Ariel Owners Motorcycle Club

Thanks Paul, that is a possibility, but today, I shimmed the gap between rocker box and exhaust lever out to 0.160", put the rockers back on and was turning the engine over very slowly with my finger on the exhaust tappet adjuster - ouch! Every now and then the valve must be sticking as it comes up with some force, hits the tappet adjuster (hurts my finger) and makes the familiar metal clang I have been referring to and calling a mechanical clang for ages.
So the question is why does the valve stick - I am guessing a bent valve stem?
If the valve is sticking, this must cause slack between the pushrod/rocker and pushrod/cam - I wonder if that is what occasionally causes a mechanical jam?

From what you say, it seems that it has to be a valve sticking problem. The only solution to that, usually, is to take the head off, take the valve out and see why it sticks. A bent valve stem is one possibility but debris on the stem of some sort is another. Obviously, you need to end up with the valve moving totally smoothly all the way in and out.