I'm also confused about the fuel tank location. Looking at the Tiger Trainer manual, the fuel tank is not specified, but appears to be mounted directly behind the firewall, which would put the back of the tank near the leading edge of the wing, and well forward of the CG. With a TT 54 Four stroke on the nose, as well as added nose weight, this plane should be nose-heavy as all heck!

I seriously suspect its either wing incidence or horizontal stabilizer incidence. Try the popsicle stick under the trailing edge of the wing, as well as making sure the ailerons are not reflexed down.

Just as a note, the tail heavy plane will not just climb with wind and speed. It will have a tendency to snap up or down with wind gusts, and be VERY hard to control. Small movements of the elevator either up or down will cause rapid changes in the planes attitude. Further, when banking in a turn, the tail will tend to drop. The wing incidence however, could easily cause the ballooning behavior.

Brad