I have had 2 LT-40s (first a kit then an ARF) and the charicteristic you are describing definitely isn't normal for this plane. When reading your post my first thought was that the plane may be tail heavy buy it seems you have ruled that out. These planes do float forever since there is so much wing area. I can't tell you how many times I watched mine float on by and had to go around for another try while learning to land. They simply cover a lot of air space for every inch that they drop closer to the ground. A flat bottom air foil will produce more lift when you increase speed but it sounds like your plane is excessive. I have flown other trainers that have this charasteristic and it can be really annoying as you feel like you are constantly trimming and correcting the plane.

I would give the above suggestion to raise the trailing edge of the wing a try. It may not be the exact answer to the problem but it could very well help. If the balance is OK as you say my next guess would be a wing incedence problem. Of course the problem may not be with the main wing as the horizontal stab could also be the culprit. If you fly with a club you may try asking around to see if anyone has built the kit version of the LT-40 and borrow their plans (assuming yours is an ARF) so you can check the thrust line and wing incedence of your plane against the plans.