RE: O.S. 91 Surpass - New - loads up/stops
"Loading up" is caused by a too-rich idle mixture. You need to work with the idle mixture and idle RPM. Start out with the idle set to "stupid rich", get the engine to full throttle and set the high-speed needle so that the engine is leaned to just rich of peak RPM. Now retard the throttle and lean the idle mixture as needed. When you find a good idle RPM and mixture combination that works, check it with acceleration.
Many times, "loading up" is caused by trying to have an idle RPM that's just too low. Anythng lower than about 2,500 RPM would be considered good.
You may be trying to use too much propeller. We like to run a 12 x 8 or 10, and prop for full-throttle RPM near 10,000 when the engine's leaned to just rich of peak RPM.