Jayseas,

Don't let that discourage you...

The Fox 35 has been around a lo-o-o-ng time. It isn't perfect (understatement?) but it can still do a good job.

A uniflow tank, running properly, can be dead solid down to the bubbles at the end of the flight. Slightly off, and particularly with a Fox 35, it can cause a very noticeable difference in the run between upright and inverted, insides and outsides...

General rule that works pretty well: the end of the uniflo tube inside the tank should be about 1/4" above the center of the intake. It may take a bit of tweaking on a particular model, and with a particular example of the Fox. (Some of them differ.) I notice, I think, that the tank is/was bolted to the fuselage in a way that you couldn't adjust its height up or down. Suggest that next time you build in a way to let you do that...

HOW you run a Fox 35 can make a difference, too. It is famous for the '4-cycle/2-cycle break' type of run. That can be done. If the engine was too lean, going gangbusters in 2-cycle all the way, it may have leaned out high on the flight circle, or reacted too much to a slight misalignment of the tank height. The Fox also has a nasty habit if it is run too rich... Nice grunbling 4-cycle, but when you loop it, it can break too much into 2-cycle and cut out.

An ideal launch needle setting, if everything about tank height is close or right on, is warbling between 2-cycle and 4-cycle, mostly in 2-cycle with fairly frequent 4-cycle burps. When everything is right, the model settles to a strong 4-cycle within the first level lap, and breaks into 2-cycle when you maneuver. (It should also break back to 4-cycle after a maneuver is finished.)

If you have fuel with at least 25% castor, and from 5% to 10% nitro, a good hot plug and a prop that is neither too much or too little load (say a TF Power Point 10-5 or 10-6), you should be able to dial-in tank height so it will fly well and consistently.

Edit to add: That 1/4" starting point is for profiles. For upright or inverted engine, full-body models, a 1 inch high tank sitting on the mounts (uniflow vent end inside at 1/2" above the mounts) works very well.

Besta luk towards that!