Joe,
I replied to yours of the 23d on the redirect to my email. Thought others might use the info - thus,,,

The Fox 35 Stunt engine has a quirk about running right when mounted sidewinder, as on a profile. With a usually 1" tall metal fuel tank, the center of the tank's height should be about 1/4" above the shaft centerline. Individual engines differ; it may take a bit of fine-tweaking up or down to get the in-flight run equal upright and inverted, and in inside and outside maneuvers.

Set up "in-line," instead, the engine will go richer when inverted or in negative g's. That indicates that the tank is " too high" when the model is in negative g conditions; fuel is feeding "downhill" to the needle valve jet. Gravity feed, IOW. This also means that the tank is too low for when the model is upright - the engine has to draw fuel "uphill" to the spraybar jet. Setting it richer to draw enough fuel makes the reversal to 'gravity' feed even worse.

Raising the tank that 1/4" (or so) just about balances the "feed" tendencies.

In days of old, we ran the engines harder. More RPM helped provide more fuel draw, and the tanks were all "open vented" - two vent tubes so that one was open to dry space inside the tank whichever side of the model was up. With that venting the engine ran progressively leaner as the fuel ran down, and there was considerable spray out the vent tubes from siphoning out the 'wet' vent.

Today, uniflo venting is popular. Set up right, the run is very consistent down to the final bubbles before empty. The only problem with uniflo is that it is more sensitive to tank height. When it is right, it is great, though. It has only one working vent tube, which usually ends inside the tank at half the height of the shell. An overflow tube allows filling the tank, and it is capped before flight.

That open end, where vent air enters the shell, becomes in effect, the liquid 'surface height' relative to the spraybar jet. It doesn't change height. In the over/under two tube venting, the fuel 'surface height' changes from the inboard wall of the tank to the outboard wall as fuel burns off. Usually a distance of about 2". The centrifugal load on the fuel - around 3g for most CL sport models - makes the effective height change several times larger. Think how the engine would react on the bench if you raised and lowered the tank 5 or 6 inches each way...

With uniflo, the height change is only the distance - spanwise - from the vent tube end to the fuel pickup tube end inside the tank. Both are usually soldered alongside each other at the same height inside the tank, near the back of the wedge. This allows us to tune a leaner takeoff run - more useful power - that won't go overlean at the end of the tank. It allows setting that rumble and scream 4-cycle/2-cycle run so many like. It screams into 2-cycle when maneuvering drag loads force the change. AND with uniflo, it usually drops back to 4-cycling as soon as the loads end.

Control line uses such simple machinery, which also means that if you don't get something simple just right, you got trouble... Part of why we enjoy it??