Many new engines won't run well (or at all) at full throttle until they have been broken in. If the manual says to start at 2.5 turns out then next step in break in will be to lean it out. You should have to open the needle at all. Since you've run the first two tanks already close the needle (gently) and turn it back out 2 1/4 turns and rev it up again. It should get closer to full throttle. I'm not a master with airplane engines yet so I won't give you a full break in procedure. Keep leaning the needle slowly until you can run at full throttle without it 4-cycling but keeping it rich so there is a good trail of smoke coming out of the muffler.