Surging or pulsing

[Great machine]

Hello, I have a John Deere 420 garden tractor. 2 cylinder, it is a great machine ! However it has started to surge or pulse ~ rise and fall of rpm's. It does it whether it's under load or at idle.

I've cleaned the air filter and prefilter, changed the spark plugs (gapped them) Didn't make any difference difference.

Any ideas or suggestions ?

 

[BlazNT]

Dirty carb
Down and dirty: https://www.youtube.com/watch?v=VBL-BUj0c6E
More technical: https://www.youtube.com/watch?v=nAk_WDt3AD8