You should try your stack just to see how your 125 responds to it. I have two Saito 125s in two different warbirds. When running right side up they do not like the stack. However I run my engines inverted and tyhey love the stacks. I get more consistent throttle responce and about 300 more RPM's out of them. Now I will admit I placed an ''O'' ring between the Stack and the Carb. and I pollished the inside of the stack so it matched the throttle opening. However without the stack my engines would hasatate a bit on throttle up, andidel was not consistent.
Sharpeye
The engine will be side mounted in the Stearman. I do plan on trying it both with and without the stack. If there is an RPM drop, I will need to weigh it against the fuel economy and cleaning up the plane.