I have a tt four stroke and I gave up on it. Mine has the exact same symptoms that you describe. I have changed tanks....ran it on the bench
checked valve lash...used every trick on the motor I know as far as carb adjustment and enlisted the aid of all my knowledgeable friends and
this motor will not run right. It acts like it is over heating. One thing that I will try now that someone has mentioned it is checking the cam timing.
I had this motor installed in a u-can do and because the motor is shorter than an o.s. or a magnum I had to put a half inch playwoood shim behind
the motor mount to get the prop in the right spot. These engines are just not worth the trouble because they are no cheaper than a magnum. I luv all
my magnum engines.....they are basically a set um and forget em. The only thing I have to do is check the valve lash from time to time.