The 3250 is a great big cheap engine.
Mine ran great, didn't changed nothing.
I used 5% nitro and 18% castor oil, this was the best mix I found for it.
OS 8 plug and bisson pitts muffler gave me about 7800 rpm with 20x8 wood prop and that's was only at the half of the second gallon.
For your profile the best muffler is the bisson side type.

http://bissonmufflers.com/en/muffler...ting_id=114077