Yours should be 2H500 Clive, if it has a 3U500 fitted that's your problem. You are seriously risking major damage with large throttle openings and only a 90 jet fitted!
I would prefer to buy a used carb Mikuni rather than a copy RT?? will be much cheaper too £30 - £40 I would say
This is what you need
These are the spec's your carb should have
Mikuni VM24SS, Number 2H500
Standard Set up with factory porting, standard exhaust and air filter
Throttle Slide cutaway, 2.0
Needle, 4j13 Clip position 3 from top
Needle Jet, 249 0-2
Main Jet, 130
Pilot Jet, 25
Reed side diameter, 35mm
Filter side diameter, 43mm
Length, 85mm
Float Height, 21mm