Camshaft

Les Emery's Norvil site has detailed instructions about running in cams.  Maybe the reason is in there?

Unfortunately all too common, seen on triumphs as well,even on a Yugo where it had one lobe worn to a circle but still ran on 3. Camshafts were made soft then case hardened to give thin surface layer that is wear resistant, longer you leave them in the baking process thicker the layer. Commercial constraints mean results were variable particularly on Mk3 Commando, quick test on any second hand cam is try and file it, if you can then discard. Back in the day you would get a friend in the toolroom to build up lobes with stellite and reprofile by hand, making sure base circle to peak hight same as unworn lobes, done that myself!

However we are now more affluent so you need to find another cam, new or second hand with four equal lobes, check your cam followers are ok before reassembly, if scored they will score new cam to match. Also unless you have an oil filter fitted it is time to take the oil tank off and remove that ferrous sludge.

Good luck with your rebuild, will be doing exactly same with recently bought MK3 which is well down on power.

Regards Martin

The cam lobes/followers has the highest loads in the engine and is one of the first areas to show trouble if oil is not good enough. The Commandos had a lot of failed cams in the 70s due to incorrect hardening. I did not think that issue impinged on the Atlas or earlier but then the 70s was a long while ago and parts have been remade. Also when the cam is part worn on one lobe the apparent performance often remained still ok.

In a moments 'jollity'  I once saw in Motor cycle Mechanics the observation with regarding a failed cam lobe, 'the owner should have noticed due to the increased tappet gap!?'

The casting number suggests that this is an early Dominator camshaft. I wonder if it has been re-profiled but not sufficiently re-hardened???

While you have the motor apart I would check the cam trough is intact as it provides lubrication from the word go. I would also be scrupulous in cleaning out any cam debris. Fit a sump magnet and drop a couple more into the oil tank. As the engine oil should not get into the clutch I would use a fully synthetic oil , 20/50 if possible  to give the best protection. The followers if still reasonable will need re-facing , I would also be looking at the valve action for the lobe that went AWOL in case there is a resistance somewhere.