Had that problem with my John Deere X300. After a lot of google research, I found a possible cause as "clogged air filter." I removed mine, washed out the foam prefilter and tapped out the paper filter. Reinstalled, and everything ran fine. Started surging again about a month later, so I replaced the paper filter. Been fine ever since.
-dan z-
You get lean hunt and rich roll. With rich roll the muffler turns black. With lean hunt pulling on a bit of choke makes it better. Rich roll gets worse. Generally the "period" of lean hunt is longer than the "period" of rich roll - or put another way the lean cadence is slower.
A bad spark plug did it on my lawn tractor. I guessed both the fuel and air filters before settling on the plug. The next step was to pack it up and take it to someone who knows something about engines. ;-)
But "generally" if it starts OK but rolls at idle, the plug is not the problem. A plug will - and again I say "generally" either cause difficult starting or cutting out under load -VERY seldom an idle surge, or hunt.
If allowed to run too long with a "rich roll" the plug will eventually foul.
If run too long too lean under load a plug will "glaze" - but MUCH less likely on unleaded fuel than it was with leaded gas - particularly high sulpher leaded, which produced a yellow-green lead sulphide glaze - also known as Galena - one of the earliest used semi-conductors.
Used to see a lot of "4 transistor" Toyotas back in the day - - -
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