that is harder to say these days
some motors out there still seem to need 40 to 100 miles for break in, while the ones I was getting only took about a mile or so to get to good power and another 20 or 30 miles to get to full speed
on mine, I started using only a pint of 16:1 to get it going, then fill with 32:1 thereafter - some motors may still benefit from two tanks (1 gallon) of 16:1 before switching to 32:1
since your plug may be fouling, I'm guessing you have 40 or more miles on the motor - if not, then it might be that you are way too rich in fuel/air mixture
Sometimes I sets and thinks, sometimes I jes sets.