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E: Engine Series ‘C’ Comet Camshafts


aciera

Website User
VOC Member
Hello, I'm restoring a Comet of 52, I'm not a mechanic, but in principle I'm doing pretty well.
When I stalled the engine, the rigging did not play, by trial and error I arrived at this result, which seems odd. Exhaust valve opening 46 degree, opening valve injection 29 degree, closing valve exhaust 34 degree, closing valve injection 90 degree. View the photo. I think this camshaft does not match this engine, or it possibly corresponds to a race version?
I bought this bike in 1987 from a person who was shopping.
I think I have to order a new camshaft, MK1 seems to me original? I'm not sure, is someone can help me? thank you in advance
Patrick
 

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timetraveller

Well Known and Active Website User
VOC Member
Hello Patrick, I do not understand some of the figures on your degree wheel. The value of the inlet closing at 90 deg and opening at 36 deg do not seem correct. The opening is about right but the closing should be about 64 deg whether it is a Mk I or a Mk II. If I take your figures of inlet opening, 36 deg, and exhaust closing at 34 deg then that gives an overlap value of 70 deg and that value for a Mk I cam should be in the range of 68 to 75. I think that you have a Mk I cam (or a Mk III which has the same timing but is supposed to have quieting ramps). The racing cam for Grey Flashes and Black Lightings have an overlap value of 105. You should know that over the years there have been some very poor cams made for Vincents and possibly you have one of those. Good luck with the rebuild.
 

aciera

Website User
VOC Member
thanks for the answer, I also think that this cam shaft is not correct, closing the intake valve at 90 degrees is much too late. I'm going to order a new cam shaft.
 

davidd

Well Known and Active Website User
VOC Member
Patrick,

I think you are doing the right thing, but the height at which you measure these openings and closings will cause the numbers to change a large amount. Often when a valve is late closing it is because it is down at 0.001-2", which is effectively closed (although it shows the the cam is not ground correctly.)

If you measure the events at 0.040" and 0.005" it will be easier to identify the character of the cam at the 0.040" and the correctness of the grind at 0.005".

David
 

timetraveller

Well Known and Active Website User
VOC Member
Patrick, David is correct and the figure of measuring at five thou lift is in all the books and instruction sheets on Vincents. If you do not have any of these then you will find it much easier to work on the bike with one of these books to hand. The book by Paul Richardson one is probably the best for a newcomer to Vincents and might be available from the Spares Company or other on line source.
 

aciera

Website User
VOC Member
The book KNOW THY BEST, I have it, the book of Paul Richardson I just ordered it. The problem the English language is not always easy to understand for me.
The measurements, I put two comparators on the valve head, and I start at the top dead center TDC, and two crankshaft lathes. I think it's the right method.
I am waiting to receive Paul Richardson's book before making a decision.
 

vibrac

Well Known and Active Website User
VOC Member
You learn so much on this forum years ago I found out about the valve equally open at 4 degrees before TDC and I have never worried about open and closing points since. Then only last month Bill mentioned the cam 'keyway' pointing between the rear push rods and the front camshaft keyway pointing straight down at TDC Ben actually used that last week to initally set up a new twin motor and when he got down to detail it was almost perfect
25833
copyright BSKspeedworks
 

Bill Thomas

Well Known and Active Website User
VOC Member
I can't be sure ??, But I think on a Comet, That slot should be in a straight line between the pushrod tubes and the middle of the cam spindle, Slot at the top, Like the rear one on Vibrac's photo, With both valves OPEN !!.
TDC ish. Cheers Bill.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Patrick, Can you show us a photo of the Cam and the followers, There are some funny ones around, I have a set with big round lumps on the followers !!. Sometimes there is a number stamped on the inner end of the cam, 1, or 2. Cheers Bill.
 

chankly bore

Well Known and Active Website User
VOC Member
I can't be sure ??, But I think on a Comet, That slot should be in a straight line between the pushrod tubes and the middle of the cam spindle, Slot at the top, Like the rear one on Vibrac's photo, With both valves OPEN !!.
TDC ish. Cheers Bill.
You're right there Bill. The timing diagram in "Richardson" for the Single is wrong. Slot pointing between pushrods at 4 degrees B.T.D.C. is right as long as the cam is original or has been ground to the same profiles using the original slot as the datum. There are, of course, cases where this is not so!
 

