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Vincent Comet Timing

Matty

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VOC Member
The basic query I have is about setting up the timing gears as shown by the diagram on page 120, Fig 59 in the Paul Richardson book.

Is the engine at TDC for this diagram?

The reason I ask is that I have had the bike for 52 years and used to set it as the book and have no reason to think that the crankshaft pinion has ever been changed. Some years ago I found that the dot on my crank pinion was not in this book position at TDC and in fact was one tooth different.

Because the performance was not quite as good as I would like I therefore set up the timing with the engine at TDC with the other gears as the book - ie. one tooth different.

There was little or no difference in the performance (I did also reset the ignition timing)!!!!

The cams are I believe correctly marked with equal valve lift on the overlap with the dots lined up on the cam gears in line with the spindle centres.

I have driven the Comet to Liverpool for the Manx GP and Vintage Rally several times now (about 1200 miles each time) with my TDC timing and it will cruise at 60-70 mph fairly happily and ticks over very slowly and evenly.

However it seems a bit gutless up the mountain (down to 35-40 in 2nd gear after the Gooseneck) and I wonder if I should reset it to the book with the crank pinion one tooth away from TDC.

The cams do not look worn, but I seem to remember it went much better 48 years ago when I went 2up to Paris, Down the Rhine Vally to Austria, then round the Dolomites, over the Grossglockner Pass and back home. - Perhaps I was lighter then (but not much) and I seem to remember cruising at 80mph down the Autobahns with little effort and it pulling very well up the mountains. It also has a slightly higher compression ratio now (from 6.8 to 7.3) so I would expect it to be EVEN MORE POWERFUL !!

Can anyone advise on the correct method of timing please because the books I have do not specify the cam settings with respect to a timing disc, TDC etc. only lining up dots which could have been mismarked by the factory in 1952.
 

alscomet

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VOC Member
Comet Timing

What I wouldnt do is disturb the timing gears without first installing a timing disc and dial gauge and writing down when the inlet and exhaust valves open and close.I went through this process with unmarked timing gears and the marks ended up 3 teeth away from their designated position.
Search on Unmarked timing gear on the VOC site also lots of Ideas come up
and all have good information and tips in them.Then after you have the information put another posting on as the problem will be more evident to the
experts. al
 

Matty

Well Known and Active Website User
VOC Member
Thanks, but where can I find the technical information about the cams which I need in order to set the position with a dial gague and disc please - it is not in any of the books I have.

The bike has done about 70,000 miles and I rebuilt the engine last year with bearings, new liner and piston, and steel main idler. This is the first rebuild of the bottom end since I first did it in 1959, but the timing gears are otherwise original and marked ex-factory.

The question remains however about the diagram in the books - ie is this diagram for the engine at top dead centre or as the gears are marked for correct timing ex factory? - the camshaft marking is certainly where I would expect for the engine at TDC.

Matty
 

pifinch

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VOC Member
I dont have the book with me but I'm sure the technical section of Richardsons book has valve opening/closing detailsin it.
Steve
 

alscomet

Well Known and Active Website User
VOC Member
comet timing

what i would do is
1,make a top dead centre stop out of a used sparkplug and 1/4" rod so that
TDC can be accurately fixed.
2,fix all your pulleys as detailed in Richardsons book p107 leaving the half time pulley out
3, put a dial gauge onto inlet valve and zero
4, turn your engine back beyond44deg btdc and then forward to 44deg BTDC
5, at this point inlet valve should be just opening
6, Install your half time pinion with a temp key that you can get out easy
using its original mark which should be ok
7, turn the engine and see at what degrees the inlet is opening but doent keep turning over and over just the once.
8 if its not opening at the correct time turn back and adjust the position of the half time pinion until it opens correctly.
9 then check the rest of the readings to see if the valves are doing what they should.
Thats how i would do it but i am open to suggestions.al
 
Last edited:

Graham Smith

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VOC Forum Administrator
VOC Forum Moderator
The basic query I have is about setting up the timing gears as shown by the diagram on page 120, Fig 59 in the Paul Richardson book.


Which edition of the Richardson book have you got?

The reason I ask, is because you may be interested in a paragraph from The Technical Officer's report in September's MPH (yet to be posted).

It has come to my attention through a Member, that the second edition of Paul Richardson's Vincent Motorcycles contains an error in one of the illustrations.

This error was put right in the third edition. The error is quite serious; the illustration appears on page 120 of the second edition, and depicts the timing gear markiongs for 500cc models; it depicts ERRONEOUSLY the camshaft marking as being 17 degrees 25 minutes in advance, clockwise, of the camshaft slot. The third edition depicts CORRECTLY the camshaft marking as being 197 degrees 25 minutes in advance, clockwise of the camshaft slot. If you timed a machine according to the second edition, it would never fire!
 
