You're leaving power on the table with the rich jetting and your pre-mix is way to rich for that oil.
This means you have less fuel parts per million and you have to jet fat to try and make it up.
Click ---> http://www.amsoil.com/storefront/tdr.aspx scroll down to "Applications", second paragraph.
Says, "In pre-mix applications, DOMINATOR delivers outstanding protection at 50:1 pre-mix ratios (2.6 oz. oil per U.S. gallon of gas).
That will give you more Fuel parts per million and you won't have to Jet so Fat.
I recommend draining all of your fuel and mixing a fresh batch at 50:1, which is 13oz to 5 gallons of premium pump gas.
Leave your jetting as it is right now, (27.5 pilots, 320 mains and needle clip in 2nd groove from pointy end).
Go do a perfect plug chop with a fresh set of B8ES' gapped to .028" and report back.
http://www.dfn.com/agservices/plugchop.html
I bet it's rich as all get out now.