oexing

Well Known and Active Website User
VOC Member
You better not rely on any marks on camshafts or cam gears . How can you know if these are still exactly as designed ? It has been written numerous times that the ONLY safe way to find a correct timing is to set even valve lift at overlap and 4 to zero degrees before TDC. So why mess around with questionable numbers of valve timings from old books or records from factory findings like those noted down after assembling Lightnings - which have quite some variations, not because they were in a plan but because they were found when measuring up later.

Vic
 

MartynG

Well Known and Active Website User
VOC Member
thanks for the answer, I also think that this cam shaft is not correct, closing the intake valve at 90 degrees is much too late. I'm going to order a new cam shaft.
Do yourself a favor and DO NOT order a Mk 2 camshaft as they have lots of lift which means you need to make many modifications to the cases and the head to accommodate that extra travel and also they are down on power at moderate revs though they are great for race track work where the revs are much higher. For road work a Mk1 or Mk3 cam is the way to go.

As Bill advised - ignore ALL timing marks and set the valve timing so you have equal lift at 4 degrees BTDC - if you cant get 4 then go towards TDC not away from it, such as 3 BTDC.

Martyn
 

Bill Thomas

Well Known and Active Website User
VOC Member
I should have remembered to tell people to shorten the top of the lower valve guide, With Mk2 cams,
But after all these years, I forget !!, I always use race cams.
Cheers Bill.
 

aciera

Website User
VOC Member
Hello, a photo of the camshaft, and followers. On the cam shaft, I did not find any number,
On the followers the part number ET 29 and C 3.
I bought this bike in 1987 from a man who was racing,
He talked a lot, he told me that on his bike he was riding pistons of FIAT and other editing a little weird.

Patrick
 

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timetraveller

Well Known and Active Website User
VOC Member
Patrick, the picture of the cam is very useful and it certainly is not the Mk II racing cam. However, despite what some of the others have written do not just use this cam and set it up so that there is equal lift at 4 deg BTDC. This is not getting at the experts, it is because you have no idea what this cam is. It could be a perfectly formed Mk I or it could be a total mess. Once you know whether the cam is a normal cam then you can use the equal lift method. Before that you need to find out what the lift profile is. This sounds tedious but in fact can be done quite quickly provided that you can set up a good test system. The ideas is that you should measure the lift on both cams every ten degrees of engine rotation and at least check that the base circle is round. I have seen base circles with a 20 thou dip in them. When you have the measurement plot them on a graph. If you use Excel, or some other spread sheet, then this is very easy. If not then if you send me the measurement by private message (seek instructions on how to do this via the forum or use my email address enw07@btinternet.com if you wish) and I will do it for you and put the resulting graph on the forum here. This way you can find out whether the cam is properly formed and from then on you can use whatever method you find most convenient to do the final timing.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Yes Patrick, They are standard followers, And Cams look like Mk1 or Mk3, I have never seen a Mk3 !.
I get the Piston at TDC, Not on the firing stroke, Or the mag will be timed wrong !!. On the other side of the Camshaft There is a slot, Have that slot upper most, Pointing in between the push rod tubes, Pop the pushrods in, With no adjusters, And turn the Cam by hand, Forwards and backwards, Just by looking, Find the spot where the pushrods are sticking out the most at the same time,
ie the overlap, Then check with your degree plate what the numbers are, The inlet opening number should be a little higher that the exhaust closing number, For best performance !.
Good Luck, Bill.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Just thinking, Don't worry about inlet closing and ex opening, You can't do anything about it because they are welded together, Just think about Inlet opening and ex closing, And as I said the first number should be a small bit more than ex closing.
For a Comet with Mk1 cam, Inlet open 40 ish and ex closing 33 ish.
You can be a lot out with a Vin' and it will still run OK, And as Timetraveller says there are a lot of badly made cams sold, Another reason to do it the overlap way ??.
The Mk3 has ramps to help it run more quiet, But I don't know those numbers.
Cheers Bill.
 

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