ok thanks, just trying to wrap my mind around what the issue could beI would think a worn out cam would have more of a timing issue, causing sputtering and back firing.
no solution for the 15.5 hp engine, ended up scrapping itSurging on a goverened engine is always cause by an air : fuel ratio that is too lean to maintain a constant speed.
For that to happen either the carb venturi is not passing enough fuel into the air stream or more air is entering after the fuel has been added.
IT has to be one or the other or if you are really lucky, both .
So you are looking at sources of induction leaks or fuel restrictions.
IT can be as obsecure as the carb mounting stud/ bolt causing the intake tact to crack or being drilled so deep it intersects the inlet tract.
A cracked head, leaking gasket badly worn butterfly pivots, cracked manifold, porous manifold .