Well it does have twin 40's, so could be a mixture thing!

but why the tapping if it's just running lean/rich??

how long should it be before the followers quieten down with a kent cam? My dad rekons normal ones take seconds to quieten and that you should only leave it idle for the first 20 mins.

But the kent manual says keep it above 2500rpm??

I'm Pickled, could it be a badly seating valve?