My 4kw Onan does that if the altitude (mixture) screw on the bottom of the float bowl is set too lean - for too high an altitude. Try turning it a bit at a time towards the lower altitude direction - mine smooths right out and puuuuuurs like a kitten when adjusted properly.
Along those lines, especially for generators and other smaller gas engines that only run occasionally, then sit idle for long periods of time - that tends to allow fuel sitting in the carb float bowl and up in the jets, to turn into a varnish-like material that's pretty much impossible to remove without completely dis-assembling the carb.
Do your gensets, lawnmowers and other small engines a break, and either RUN them occasionally long enough to come up to normal temperature - and/or treat the fuel with Stabil fuel conditioner to extend the life of the fuel. It's LOTS easier, cheaper, and less frustrating than trying to correct problems AFTER the damage has been done!
John Day....|'88 Winnebago Super Chief 27ft. Class A
Eastern .....|'88 KIT model 240 24 ft. 5er
Oregon ......|'02 Dodge/Cummins 2500 Quad Cab