Wozza If a bottom fed carb, try it with the float chamber top removed. If the valve seals, then the problem is that the needle can't rise enough, and is fouling on something in the guide on the underside of the lid, muck or corrosion, easily removed with a small drill. Float locates on the needle OK?
An original needle seating may prove difficult to remove from the float chamber, the two metals amalgamate to a degree over time. Most folks don't bother and replace the whole thing. Best attack is a little bit of warmth, say half an hour on the barby, on a nice even heat, thick gloves, firm support, good fitting tool. Check availability of the replacement parts before you go to this bother, as in the worst case scenario the valve seat will be damaged and the float chamber crack.
Grinding paste is far too coarse, Brasso metal polish was the medium of choice, these days try bath cleaner, T Cut or even toothpaste, in other words a very mild abrasive.
Alternative is to try a different needle first, or another float chamber, but with a used one the same worn valve seat is a risk.
This is where the Monobloc scores, the float arrangement is a whole lot simpler to service and an improvement on this design.
Swarfy.
