I have a profile style uni-flow tank that has a weird problem.  Set the needle on the ground release the plane and within a half a lap the engine goes into a hard two cycle and stays there for the rest of the flight.  You set the needle to a blubbery rich and it will still go a hard two cycle.  I changed motors, props, fuel, and as best I could made sure there were no bubbles in the fuel line.  In sure desperation we changed the tank and problem seem to go away.  The plane is my profile Trophy Trainer, engine is a Brodak .40 with a 10.5 x 4.5 APC.  Fuel is 15% nitro with a blend of caster oil and synthetic oil.  I plan to open up the tank but I would like some ideas for what to look for.
Andy

Did you pressure test the tank? I have found pin hole leaks in recent tanks that I have purchased from both of the major tank suppliers. Not a big deal to fix but will cause runs like you describe if you don't. 

I have a profile style uni-flow tank that has a weird problem.  Set the needle on the ground release the plane and within a half a lap the engine goes into a hard two cycle and stays there for the rest of the flight.  You set the needle to a blubbery rich and it will still go a hard two cycle.  I changed motors, props, fuel, and as best I could made sure there were no bubbles in the fuel line.  In sure desperation we changed the tank and problem seem to go away.  The plane is my profile Trophy Trainer, engine is a Brodak .40 with a 10.5 x 4.5 APC.  Fuel is 15% nitro with a blend of caster oil and synthetic oil.  I plan to open up the tank but I would like some ideas for what to look for.
Andy

    The tank may be too wide. When you take off, you have to suck the fuel "uphill" from the wedge to the spraybar. Try running it on suction, if it works at the beginning but then goes lean somewhere towards the end of the tank (anomalously lean) that's probably the problem. With suction, a typical profile arrangement has atmospheric pressure or slightly higher at the beginning, and then the pressure falls until the end. With uniflow, the pressure is at the "end of run" pressure all the time. If you have enough fuel draw, all you have to do is set the needle a little richer on the ground, and count on it leaning out in the air. If you don't, you can't open the needle enough to get it to run properly in the air, which is what it seems like here.

    Solutions could be a clunk tank (with the flat side against the fuse) or an inboard tank, which will have positive pressure (more than atmospheric) in the air (and will have to be set lean in anticipation of going rich in the air). Or a narrow conventional tank.

    Brett

Dreaded profile bad vibe. Was it the same style tank or different. Plane/engine combinations vibrate differently on the ground and in the air. Not uncommon to find this happening. Profiles especially are known for this. Might simply be a case of bad harmonics with that specific tank/engine/plane combo.

Could be a loose pick up tube that rattles around when in flight. Could be debris in the tank that gets sucked back towards the pick up when centrifugal force shoves the fuel over.

Profiles in my experience are not a science. Engine run problems are solved by guesstimating and trial and error. Congrats if you've solved the issue to your satisfaction. I was going to suggest steel plates (or thick aluminum) under the engine and on the inside of the profile. Tightly cinching bolts often helps with vibrations issues. Rigid noses are generally a plus for modern schnerle engines. Fox 35s often respond best to the classic wifty Ringmaster idea. Tank style definitely a variable that can make a difference. I like the long uniflo RSM profile tanks and chicken hoppers on profiles. Clunks set up for uniflo or straight plumbed can also work very well.

Not uncommon to have a tank with a defect. Solder moving around, leaks, wrong placement of pickup, bad location of uniflo and so forth. Leak check the tank underwater. Seal all tubes but one. Pump air in hard with a syringe. Even a small tell tail stream of bubbles can create havoc. Seal up any leaks, of course. Pop the back, take a look. Anything where it shouldn't be... Placing the pickup poorly could possibly cause the symptoms you observe.