Well, it's another theory, and thanks for your input. I didn't actually say it was coming from the starter area, but just
when starting. It's not ocurred for the last few days, and I've started up repeatedly quite a bit to see if I can repeat the fault, so naturally I'm hoping the happy bike fairy has made the problem go away
It is quite a disconcerting THWACK, as you say, or wheeze-thwack, wheeze-thwack more like, as the thing turns over, but seems from deeper in the bowels than the starter itself. But yes, since the starter is pretty easy to get off I'll check it out. I am more inclined to the auto decompressor theory though because of others' reports of such a fault, and because it just
sounds like something more out of kilter than a broken tooth. I'll let you know.....