Hi and welcome to FixYa. I am Kelly

Surging can be caused by several things. (in no particular order)

1. A minor gasket leak in the carb manifold. See if the carb is loose....
2. A worn butterfly shaft that is temp binding in the body of the carb
3. A worn governor spring or carb linkage spring.
4. A dirty carb float bowl. ( I recommend you remove the float bowl and check for debris in the bottom of the carb. ) Clean everything with carb cleaner but do not mess with the float.
5. A dirty fuel filter
6. A VERY dirty air cleaner.
7. A fuel selector that is not all the way on or open.

These are the most common causes.

I would start with the fuel bowl, then inspect linkage springs and go from there.

If we need to get heavy into troubleshooting, I will need your complete engine model number so that we can refer to the same diagrams and procedures.

Thanks for using FixYa.

Kelly