The advice u got on going to the 12/7 prop is correct.The 70 surpass runs great on a 12/8 Master airscrew Glass prop. I've been running 4'rs for over 10 years now,and learned a very costly lesson that no one seamed to mention in the post's. Because a 4 stroke has a dead stroke between the compression stroke.you should NEVER run a wooden prop on them. The wooden props dont have the nessary weight and mass to carry the motor through the dead stroke.Yes i know,ur engines are running ok it seams,but without the load of the heavy prop,they are free wheeling and this causes the compression stroke to DETONATE when it comes up because it's not having to work to get the fuel load past the valves thus it pre-ignites causing back fires and basically a lean acting run. One other bit of advice BLUEMAGIC,since u have a gallon of fuel through the 70,it's probly a good time to check the valve settings on it. Set them with piston at TDC in the center of the compression stroke with motor cold. Set them between .004 and .01 thousants clearance between the valve spring and rocker arm.

good luck flying