I do not want to discourage anyone but some of the above remarks should be noted. If the cams have ever been replaced then the marks are not necessarily any use at all. Never trust the marks unless you know the to be correct. I spent seven and a half hours last weekend doing the valve timing on Norman Lord's twin which had had everything removed from the timing side etc. The 'rocking' method is certainly a way to get things about right but as far as I am concerned the only correct way is to measure the valve lift every ten degrees of engine rotation an all four valves (or both valves if it is a Comet) and then to plot graphs which is now easily done with Excel or any other spread sheet. When the graphs are plotted you should mark off the curves where there is five thou lift at the start and end of each lift profile and then see what timing the cams can give. Not all cams time up as they should and you will have to compromise. While you are at it you should also check the base circles. Two of Norman's base circles were not good to a couple of thou and if you set the tappet clearance to zero in the wrong place then the valves concerned could be lifted off their seats for much of the time. I have also seen new cams with a twenty thou flat in the middle of the base circle. Not good for valve, seat or power. With a Comet you should be able to find a happy compromise. Using Norman's twin as an example we could get the rear cylinder just about spot on but with the front cylinder it would have meant pressing the cam out of it's pinion and then reinserting it again a few degrees rotated. It is a touring engine and we left it alone and it should be good enough. When we finished both cam pinions, the large idler and the half time pinion were all engraved so that in future the marks can be used to time this particular set of cams. Remember, seven and a half hours! It is not a job to be rushed but rather one to be done correctly.
Comet Valve Timing
- Thread starter redbloke1956
- Start date
Hi Guys, just a little feedback so far....I heated the cases with a hot air gun for 2 hours or so and then spent 30 minutes locally heating around spindles with a butane torch, i removed the spindles with a slide hammer and it still took some effort with all that heat.
I cooled the new spindles in the freezer overnight, reheated cases and inserted them using the same slide hammer and loctite.
I purchased a number of 1/2 time pinions and found that a +.004" removed just about all backlash, I am now fitting the Alton alternator and the B-TH mag before I have a crack at setting up the timing of the gears, also replaced the cams, cam followers small idler, large idler and large idler spindle.....if the missus ever discovers how much I have spent on this project so far she is going to want a new kitchen.
will keep you posted, thanks for all advice and feedback so far guys.
I cooled the new spindles in the freezer overnight, reheated cases and inserted them using the same slide hammer and loctite.
I purchased a number of 1/2 time pinions and found that a +.004" removed just about all backlash, I am now fitting the Alton alternator and the B-TH mag before I have a crack at setting up the timing of the gears, also replaced the cams, cam followers small idler, large idler and large idler spindle.....if the missus ever discovers how much I have spent on this project so far she is going to want a new kitchen.
will keep you posted, thanks for all advice and feedback so far guys.
I have to say I agree with timetraveller in some respects,I changed my comet cam for a spare mk3 I had and it felt totally different, nowhere near as good.When I put the old cam back and checked the opening and closing it timed up like a grey flash inlet with a standard exhaust! So you have to know what you have before you can get it right,bearing in mind the factory figures are only a guide as even the original cams can vary several degrees.
I have had my Comet for 55 years and a while ago thought that the marks on my gears were incorrect because the performance had always been rather flat. I moved the cam timing one whole tooth on the crankshaft pinion, to get the valves with roughly eqal lift at TDC, which really set the valve lift equal about 5deg before TDC. One tooth of the crank pinion means 15degrees change and I could not detect any change in performance at all !!!. So the valve timing can not be very critical on my "fluffy" old Comet which I cruise at 60-70 on the motorways. The lift of the valves is about right at 0.33/0.34 inches, so the cams are not very worn.
However setting the timing back to the marks on the timing gears (all original from new but with a new big idler) ie. back to how it was when I bought it in 1956, puts the valve timing so that the valves have equal lift at about 20deg before TDC which I believe is about right for Mk III cams.
Have just done 1100 miles last week to and from the Manx rally with no problems for about the 36th time so there can not be too much wrong - perhaps the poor performance is the reason for its great reliability?
