Large engines do not usually need 20% oil, that may be part of your problem. The store bought fuel likely had 17% or 18% oil. Some of the large Supertigres recommended only 9% oil.

I now recall haveing a similar problem when I ran out of fuel except for some old control line fuel with 25% castor oil. It ran but had problems with the transition, swithching to an Enya #3 plug did the trick.