I've got a 7 yr old Tech OVRM120 that surges , which is worst on the low speed setting.. If I spray the throttle shaft with carb cleaner the surge is gone and the rpm smooths out. I can feel the extra play throttle shaft. I have also checked and cleaned the governor linkage and found it OK. Based on this, I think the throttle shaft is the source of the vacuum leak and causing a lean surge. Does the carb body or "dust seal" on the carbs get worn? Is there a spec for the diameter throttle shaft or a rebuilt manual with specs. Rebuild or new carb?