Test rode the bike today. The problem remains, but the symptoms are more clear. As the engine heats up the idle speed begins to hang at 2,000 rpm. Hotter, and it goes to 3,000 rpm. I had to turn it off after that.

Following the feedback above, my guess is air is getting into the carbs somewhere, and more air is getting in as the bike heats. That would suggest heat expansion on a crack or fracture.

By way of a guess, I'm going to take off the carb rubbers and check them very carefully. Where else could O2 be getting in? It's possible there's a crack in an alloy part or, less dramatically, perhaps something has come unscrewed inside the carbs. I'll double check the idle screws, and if there's still no clue, strip the carbs to check all the parts.

Is there anywhere else I ought to be looking? Do these symptoms sound like anything else? Any and all ideas gratefully received!

easiest way to check for air leaks is to spray some carb cleaner at the rubbers - if the revs rise...it is getting in.

arite mate did you ever sort the problem with ure bike