Eng. surging

It does this at lo & mid throttle / stops a high / Thoughts

Surging is almost always caused from too lean mixture.

Try a partial choke to see if that levels it off. If so, the carb may need to be gone through.

Try an idle mixture adjust first. Turning the idle mixture screw in enriches the mix. It will have minimal effect though, all it adjusts is a small air bleed, so not much to gain there.

Pay particular attention to the 2 tiny holes in the side of the bore where the throttle plate closes. One is the idle feed, the other is off-idle feed. The throttle plate must close tightly and completely with the stop screw backed off. If the plate is bent or crooked in the bore, the shaft worn, will cause lean mixture beyond ability to adjust out.

There is also an idle jet between the upper housing and the bowl that needs to be scraped clean. Be sure the gasket seals around the small hole beside the venturi opening. That's where idle/off idle fuel is drawn up.

Look for vacuum leaks.

Be sure the float level is correct.

Also with the dash lever in the idle position, the plunger inside the governor spring should be pushing down on the governor arm, and the link between the governor arm and the carburetor should be holding the throttle firmly against the idle stop. If these 2 things are not happening the linkage will need to be adjusted. The link from the gov to the carb can be lengthened, but if the U bolt is loosened the governed speed will be effected, proceed with caution.

There is more on governor adjustment a few posts down.