Problem with surging

A buddy asked me to winterization his 06 John Deere x300 with a Kawasaki. I asked him to starting it to reach operation temperature. I noticed at slow rpms the engine was surging but fine at high rpms. So, I sprayed some seafoam in the air intake while at low rpms and the surging stopped but only while I was spraying. Once I stopped spraying the surging came back. I gave a few more quick burst of sea foam then sprayed until it stalled out. Let it hot soak for 10-15 minutes and restarted. After the white cleared the engine is now surging at all rpms. Did I or sea foam make matters worse. I read on here it might be a vacuum leak?
I then fogged the engine, changed the spark plugs, put in new air filter, changed the oil and filter and installed a new fuel filter. I did notice the plugs were out by.005-.007 inch. So I properly gapped the new plugs, sprayed fog oil in each cylinder and installed those. And the fuel filter was quite full of debris. Oh, I forgot to say that before I sprayed the sea foam we drained some oil out and put in marvel mystery oil and put in 3 oz of sea foam on the gas tank with 3 gallons of fresh gas.

Thanks,
Tom

Problem with surging