OK here's what happened yesterday: This is a restored old trainer with a Magnum 40 GPA. The plane is a bit heavy at 6 lbs. My instructor tried twice to get it up, but we used the whole grass runway and it never had enough oomph airspeed to take off. The engine is used, but sounded good, actually kind of quiet and purrs at idle, tuned well, the fuel system seems sound. Foam wheels which didn't seem to be hanging up on the grass, grass it was pretty short. I'm sure at the other field I use, it would have gotten up off the asphalt runway, but not quite enough thrust on the grass. It is swinging a 11 X 6 prop. I took it apart last night, and there was a baffle in the muffler, which I read in a another post when removed could give more power. I'm using 10% nitro fuel.
So what would you do first to get more thrust? Rubber wheels (heavier)? Different prop? Take baffle out of muffler? 15% nitro? Something else I'm missing?

Oh I forgot to add the spinner is heavier thatn plastic, has an aluminum backplate and a big aluminum bolt, I assume it was for nose weight, there is qit a bit of lead glued to the back of the firewall for balance.

I also have a .46 Magnum GPA I could switch out to. Someone told me the .46 is not that much more powerful than the .40. I have 10 x6 and 10 x 7 props I could switch out to on the .40. Shouldn't the .40 get the plane in the air on a decent short grass runway?