I had a problem recently with our Onan 5500, hard to start, surging and dying, and originally suspected varnish, clogged jets, etc given it only has 150 hours on it and is 12 years old.... so I tore the Carburetor apart.
Carburetor innards looked pristine jets all clear etc - but low and behold the float was binding in the housing, and honestly looked like it had either expanded or was not cast to the right dimensions in the first place. A little filing and sanding cured the problem 100%.
Haven't seen that. You were really fortunate in being able to find it and and correct it without buying parts. ONAN tries very hard to sell nothing less than a whole carb. If I had to guess the cause... Ethanol...
God Bless, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100