3.5HP Briggs and stratton?

Hi, some advice needed please. My 3.5Hp has worked perferctly for 10yrs, i come to use it on Sunday and it's fine.
I then refill with petrol and the problems start, i started it and it turns over fine, but it sounds like it is being revved up and down when it idles, you then try and use it and it has virtually no power.
So I've drained the oil and petrol out and will refill with new stuff aswell as putting in a new filter. My question is will this sort it. The only thing I saw was that the idle governor spring(?) seems to be being 'pulled' open and closed in time with the engine revving ast idle.