Some 4stroke engines don't like 2stroke fuel with 20% oil. The "excessive" oil cools the glow plug too much. You can easily tell if this is the case when you remove the glow clip. If the RPM drops substantially and the engine does not idle and transition well, then you need to reduce the oil content (i.e. use a "4stroke" fuel). Otherwise, use the same 2stroke fuel on your 4strokes.