Your ST 61 may need a little adjustment to the spray bar. I helped a freind with his ST 61 which he had purchased used
with Hanger 9 Pizazz. His problem was lack of power and
unable to lean out the engine enough. The instructions with
my ST 3250 make a small reference to adjusting the spray bar.
The position of the cats-eye in the bottom of the spray bar
affects the fuel draw. The two small screws on the left side of
the carb. allow you to change this. You may need to mark the
current posistion of the spray bar as your starting point then
adjust two or three degrees and then run your engine. My
freinds Pizazz was a little heavy too, but the ST61 pulled it
fine after we made one adjustment at our flying field a couple
months ago.