There should be quite a bit on the forum on this subject. The Tower .40 is a clone of the FP.40 but with much better fits and a true ABC construction. They tend to varnish up and get sticky. A friend of mine here in town had this very thing happen with his. You can tear it down and physically clean everything with some elbow grease, solvent and a scotch-brite. I think the problem area ion my friends engine was in the crank bearing area. Another thing you may want to try is to bench run it with a smaller prop, like a 9-5 or 9-4, and run it back in with some fuel with more synthetic or all synthetic oil in it, and run it at peak RPMS for a few tank fulls and see if it loosens up. It may not be running as hot with the Dinger muffler because it can breath a little better. Those 762 muffler were pretty restrictive and you just don't have it as open as the Dinger muffler. If you do the bench run method, I would do that without the muffler, just to give it a better chance of running a bit cooler, and less loaded down so all the crap can work it's way out..
Type at you later,
Dan McEntee