom Gaynor

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VOC Member
Manx Nortons are (or can be) timed on "valve drop", making sure that there is 0.04" clearance between valve and piston. This is the way the instruction sheets (think Vin instruction sheets, typed on foolscap.) Even if one did NOT time them that way, it would be imprudent not to check. One tooth out on the bevels and the inlet valve will, just, touch. Amazingly, it didn't bend. Don't ask how I know this.
But if the timing IS done by valve drop, then the point of equal lift is - you've guessed - 4 to 6 degrees BTDC. I take that to mean that Norton wanted the maximum possible lift consistent with safety, and the maximum practicable compression ratio, and designed the motor that way. A Norton head has a cr of around 11.5:1. (A Summerfield "bathtub" head has 13.8:1 using the same fuel (and ignition advance) but does have different cams, said to be higher lift.) A standard Vin has a cr of about 7:1...
If at maximum valve drop you had a couple of feet of clearance, Vincents (and Rudges), having combustion chambers like the dome of St Pauls, then obviously there would be far more latitude for altering the overall valve timing. And some good might come of this, in some circumstances.
 
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