It is on a MTD/Yard Machines Lawnmower. It starts fine however sounds like it is missing about 15 seconds after startup at all speeds. Might be surging a little. Carb has no adjustments. Governor springs (2 of them) seem to be correct and ok although looser than I would like. Cleaned and regapped plug - helped some but not much.

If cleaning and gapping the plug helped some, I would try a new plug. Some plugs test good outside of the engine, but will not fire under compression.

Well, I ended up doing two things to solve the problem. 1) - new plug; 2) - downloaded the engine manual which recommended adjusting the throttle cable. Problem seems to be solved - engine running smooth