I think you unclip the silvery bit (2 clips toward back and another latch at front) then a few 10mm bolts under it.
Despite all its problems, at least you can get a g4/lm carb off pretty quickly (although some FC engines are a bit of a pain with the snorkel in place).
Maybe the paper air filter has somehow got soaked in clean oil mist - any blow back through the carby?
If I were to buy a new mower today I would have to go for the Masport President 2000 - 19in, 4 swing blade, ball bearing wheels, decent handle mount, and the Alko Loncin 160cc Chonda with a cast iron liner (would be my main selling point).
