I've built a lot of Control Line profiles and that fuselage design is pretty anemic. I haven't built my Fun 51 yet but I remember studying the plans and thinking it's all wrong for strength. You're laminating balsa over a double lamination of Lite Ply. Would be better with the Lite ply being the outside lamination. Profiles suffer from a lot of flexing and will commonly break right at the wing leading edge. This flexing could be a cause for your excessive foaming. I've been adding 4-7 drops of Armor All to every gallon of fuel as an anti foaming agent with no ill effects.

So you might consider strengthening the front of that fuselage, eliminating some of the flex and maybe the foaming wont be as significant.

Food for thought?