I am bumping this old thread just to close the loop on a "no start" problem I had that fits in the the discussion topic.
I installed a new fuel pump & filter my generator about a year and a half ago.....that cured its "run for 20 or 30 minutes and then stop" problem. (Unit has just over 400 hours on it.) Shortly thereafter, I installed a new carb and it "almost" completely cured a light surging problem. But my 4KW Onan (Spec K) eventually got to where it would not start. I pulled the plug and the only way I could get any visible spark (even with a new plug) was to gap the plug down to about ten thousandths......then I had a very anemic, blue spark. Even with the plug gapped down so much (supposed to be .025") the generator would run fine but was v-e-r-y difficult to start. So I put repairing it on the "round to it" list. I "figured" it needed a new magneto assy and that requires dropping the unit in my case. Finally, when starting it became nearly impossible, I ordered a new coil/magneto assembly ($56.68 incl shipping from www.partsfortechs.com) and removed all the goodies from the Onan (wiring, controller, solenoid, etc) to ready it for lowering and removal.
I live in a retirement village so I have to work on a very bad surface at my storage facility.....and when I found I would have to raise the rear wheels off the ground about 8" for clearance in order to use a hydraulic table for lowering, I got cold feet and and got an appt with the $156 per hour folks at Cummins Onan here in Denver. Waited 6 weeks, then another week in-house before their "generator man" put a wrench on it. When Cummins got the new mag/coil assy installed I reinstalled all the (make-ready) items I had removed and the Onan now starts quickly and runs like "gangbusters". SOooo yes, the engine mainly needs Fuel, Air, and Spark......... but make that a good strong spark, IMHO.
The invoice from "ASAP Parts for Techs" outfit says the assembly they sent me is for Spec J and after. So logic says if your Onan is earlier than a Spec J, you probably have a separate magneto and coil (and I believe the coil is accessible without removing the whole unit.....a real plus).
BTW- The "Spec" designation is the last character in your Model Number; e.g., mine is 4KYFA26100K so mine is a Spec "K".
I have enclosed a pic (I hope) of the part that was removed. Might have just been the plug wire, but there is no way to change just that part; it is one assembly. There is a fair amount of surface rust visible but at 10 years, perhaps that would be expected?
Hope this rambling post helps someone................ ed s