Matty
However setting the timing back to the marks on the timing gears (all original from new but with a new big idler) ie. back to how it was when I bought it in 1956, puts the valve timing so that the valves have equal lift at about 20deg before TDC which I believe is about right for Mk III cams.
Have just done 1100 miles last week to and from the Manx rally with no problems for about the 36th time so there can not be too much wrong - perhaps the poor performance is the reason for its great reliability?
Matty
Now divide that 15 degrees by five, and you will find you can play around a lot more yet.
Thanks - I realise that there is a "vernier" adjustment for valve timing by moving the key in the half-time pinion, but if a change of 15degrees does not seem to make a difference from the old factory settings it does not seem worth playing around with 3degree changes.
However I suppose that if a move of 15 degrees does not seem to make a change, perhaps the "sweet spot " is half way between and an error either way of this optimum setting degrades the performance by an equal amount.
Mine seems to have worked OK on the old factory settings for my purposes for 55years and it did around 80mpg on the last trip to the Isle of Man, so I think I will live with it as it is - maybe all I need is a lot of time and a dyno to find the real answer!!
Matty
However I suppose that if a move of 15 degrees does not seem to make a change, perhaps the "sweet spot " is half way between and an error either way of this optimum setting degrades the performance by an equal amount.
Mine seems to have worked OK on the old factory settings for my purposes for 55years and it did around 80mpg on the last trip to the Isle of Man, so I think I will live with it as it is - maybe all I need is a lot of time and a dyno to find the real answer!!
Matty
Hi - 15degrees is what you get by moving the mesh of the half-time pinion one tooth with respect to the idler - But what I am trying to say is that this large change seemed to make no difference at all to the performance of my Comet that I could tell.
So it now runs with the factory settings again instead of roughly equal valve lift at TDC which is what resulted from moving the mesh of the half time pinion one tooth.
So playing around with much smaller increments by moving the key on the half-time pinion would seem to be a pointless exercise because the valve timing does not seem to be very critical.
I had been through a big exercise on this subject a while ago on the forum - plotting valve lift profiles against crank angle etc. This was in an attempt to find why my bike was not very "pokey" - but all seemed to be OK technically with my set-up. For some reason which has not been resolved, some people with MkIII cams claim they can reach 90mph and cruise at 80 while mine and many other Comets I know struggle to get much over 75mphon a level road.
As I said before the lift on my cams is good and I do not think they are significantly worn.
If anybody has an answer I would be delighted to know.
So it now runs with the factory settings again instead of roughly equal valve lift at TDC which is what resulted from moving the mesh of the half time pinion one tooth.
So playing around with much smaller increments by moving the key on the half-time pinion would seem to be a pointless exercise because the valve timing does not seem to be very critical.
I had been through a big exercise on this subject a while ago on the forum - plotting valve lift profiles against crank angle etc. This was in an attempt to find why my bike was not very "pokey" - but all seemed to be OK technically with my set-up. For some reason which has not been resolved, some people with MkIII cams claim they can reach 90mph and cruise at 80 while mine and many other Comets I know struggle to get much over 75mphon a level road.
As I said before the lift on my cams is good and I do not think they are significantly worn.
If anybody has an answer I would be delighted to know.
With Mk2 cams and standard compression I can get about 80/85mph from my Comet but this is wringing it's neck. It will cruise 60/70 1 or 2 up and get over 90
coming down by Kates Cottage on the Island.
I found during my recent strip down that the gearbox sprocket was only a 17 tooth, this may be a small factor with the top speed, but acceleration is pretty good.
I found no better performance with my first Comet in the late 60's.
Are 90+mph bikes fitted with a more optomistic speedometers ??;-);-)..john
coming down by Kates Cottage on the Island.I found during my recent strip down that the gearbox sprocket was only a 17 tooth, this may be a small factor with the top speed, but acceleration is pretty good.
I found no better performance with my first Comet in the late 60's.
Are 90+mph bikes fitted with a more optomistic speedometers ??;-);-)..john
I've tried all the so called shortcuts, from propane brush burners to blow torches..... Nothing comes even close to popping the cases into the kitchen oven when wifey is out for the day. Your job is twice as easy with a Comet, you might not even have to remove the crankcase studs.
Cheers, John
Cheers, John
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