Join Date: Feb 2015
Location: Northeast, Easton, Pa
It sounds like the slow jet is clogged. Most people do not know about the tiny hole in the bottom of it, that is where the fuel goes through, the big hole is for the air to mix with the fuel.
As O.P. mentioned, check the tiny "O" ring on the jet, many times it gets damaged when re-installing the little black plastic slow jet. I always put a light coating of silicon grease on it before I install them.
The engine might sputter a little bit until it rinses off the grease at first, then it will clear up and idle fine, and no surging at full throttle.
When the engine is running full throttle with "No Load" it is taking most of its fuel from the slow jet because the throttle plate is barely open, when under load it opens further and then it takes fuel from the main jet due to the governor opening the throttle plate further.