HELLO mike just read the thread (mustabin asleep?) looking through it no-one mentioned about checking the flange on the carb for being bowed,,,,use a steel rule to check,it can happen if to thick a gasket is used,nipping down at the ends and bending the flange ,,,,,,,it can be cured no bother by rubbing the flange on a sheet of glass with grinding paste only thing is it requires a carb clean up afterwards to get rid of the grinding paste,,,,,,if its bowed it will cause an air leak at the joint and give the sympoms you describe,,,im not saying this is the cause but its worth checking mate ....best of luck dave