912 ULS Misfire

Hi, my 912 ULS has developed an intermittent misfire, one cylinder head running cooler than the other, I thought it might be the float issue, I took them out and weighed them, one set was over 8g and therefore replaced both sides, however the problem still exists.

I have changed the spark plugs and checked the fuel all seems to be in order. Does the same with either circuit on the mags, I am thinking it is a fuel problem? Any help much appreciated.