The Lycoming peple will tell you "If it is not under full power, lean it" I use full lean on takeoff and lean as I reduce power. even when taxing lean it.
Do you see a change in RPM or manifold pressure?
Different engine could cause harmonics in the fuselage giving the feel of surge, exhaust hitting cowl etc.?

If you feel a surge but don't hear it, or no other visual indication, it may be the fuel sloshing fore and aft in the tank. I have felt that numerous times in the past. If the tank is low, it won't do it as not enough mass to be felt even if it is sloshing. Maybe try flying with a 1/4 tank and see if it still does it. Also, if in smooth air, fly straight and level for at least a couple minutes with no movements to give the sloshing time to die.

Bernie Fried
N18XS

Weird, I had the same suspicion on occasion over the past few months, yet could not see/detect an RPM fluctuation - I have a C/S prop....my plane is just out of conditional - the throttle armature was a little loose, so I ended up pulling the (recently rebuilt) carb and replacing the bushings and putting a new arm in - in the process, we noticed the screw that goes in the front of the carb near the top (I don't have it in front of me, so pardon the generic description) was sticking out into the carb throat and it was obvious there was an "eddy" being created when the fuel was atomizing while being jetted up the carb....We fixed that condition and I just flew it for the first time Tuesday evening and noticed my idle had improved - ticking smoothly along at 700-725, vs 800 RPM previously - I did not look for the "surging", but am suspecting that the slight disruption of the air/fuel mixture through the throat at that "eddy" may have been a cause. Think of an eddy or standing wave in a river, and how it "ebbs and flows" irregularly.

Food fer thinkin....