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#680479 - 01/08/17 2:02 pm cam profiles  
Joined: May 2006
Posts: 148
TriVin Offline
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TriVin  Offline
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Joined: May 2006
Posts: 148
Warrington, Cheshire, UK
Hi

Am playing with my xmas present of a DTI to generate a graph in Excel of my Comet cam profiles. This I have now achieved, after much mucking about, but would like to import it into this site to show more experienced members what I have. I deduce that the valve overlap point is not at 4 deg before TDC bit about 15 deg after!
However, how do I import from excel into here?

cheers


Dave
Tiger 100SS 1966
T120V 1972
Vincent Comet 1952
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#680482 - 01/08/17 2:20 pm Re: cam profiles [Re: TriVin]  
Joined: May 2006
Posts: 148
TriVin Offline
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TriVin  Offline
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Joined: May 2006
Posts: 148
Warrington, Cheshire, UK


has this worked? we shall see

Last edited by TriVin; 01/08/17 2:41 pm.

Dave
Tiger 100SS 1966
T120V 1972
Vincent Comet 1952
#680518 - 01/08/17 7:01 pm Re: cam profiles [Re: TriVin]  
Joined: Aug 2010
Posts: 312
David Dunfey Offline
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David Dunfey  Offline
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Posts: 312
New Hampshire, USA
Well done Dave! It does raise a few questions. This is a MK2 Factory cam, by the looks. How did you establish TDC?

It appears to be retarded 15 degrees or so, which is quite a lot in the wrong direction, under normal circumstances. Typically, the best performance on the Factory dyno set the cam at 4 in advance of TDC. This became the standard for Vincent. In fact, if you have seen the diagram of the Vincent timing marks on the gears all lined up...that is an illustration of the timing marks at 4 BTDC, that is with the EX and IN at equal lift.

On my racer, I set the cam at 7 ATDC, which is retarded, but I do this so the valves do not tangle (my cam has about .5" lift) and I shift at 7000 RPM. From what I understand, this would not be unusual in these circumstances. However, I would not expect this on your bike.

Finally, are you taking valve lift readings off the rocker arms or the valves themselves. Normally, the intake lobe is a little taller than the exhaust, but it may be lost in the measurement.

All the best,

David

#680566 - 01/09/17 6:55 am Re: cam profiles [Re: David Dunfey]  
Joined: May 2006
Posts: 148
TriVin Offline
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TriVin  Offline
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Joined: May 2006
Posts: 148
Warrington, Cheshire, UK
Hi

I found TDC using a piston stop made from an old plug. I took the readings at the tappet end since I just wanted to try out my new DTI without having to remove the UFM.

How can you tell that they are MK II cams? I thought MK II's were for racing? Are the profiles so different that you tell easily from looking at the graph?

I shall repeat the exercise just in case I did something wrong.
Interesting that you say that the illustration of the timing marks in the books is at 4 deg BTDC. I had always assumed it was at TDC. I don't know why I thought that, just did.

If the second set of measurements are the same I shall take the cam out assuming it has rotated on the shaft. If I get it welded up do you weld all round or are say, 3 spot welds sufficient normally?

Last edited by TriVin; 01/09/17 7:00 am.

Dave
Tiger 100SS 1966
T120V 1972
Vincent Comet 1952
#680706 - 01/10/17 12:36 am Re: cam profiles [Re: TriVin]  
Joined: Aug 2010
Posts: 312
David Dunfey Offline
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David Dunfey  Offline
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Joined: Aug 2010
Posts: 312
New Hampshire, USA
Well, I should have been able to tell by looking at the graph, but it turns out that upon closer examination it appears to be a Mk3. I was thrown a little by the lift, which is a little higher than MK3's, which are normally at .330" and yours look closer to Mk2's at .365, but I see that they are more like .350". I also did not recognize the opening and closing points, but that may have been due to my not accounting for the 15 of retard.

The Mk3 is a 30 44 44 30 cam.
The Mk2 is a 37 51 51 37 cam.

That is, IO IC EO and EC in that order marking Intake and Ex opening and closings. These would be target numbers and often are not hit exactly.

It is probably easier for you to read and note the opening and closing events with the original at full size. If you can move your X scale you can see what the cam does at 4.

Before you remove the cam I would ask a few more questions. Often the the cam ID is marked in the grove in the cam itself. There might be a "1" or 2 or 3 marked in the groove at the center of the cam.

Do you know who made the cam or how new it is?

The cam can be re indexed before you remove it. This might show that it did not spin in the pinion, but it may have started out in the wrong position. If it is set up with the marks, but fails to measure correctly, then it is likely it shifted.

The reason for shifting is usually seizure of the cam bushings. This can be momentary, so it can grab and let go. This is a certainty with a MegaCycle cam as they install an incorrect cam bushing, which seizes.

It is also possible that the cam was pushed into the pinion in the wrong position, but set up or reset according to the marks by mistake. I never set the timing according to the marks, for this reason alone. This is a good exercise that you may never want to repeat!

I do have Excel graphs of the cams I have timed. I will try to PM my email.

David

#680934 - 01/11/17 3:49 pm Re: cam profiles [Re: David Dunfey]  
Joined: May 2006
Posts: 148
TriVin Offline
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TriVin  Offline
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Joined: May 2006
Posts: 148
Warrington, Cheshire, UK


Well, there is no marking in the keyway to identify which cam it is.

