The only Fox engine of mine, out of maybe three dozen the past 55 years or so, that ever "burped" some was a newish 35 stunt (newer than most of my pre-1970 oldies, anyway). The 3-bolt 19 and its 25 size sibling not only never burped (and I've had quite a few of those, at least three of each, probably more, when they were still being made), they offered better power to weight than the 29 and 35 did. The old 35s didn't burp unless matched to a bad tank, and the 19 and the 25 definitely quality as old. Fox stopped making those before 1970 (the 25 was out of production before 1965).
I think you have a bad tank. Classic style 4-2-4 engines do not like a great many "profile" tank designs that newer engines run fine with (including OS Max-S 35s, Max-H 40s, and Johnson "S" engines, to name another trio).