I took off my carb yesterday to do service, only to find that the carb runs on almost full choke all the time, i cleaned it and refit everything, but for some reason the choke needs to be nearly closed before it runs, if the choke is left open and you try to rev it up, it just stutters and dies, but with the choke almost fully closed, it drives fine but is quite heavy on petrol. anyone have an idea what the problem might be? the engine, plugs, plug wires, points gap is all in good condition and set correctly, as is the timing