Last edited by a moderator:

alscomet

Well Known and Active Website User
VOC Member
comet timing

Yes the second edition i have in front of me and the camshaft is shown differently to the third edition but that doesnt matter if the breather is in the right place you can get rid of all the timing marks as long as the inlet valve opens at the right time btdc by readjusting the half time pinionto suit.al
 

Matty

Well Known and Active Website User
VOC Member
Thanks to all for the advice - in fact I feel rather stupid, because I failed to notice that the information I needed on valve timing is given on page 117 of Paul Richardson's book. My version has no edition number by the way and I bought it in 1956 with the bike.

I had missed this because pages 115-118 are in the 1000 cc part of the text which I had never read, assuming they did not apply to the 500cc Comet. (I am an Engineer after all and so hardly ever read instructions in any case!!!!)

With your help however and the cam profile figures given I can now easily sort out my timing, but it will be interesting to see how the marks on the crank pinion line up with the correct cam positions.

Thanks again.

Matty
 

Matty

Well Known and Active Website User
VOC Member
Timing OK

Checked my timing measuring valve lift with clock gagues etc. and found that with the engine at TDC I had (within small limits) equal lift on the exhaust and inlet valves.

This does not conform to the data on page 117 of Paul Richardson's book.

So have reset the valve timing "to the book" and it then agrees with the timing marks on the gearwheels. This was not too easy to check because I suspect the cam profiles are not perfect and one is measuring "up a ramp" so a small change in angle will produce quite a large change in the lift.

So. The diagram on page 120 is NOT shown at TDC but 15 Degrees (one tooth of the halftime pinion) away from TDC.

Note :- where the diagram is positioned the valves will have roughly equal lift.

The bike is running again and I have not even had to adjust the tickover - will see later if ther is any change in the higher end performance.

Matty
 

timetraveller

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VOC Member
If when you road test the bike you are happy with the performance then leave well alone. If not then do the job properly. Assuming that you have either MkI or Mk III cams then you can check for wear as the lift on both inlet and exhaust should be about 330 - 340 thou. Significantly less than this and the cams are worn. Then as to the timing. You do need to ascertain TDC properly, not just 'about'. Accuracy of one degree is quite reasonable. Then, at TDC, there should be more lift on the inlet than on the exhaust. The reason for this is that the 'degrees of inlet opening should be greater than the degrees of exhasut closing'. It says so in the book. Depending upon the cams and whether the followers are still flat or have worn concave you should have 10 - 20 thou more lift on the inlet than on the exhasut at TDC. The correct way to do all this is to remove the oil tank and front end and then use the DTI on the top of the valves and then take a reading all the way round each lift profile at 10 degrees of engine rotationi .e. 5 degrees of cam rotation. Check the base circles first by nipping up the tappets by about 20 thou and leave the tappets tight for doing the lift profiles. When you have the measurements plot them on a graph, a spread sheet makes it easy, and then draw a line at 5 thou lift. That is the figure at which Vincent valve timing is given. Work out what is the best compromise for your particular cam profiles as many Vincent cams are of really poor quality when it comes to the profiles. Your cam and follower hardening must have been right for them to have lasted so long. Pain in the posterior I know to have to do all this but if you are proper engineer the you know that you have to do things correctly otherwise your customers come back with the items you have made and threaten to use them as suppositories on you. Good luck.
 

Matty

Well Known and Active Website User
VOC Member
Thanks Timetraveller for the informaton. I have 340thou or so lift on both valves, so this looks OK and have used a tool which screws into the plug hole to find TDC accurately.

When the rain stops I will check out the performance - but perhaps I expect too much now because even at my age (72) I ride a Honda VFR750, and my son's RD350 LC Yamaha (headbanger job) and Fireblade regularly!!!

However after 47 years as a Professional Electrical, Electronic and Mechanical Engineer (Now long retired) my approach is for pragmatism rather than perfection which is I believe a very expensive option - in other words the old Comet is probably "good enough" and cost effective for what I want.

Matty
 

Matty

Well Known and Active Website User
VOC Member
Have tried out the Comet and it is a little improved in performance with the crank moved 15 degrees and seems to rev more easily with a bit more torque out of corners.

The main change seems to be that the engine seems smoother at higher revs with less vibration and harshness - I do not really understand this because the dynamic balance has not been changed !! Perhaps it is some complicated effect of the combustion process or gas flow. However the change is not really very great and air temperature, humidity etc. sometimes seem to produce simular effects.

Hope this little investigation may help others with their cam timing knowing that the diagram on page 120 is NOT at TDC but 15 degrees away from it.

Matty
 

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