No, I haven't looked at that. What it does is spitting and farting at constant throttle between 2,000 and 3,000 r.p.m. , as far as I can tell back through the throttle bodies. It did that at about 2,800 from when I bought it, and the rev. range in which it happens has spread.
The reason I've got the replacement timing chain is because @pete roper suggested that it would not be a bad idea at the mileage that the bike has on it. Maybe it isn't necessary, but I don't question expert advice like that. On top of that, there is no solid evidence that the 70,000 odd km on the clock are really 70,000, or might be 170,000, or maybe even 270,000. Who knows.
Apart from that, as I mentioned, there are two broken screws in the timing cover. I figure, If I take it apart to do that, I can change the chain while I'm in there. Or vice versa: if I do the chain, it is a good opportunity to fix the screws.
Whatever, the bike has apparently had one or more previous owners who have been satisfied with somewhat slap-dash maintenance and problem solving, although at least one was obviously keen on good solutions. I'd like to get to a point where I have the feeling that there are no more problems lurking in the wings.