Hello, Ive been fighting mine for years, difficult to get parts for the Tecumseh engine.
When I had the same problem, I found the advance acting up, there is small spring attached to the throttle mechanism and them to the butterfly on the carb, is it in place and in good shape? Can you manually actuate the butterfly? it should scream wide open when running
There is an idle screw adjustment but I would concentrate on the throttle/carb first. Other than idle, does it run correctly? i.e. do you get enough RPMs to turn the clutch? Any small engine shop should be able to help