Hi all
Like all fault diagnosis it is a process of elimination starting with the simple. The air filter looks ok in the picture you posted Ollie but I would clean and oil the filter ( unless this has been done ) so it is as the manufacturer of the engine intended it to be to see if it makes any difference. Not everyone oils the foam filters for various reasons but it is interesting to note on the Briggs web page for servicing of air filters it states in capitals that foam air filters must be oiled to trap smaller dirt particles. I think a dry filter will allow a slightly greater volume of air to pass as opposed to an oiled filter.
As Bigted pointed out these engines seem to be set very lean so any imbalances of ratios will have the potential to cause issues. If it makes no difference to the surge then it is ruled out as the cause and step 2, jet cleaning is done.
Cheers
wce
