TKB, I think mine was around the same time as yours...
Real bugger of a thing. I didn't realize at the time that it was a vacuum driven unit, and that fuel would only flow when vacuum was applied to the petcock. Mine didn't get to the point that it wouldn't run, but it was nearly impossible to start without flooding and absolutely would -not- idle.
Simply put, with the petcock set to 'on' or 'res', no fuel should be flowing unless the engine is running. This could also be your problem, Odin. Is fuel getting into the carbs?
Here's a trick I tried with some success on my bike. Get four 20oz soda bottle lids, fill each halfway with fuel. Remove the airbox on the bike (leave gastank on and connected) and pour one each into the throat of each carburator. The bike -should- fire when you try to start it, it may rev up a bit, but it should fire off at the very least. Sorta like a defibulator for the motor, with any luck she'll start and keep running (don't use to much gas, and I -really- can't recommend Ethenol based 'starter fluids', I had a friend crack the cylinder head of an old dirtbike trying to start it that way).