still searching for a Aisan Carburetor rebuild kit....
but in the meantime, I found a post that described my experiences and the bloke recommended that the water hoses must be checked.
I found that my water hoses were crusted and blocked with rust and sludge - very little water flow, the nozzles were also blocked with rust sludge - could not clean the hoses - so I replaced them with after market ones from the local spares shop and flush the cooling system and replenished it with 50/50 Evron
I am certainly receiving better performance, but it can get better with a carb rebuild.
Here are my pictures of the rust mayhem - one with the jet completely blocked and the other showing the rust/sludge that was inside the hoses...