How can the “Fox 35 burp” be related to Schnuerle porting when the Fox 35 is not Schnuerle ported?
The problem in either case is that the gas flow inside the engine is too slow and is affected by the acceleration to a disproportionate degree. That's why the Fox "burp" is conclusively solved by speeding up the flow by making the bypass area smaller.
"Engine experts" will tell you how this will kill the power by restricting the flow. They are wrong, the bypass cross-sectional area is about 5x larger than it needs to be, and restricting it to way less than half the stock dimensions has no effect on the power at all - sometimes it slightly *increases* the power.
Schneurle engines are inherently more prone to changing the run characteristics due to acceleration, because their scavenging counts on the momentum of the flow to go where it is aimed by the ports. The baffle on a baffle-piston engine is there to force the incoming charge up towards the top of the cylinder, a schneurle counts on the ports being aimed that way slinging it up towards the top of the cylinder (and then displacing the exhaust out the exhaust port). It is inherently more prone because it counts entirely on the momentum of the charge to make everything go where it is supposed to.
In either situation, running it faster helps by speeding up the flow. But you don't want or need it to run faster, so an alternate solution is to reduce the bypass and port size to speed up the flow and give the charge more momentum.
So, for the Fox, the definitive solution is they bypass stuffer, then you can run any speed you want with no danger of a bump, and no power loss (or even a slight increase). The net bypass cross-section required to keep from restricting the flow is tiny. And alternate solution it to put various things in the head to at least keep raw charge from blurping itself onto the plug element.
For schnuerle engines, the solution seems to be to make the ports small enough to support the *very low* power levels we run with adequate velocity. Bear in mind, a piped 75 *could* probably be made to pump out 4.5-5 horsepower, we are running at .6 hp, so making the ports and bypasses smaller would have no effect on the power and maybe (with extensive evidence of the fact) less tendency to run differently inside and out. ]
Some smaller engines seem to have nearly no problem with it, only by virtue of running at near-peaked power levels where the port and bypass area are appropriate. You can't (or at least don't want to) run larger engines at the same high fractions of the capability - stunt planes absorb about .3-.6 hp to fly normal stunt speeds, any more and you will either have to figure out some way to waste the excess, or fly it correspondingly faster.
Problem #1 - Fox burp - was discovered in antiquity, and definitively diagnosed and solved by Frank Williams in about 1994 by the bypass stuffer.
Problem #2 - schneurle engine speed/mixture differences on inside and outside maneuvers, was discovered by Ted Fancher and company (probably among others) in the late 70's and has been since at least mitigated by building custom and semi-custom stunt engines with much smaller ports than a typical high powered RC engine of the same displacement.
Of course there are other reasons it might run differently inside and outside, like the tank position, but there are at least these two that otherwise have proven intractable that are inherent functions of the engine and the internal ballistics.
Of course both problems are avoided completely by running electric.
Brett