Thanks to all who replied, finally got there in the end not 100% sure how lol tried another vacuum nozzle made sure it was nice and tight against the block and cleaned all around. Refitted original baffle plate, then thought why not check the inlet manifold, screws were completely loose, no cracks or leaks, oring looked fine, so added a smear of loctite 515 and refitted it, tight this time. Bingo! governor is now controlling revs 3400rpm, no more melted cowling.
