I haven't read the other replys but IMO my experiance is that you need less nitro and/or a colder plug with a 15-6 prop. I would try either 10% nitro or a K&B 4C plug, if that doesn't work try both. If that doesn't work try 5% nitro. That engine much prefers a 14-6 but I have run a 15-6 without issues.