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alscomet with unmarked timing gears

alscomet

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VOC Member
Have a look at my previous thread, thanks for the two replys. What I could really do with is "how to do the timing setup, with the unmarked pinions" a line by line from start to finish,come on 66 people looked at the thread and only two replys surely someone knows how to do it other than by reading the Richardsons book. its not that clear. alan
 

stumpy lord

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VOC Member
timming vincent cams

Hi ,
Rather than spend ages typing out instructions, it is a lot easier if you have a look at the works instruction sheets[available from the club shop] where you willfind a comprehensive instruction for timming your cams.
hope this helps.
norm
 

Ken Tidswell

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Timing aComet

I assume all you have is a set of timing gears with timing marks , except the half time pinnion. Are you using a dial or dial and guages to measure the lift and opening /closing points ? The instruction sheets say use 4 thou nip on the on the pushrods to eliminate backlash.
What i do is to rotate the engine and establish TDC with a degree disk and a piston stop, set disk in place with the half time pinnion ready to slip on the mainshaft without removing the disk. turn the large idler until the inlet valve starts to open, slip in the half time pinnion , and select a key way the pinnion to give the best fit with the mainshaft and gently tap the key half way in.
now rotate backwards and recheck the opening position. Check overlap at TDC and opening and closing points of both valves, these will vary within the ranges given in PR's book. Check the opening point of the breather, closing is not too important, done
Tap in the key taking care not to score it , Some people then mark the pinnon with a carbide drill, or paint or a sharp punch.
If the timing is out you can withdraw the key and do it again, until you get it right. Ken
 

alscomet

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VOC Member
unmarked pinion

thanks ken for the information,its basically what i have done ,what i must do is make a piston stop to establish TDC properly and also make sure i have half time pinion ready to fit without removing the disc. then check the overlap and adjust.
thanks again. alan
 

timetraveller

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VOC Member
You could do what Mr Boggler suggests but that will only work if you know what cam profiles you have and if they have been made correctly. Many Vincent cams are not accurate. An alternative is to do most of what Mr B suggests but near TDC on the correct stroke accurately determine the lift profiles and arrange it so that the cross over from inlet opening to exhaust closing is about 4 to 6 degrees BTDC. That is degrees of inlet opening larger than degrees of exhaust closing. For my own purposes I always plot the whole lift curve of both cams and then draw a line at five thou. lift across the base of the curves. Determine the timing of these points and then find the best compromise with the figures given in Paul Richardson's book. This way you can overcome some of the deficiencies in Vincent cam profiles. I also suggest that you nip up 5 to 6 thou. before doing all this and check the base circle for circularity. I have seen cams with 20 to 30 degree flats on their base circles and if you set up the tappet clearance on that part you will have trouble later.
 

alscomet

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VOC Member
alscomet unmarked pinion

timetraveller,
Thanks again for the information, I am unsure what cam profile i have , it is a marked cam but the markings on my cam are 4 teeth away from the markings of
a cam which i compared it with, but that should not alter the inlet lift position.
Going back to your positioning of the camshaft when setting the timing how do you overcome the force of the valve springs via the pushrods trying to push the camwheel backwards? or doesnt that happen. alan
 

nkt267

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VOC Member
Thats interesting about the 4 teeth out as peter sprott's ( Suffolk section member)was the same.We sorted that problem out on the phone.He moved the cam 4 teeth,it is still a little out but he says the performance is now 'sparkling' .There is an article on his Comet in Real Classic this month
 

alscomet

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alscomet

Thanks again, i remarked my camwheel 4 teeth as my compared one to give me a starting point.something that should be easy seems to turn out relativly hard.
alan
 

nkt267

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VOC Member
What we did with Peters Comet was to put the degree plate on the drive side, line up TDC and turn the engine till the cam just took up the play in the push rod and took the reading.
Each tooth on the cam wheel = 15degrees. Compare your reading with the 40-42 BTDC.If your timing is way of that then you will have to adjust it.If your inlet opened at say 55-57 deg then you would have to turn the cam wheel anti clockwise 1 tooth.If it opened 25-27 degrees then you would have to turn the cam wheel clockwise 1 tooth.If this makes any sense then it should help you..Peters inlet valve was opening over 60 degrees early.
 

timetraveller

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VOC Member
OK Guys, do it how you want BUT is the cam a Mk I or a Mk II or even one of these 105 degree ones which are about now. (The Mk IIIs which I have measured no more have quieting ramps than I do.) I know that it is time consuming but I still recommend plotting out the whole lift profile of both inlet and exhaust valves and then plotting the numbers up either by hand or in a spread sheet. Many old Vincent cams have a serious flat either at the start or end of the lift and this makes timing difficult. With the full information one can make the best of a compromise on the timing if the cams are not what you think or are poor quality. It takes me about one hour on my own to take a measurement every ten degrees of engine rotation of both lobes on one cam. Nowadays I then always put those numbers into EXEL. I also have set the spread sheet up so that it shows the velocity and acceleration of both cams. That can be rather depressing. I did this years ago for some specials that (an engineer) had made for Roy Robertson with an extra 1 mm of lift. The acceleration was a factor of seven times greater than what is normally allowed and resulted in the valve heads falling off and wrecking the engine. If anyone wants a copy of the spread sheet then I can send them a blank version for them to play about with.
 

Ken Tidswell

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VOC Member
Timing

I forgot to say that you should get ahalf time pinion which has the least backlash against the large idler, before you start, Some sections carry a selection of half time pinions to enable this to be done
 

nkt267

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VOC Member
I agree with time traveller that plotting cam profiles is the best way to go about cam timing.I have rebuilt 3 comet engines in the past 4 years, 2 with Mk2 and 1 with mk3.I found that none of the cams were ground exactly to the figures stated for valve timing and a 'best compromise' setting had to be used.Alscomet seems to be in a bind and needs some practical guidance before he strips the timing case.It may be that he does not have use of dial guages to set his timing 'by the book',and any advise should be seen in this light. Surely there must be a member of his local section that can give some hands on help.
 

alscomet

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VOC Member
alscomet

nkt267,
thanks again for all the effort,what i intend to do later next week is to
take the quill out, fix my timing disc in with a solid pointer, find TDC ignition stroke ,stick the dial guage on turn the engine back, then foreward to see when the inlet valve is just starting to open, then I think I will post a new thread
and ask for all your comments on what i have. alan
 

nkt267

Well Known and Active Website User
VOC Member
Great Idea.Don't forget to nip the adjusters up (refer to earlier post by timetraveller) otherwise you will get the wrong readings.A couple of thou lift on the cam can be quite a few degrees .John
 
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