I am working on an Atlas bought by a friend of mine recently. Deciding that the cam chain and adjuster could do with replacing I discovered that it has been running with only nine rollers between the sprockets rather than ten. Also that the intermediate gear is set one tooth behind that of the crankshaft pinion.
Looking into this I came across various sources that talk about fine tuning of cam timing, and so have been wondering at the likelihood of the timing having been deliberately modified by someone who really knows his stuff - or whether it's just someone's mistake?
I believe the calculations associated with the way it was set, show that the cam is running 40° advanced on the sprocket, and retarded 15° on the intermediate gear. The net result would be a 25° advance on the cam - which seems like a lot, probably too much to be deliberate?
However I also came across info about a period in Norton cam manufacture when their machinery was producing cams that were accurate as far as lobe position relationships were concerned, but not in relation to the drive position.
My questions are:
Whether it's possible this is one of those inaccurately made cams, in which case could they really have been made that far out?
Is it reasonably safe to assume that this set up is a P/O mistake?
Are there any other likely explanations?
The Atlas seemed to be essentially running ok, though since buying it, it has only been ridden about 8 miles along local lanes. It starts very well and seemed fair enough on the road, though I was a bit surprised by a sense of it having a bit of a 'soft' pull, which I had largely been putting down to carburation which I can't seem to settle properly at tick over without the choke being on?
Essentially I'm trying to avoid having to go through the process of fully checking the cam timings unless I really need to, so any thoughts would be appreciated?
OK, this is experience from…
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I read that if you set up so…
I read that if you set up so valves have equal lift on Exhaust stroke at about5 degrees before Top dead center it will run well ? not tried it though !! ,and you will need a couple of dial guages .
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An Atlas once....
I had an Atlas once that did all the things an Atlas does (as far as performance) but another person fitted it up with the cam chain out-started ran ok until you said 'let's go' then it showed up that it had lost its 'umph' big time.
I would agree, take note how it is set now. Reset it by the book and see what happens.
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off set keys
.. can be got for the Commando if you get into fine tuning of cam timing.
from JS Norton, California.
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Measure current lift
With the set up as delivered. Measure lift height and map the lift every ten degrees over the two revolutions of the crank and keep it for reference in the future. Then do the same with the new configuration. You will have two diagrams you can compare. You may see some benefits, or a lucky mistake.
Jon
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While you are there....
While you are there measuring lift 'every 10 degrees' you might checkout to see if the 'rocker advantage/pushrod' and valve lengths are correct!
On an Atlas this will be ostensible the same as the 'Commando' but of course the Commando has had compression changes, pushrod length changes, valve length changes etc so measuring these items is very time consuming, quite hard to understand, and then if you are sure it is incorrect, 'how do you put it right?'
Has any one done this for the Commando?
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John Hudson
The late John Hudson wrote many times about how poor the post-Bracebridge camshafts were. Citing that the lobes were no longer cut with any consistency in respect of position and also that the hardening process was leaving a soft surface that failed very quickly.
He also advised on how to change the camshaft timing by re-meshing the half-time pinion dots in a different position and also adjusting the cam chain.
An example of his tuning advice can be found in British Bike Mechanics, the September 1988 Issue 12. Plus in the 1996 NOC Owners Handbook p17.
Or checkout the attachments.
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Atlas camshaft timing - Thanks and update
Thank you, all of you, for your advice.
First, my apologies for not replying before this. Life took over in other directions for a while, and the issue had to take a back seat.
When I got back to it, I decided that the variance as it was set from standard was 25 degrees was unlikely to have been deliberate, unless P/O knew something was really quite odd about this camshaft.
I didn't really have time or inclination to clock up the exact timings, so I returned the intermediate gear and the cam sprocket to standard settings, and checked the timings roughly by eye. I needed to use a disk to set the magneto timing anyway, so I figured that if there was something so significantly wrong with the cam it would be obvious enough to see.
Without taking fine measurements, it all checked out much as it seemed it should so I put it all together, and except for not being quite so easy to start it now runs absolutely beautifully.
I’m not sure why it’s a bit reluctant to start from cold now though. It fires easily, just once almost every kick but takes a while to properly start on the pedal. It doesn’t seem to particularly want much choke now, whereas it did before. I’m inclined to think this probably has more to do with ignition timing, or carburation or maybe just the a weak plug, than the cam timing - because once it’s going it’s just lovely!
Thanks again, all of you.
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OK, this is experience from a Commando, but when mine was inadvertently set up one roller out, it started and ran ok but was 'soft' as you say.
If you go one roller the other way, you will find the piston tapping the valves. Don't ask me how I know.
I would be tempted to carefully set it up 'by the book', and see what you get.
Otherwise you will need to start playing with dial gauges and degree disks.