A hotter plug will help somewhat in burning-off deposits but a sooty plug is always a result of an over-rich fuel condition. It could be just a dirty air filter.
Check the left exhaust silencer for an air leak from a poor fit to the exhaust pipe (that closes up as it gets hot.) An air leak will add combustion air to any unburned fuel in the exhaust system causing it to ignite on hot parts and backfire. During warm-up, more fuel is added so there's always more unburned fuel passing through.