Hi GWA and Norm,
I'd check the easy things first ,like make sure the air filter is clean and when the motor surges you can put the choke on a little to see
if that fixes the problem if it does you must have a carby running lean problem.
Check the fuel is good and not old , check the fuel flow meaning the fuel filter is not blocked and the fuel tank breather is working.
You could remove the fuel bowl on the carby to check that the old fuel hasn't waxed up the insides of the carby.
It wouldn't hurt to replace the spark plug and check /adjust the spark plug gap.
Click on link below to see more on surging engines.
https://www.lawnmowerfixed.com/mower-engine-surging/Cheers
Max.