I came across an interesting problem yesterday which proves 'everyday is a school day'. My mate has had a faithful A10 for as long as I've known him. Last year he decided that Old Faithful was getting tired and it transpired that everything was virtually worn out. He had the engine rebuilt (most of it) by a club member. My mate was left to fit the rocker box, timing gears and magneto. When nearly everything was finished he called me to fit the mag for him, which I did, then we started the bike. It sounded dreadful, like a brawl in a Victorian pan shop. It sounded like a pushrod had come out or pistons hitting valves... I checked the tappets and noticed that the movement of the tappets looked a bit weird. As a tappet closed it 'bumped' and looked as if it was trying to open again. I phoned the engine builder and described it as 'the camshaft seems to be trying to lift two followers at the same time'. It wasn't just a case of incorrect valve timing. The engine builder and I decided to take the bike back to base, get it on the bench and put our heads together. A torch down the plug holes showed no witness marks of valves getting passionate with pistons (and compression was good with the tappets loose) and a careful check under the tappet covers confirmed that the pushrods were fitted correctly. Nevertheless we took the rocker box off to be sure. After a cup of tea (best tool in the workshop) our combined wisdom resulted in a 'light-bulb-moment' when we realised that, for some reason the camshaft was, in fact, lifting followers randomly because the endfloat was incorrect. (I digress for a moment....If you look at the followers in the barrels there is no clearance between each pair, i.e. they rub against each other, and the camshaft lobes are almost as big as the followers so there is no tolerance for misalignment of the camshaft. Correct alignment is achieved by ensuring the cam gear is tightened against the plain end of the cam and the cork washer under the top hat breather ensures there is no endfloat. ..end of digression!). So, off with the inner timing cover to find that, as predicted, Old Faithful Owner had failed to tighten the cam gear by about two or three threads, enough to cause the camshaft to float a couple of mm. to the left, so the right hand lobes of each pair (left exhaust and right inlet) were indeed lifting both followers. After assembly the bike ran as sweet as you might expect and Old Faithful Owner chuffed (and embarrassed). Engine builder and I pretty chuffed too. Job done in about three hours which included taking the rocker box off and replacing it. Time for a beer, I think!
