Mark. III Camies in `53 Shadow - What is right timing?

[Sebastian Huber]

Has anyone ever measured quieting ramps on a Mk III cam? The only Mk III cam I have ever measured up properly was a brand new one lent to me by Colin Jenner. It had just been manufactured by Gary Robinson and it timed up perfectly so I have no reason to think that the cam was in any way defective. There was nothing that could be considered quieting ramps on either the start or end of either of the lift profiles and calculating valve velocity and acceleration also did not show any difference from a Mk I. I have been told, but whether it is correct or just another Vincent myth, that the way in which the quieting ramps were produced was to grind the base circle 15 thou undersized and then to blend the new base circle into the original lift profiles over 20 degrees at the start and end of each cam lobe. Remember that this might be myth but if it is correct then it would not have been a quieting ramp and it would have been an incompetent modification. A proper quieting ramp would have had a slow lead in on the acceleration curve and it was just not there. If anyone has another Mk III cam which they think has a quieting ramp I would be happy to do the measurements again but the one example already measured did not have detectable ramps.

Hello timetraveller,
at the moment I can`t do further investigation on profile of this camy type (need them for driving) but had it my minde. There is an opportunity for me to get it done at Mahr Distribution Office near by with best available technology. Al I can say is, thet there is more freeplay at the point of start point what makes me feel that your right assuming that it`s more an incompetent modification.
Expect to do measuring with ramp/degree profile, jump Inlet to Exhaust and evan CAD image earliest in winter.
Will post results!

Assembled engine and bike and had first ride yesterday afternoon. I´m very pleased with speed, state of sparks a.s.o.
The problem was when I´ve done new push rod (one was stainless steel, three have been aluminium - one failed) I followed advice for timing according Paul Richardson in degree and this guides me wrong. Now I´m back to adjustment what has been before and it works. But don`t ask me what degree wheel shows me.
State of sparks are now ok again, expect I`ve put the breather in 180° wrong! (marks done by myself weren`t perfect)

Thanks again to everyone who helped me on my first posted problem!

Sebastian, the Bavarian
VincentOwner since 54 weeks

 

[timetraveller]

OK Sebastian. One thing to remember is that the lift profiles have to be done on top of the valves and not just from the cams. The reason for this is that the followers are the lever type and give a variable ratio as the cams touch different positions along the base of the followers. If you really have access to some high tech equipment when you do it then you need a digital depth gauge and digital rotary encoder, both able to read out to a computer file. The figures can be put into a spread sheet and the lift profiles, velocity and acceleration curves can be made to appear at the same time. If you do not have access to this type of equipment the you will have to do it the slow way, which is what I do, and use a degree plate on the engine and take lift figures every ten degrees of engine rotation or more if you have the time and patience. If you want to get accurate acceleration curves then you need to have a degree plate that can be read off to better than one degree and a dial gauge able to be read to about one tenth of one thousandth of an inch (about two and a half microns in metric measurements). At home I use a rotary table to rotate the engine so that can be done accurately and it overcomes the problem of the engine trying to rotate under the pressure of the valve springs. If when you get round to it you need any help or advice on setting up the spread sheet just ask.
A week ago when doing Norman Lord's cams for his twin I did start off with what is called 'the rocking method' where one tries to judge equal lifts on the exhaust closing and the inlet opening at 4 to 6 degrees before top dead centre. I would be prepare to concede that I am not gifted at doing this but both cylinders needed the cams advance by about 15 degrees, one tooth on the cam pinion. Not near enough for me.

 

[bmetcalf]

it overcomes the problem of the engine trying to rotate under the pressure of the valve springs. .

Another method would be to use just one spring per valve. The drawback with that for me is the chore of installing/removing the circlips.

 

[davidd]

I built a cam machine to measure Vincent cams, but I have not yet dialed it in for acuracy with my engine. Until I do that I can only comment on the cams relative to each other. I will say that the Harpers MK3 is wildly different from the Harpers MK1. The quieting ramps on the MK3 take so long to close that the readings at .005 are meaningless for timing. I read that Phil Irving chose the .oo5" number for valve timing measurement because the ramps increased at .oo5" per x number of degrees ( I have forgotten the number of degrees). Thus, at .005" he knew that he had hit the ramp.

Most modern cam measuring machines are designed for direct lift off the cam. I do not know of any cam manufacturer that can give you a print out of the opening and closing points on a Vincent because they do not know the geometry of the lifting mecnanisms. They can easily dial in rocker ratios, but that is about it.

David

 

[bmetcalf]

I do not know of any cam manufacturer that can give you a print out of the opening and closing points on a Vincent because they do not know the geometry of the lifting mecnanisms.

I may be remembering a George Emmerich sales pitch from 29 years ago, but I had the impression that the Andrews cams were developed taking the follower geometry into consideration.

 

[Sebastian Huber]

Hi timetraveller. Fine to get to the bottom of problems in this discussion. No question, you are right to do lifting profile on valve end for to get velocity and acceleration curves because of follower transformationration.
But this is at the moment to high for me. For to quote this I would try to get in contact with a guy I know since university. He is working on R&D for cams at BMW his whole life. (but can`t say if he would understand our problems coming out of the fifties)
I think my first step should be to compare profile of lobes on both cams.
It`s not a question of limited skills but of time.
Thanks for practical advises!
Will come back to you when I reached to get more facts!

Sebastian, the Bavarian

 

[timetraveller]

I may be remembering a George Emmerich sales pitch from 29 years ago, but I had the impression that the Andrews cams were developed taking the follower geometry into consideration.

I do not want to seem pedantic Bruce but one could not design a cam without taking into account the follower geometry. In the simplest case which would be a flat based inverted mushroom type then the lift on the cam will translate directly into lift of the follower. Anything else and the follower geometry will affect the valve motion without even taking into account our rockers, the geometry of their ends and whether they are exactly a 1:1 ratio.

 

[ernie]

Another method would be to use just one spring per valve. The drawback with that for me is the chore of installing/removing the circlips.

Remove the tappet adjusters.

 

[timetraveller]

ERRRRR!!!!!! If the tappet adjusters are removed what is one going to measure? Perplexedly yours. Obidiah

 

[bmetcalf]

I was thinking the same, but was responding to David's statement. A less scientific cam grinder could just copy an unworn ex-works cam and assume PEI had it right.

I may be remembering a George Emmerich sales pitch from 29 years ago, but I had the impression that the Andrews cams were developed taking the follower geometry into consideration.

I do not want to seem pedantic Bruce but one could not design a cam without taking into account the follower geometry. In the simplest case which would be a flat based inverted mushroom type then the lift on the cam will translate directly into lift of the follower. Anything else and the follower geometry will affect the valve motion without even taking into account our rockers, the geometry of their ends and whether they are exactly a 1:1 ratio.