Reading the opening and closing points off the graph seem to be....
IO (BTDC) 33
IC (ABDC) 77
EO (BBDC) 75
EC (ATDC) 45

Which are nothing like the figures you give. Have I read from a different datum than you? I think I must have.

I have no idea of the age of or who supplied the cam.

I took the photo of the cam pinion, printed it off and measured the angle between the centre line of slot and the 2 dots and found that it appears to be as standard (197 deg 27 mins) as I have just found out from reading the Instruction sheets. Not really accurate but also not obviously moved. So maybe it was just installed incorrectly.

I took the photo at 4 deg BTDC. So what is the best way to get it right now? The way I figure it the cam pinion has to rotate clockwise to get it correct. So if I turn the half time pinion clockwise for say 1 tooth then remove the HT pinion, wind the engine back to 4 deg BTDC and re insert the HT pinion it should be about right?
Or is there an easier way?

thanks


Dave
Tiger 100SS 1966
T120V 1972
Vincent Comet 1952
#680976 - 01/11/17 9:46 pm Re: cam profiles [Re: TriVin]  
Joined: Aug 2010
Posts: 312
David Dunfey Offline
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David Dunfey  Offline
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Joined: Aug 2010
Posts: 312
New Hampshire, USA
Dave,

I think you have done a nice job with this. Cam timing is always tricky to keep straight. I suspect that we are not on the same page regarding the timing events. There are a few different conventions for marking the openings and closings. Vincent measured the events at .005" lift. So, when reading their literature you will find them quoting degrees at very small lifts. The modern standard is .040" or .050". Many hot rodders use .040",but Megacycles used .050" in their listings. To compare the events I listed you would need to read your openings and closings at .050" lifts.

Additionally, if you are retarded a tooth or several teeth, then the readings will show somewhat odd openings in degrees and you would have to subtract the retardation in degrees from some and add it to others. That was what I was looking for in your numbers, but I am having trouble seeing where we are.

This is not a worry because you now know your cam is somewhere near correct and it is just a matter of lining up the marks. I suspect once it is nearer the mark it the numbers will fall in line and it will be easier to dial in.

The cam looks well lined up and if you undo the pushrod adjusters to let the pressure of the springs and cam while holding the cam pinion in this position, you can line up the other pinions so the marks are matched.

When the adjusters are done back up you can check the timing to double check that everything is OK.

Photobucket is too slow for me to post photos here. My email is in a PM to you and I can send you some graphs and photos. I would also like to know more about your DTI.

David

#681011 - 01/12/17 10:27 am Re: cam profiles [Re: David Dunfey]  
Joined: May 2006
Posts: 148
TriVin Offline
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TriVin  Offline
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Joined: May 2006
Posts: 148
Warrington, Cheshire, UK
Yes, I was reading off my graph at about the 0.005" level which is difficult so to read off at 0.050 would definitely make more sense.

My DTI and mag base is a cheap one off eBay, not a Starret or Moore & Wright one I'm afraid. I dread to think what quality metrology stuff would cost these days.

As you suggest, I might just as well follow the diagram in Richardson at this point and leave the crank at 4 BTDC. I assume that the keyway in the back of the cam is used in the grinding of the lobes as a location datum so when at it is positioned at 4 deg it would be pointing slightly right of centre of a theoretical centre line between the two push rod tubes. Looking at mine at the moment it appears to be pointing slightly to the left of it.

What in your opinion would the error of the cam position in my case have had in performance? The bike has always been easy to start with a tendency to kick back sometimes. But since I only use it for pottering about the countryside what would I expect to experience performance wise? I almost never go on motorways with it but when I have I wont exceed 70mph anyway! It has always had an erratic tickover but whether that is due to the valve overlap, carburation
or ignition I know not. One thing at a time!

I have received your PM by the way. I'll email you directly with mine.

thanks


Dave
Tiger 100SS 1966
T120V 1972
Vincent Comet 1952
#681029 - 01/12/17 11:53 am Re: cam profiles [Re: TriVin]  
Joined: Aug 2010
Posts: 312
David Dunfey Offline
BritBike Forum member
David Dunfey  Offline
BritBike Forum member

Joined: Aug 2010
Posts: 312
New Hampshire, USA
Dave,

I have not run a cam that retarded, so it is a little hard for me to know what it is doing to the performance. My guess is that it is not doing a whole lot of harm. Retarding usually helps high speed running and because you are not running at high revs you are probably not getting much advantage for the the trade off of losing a little torque. My guess is that you will find a little more torque with your timing corrected.

Many Comets top out in the high 70 mph range. My racer tops out at 101-103 mph. That is what I gear for on a straight.

The Factory chose 4 because it proved best on the dyno. When you change some of the specs on the engine it would change that recommended setting. If the engine is stock, it should do well at 4. You will find setting it at TDC or 8 will not have too big an effect. It may show up on the dyno, but in terms of running, it would be hard to tell the difference.

I would hope that having the cam in the right spot, timing the ignition correctly and having the carb in good tune would give you all the performance and reliability you need. On of the benefit of having data is that you can compare the data in the future to make sure things have not changed.

You are correct that one change at a time is the best way to go.

David


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