Glad plugs seemed to fix it!
I've got onan p series twins on my wheel horse tractors, I've been through 4 or 5 of them all with the same problem. First they start bouncing around at idle then start missing over time but still seem to run great at full throttle.
Over time the wear on the valvetrain tends to make the valves tighten up and hold open a little. If left alone it will throw a valve seat. Because of this they need valve adjustments every couple hundred hours. Onan said every 500 hours, a lot of guys like them sooner than that.
If the problem comes back that's what I would check.
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