Typically when you can not get the engine to go rich ...... there is one of a few problems

1) An air leak elsewhere in the engine crack in casting, missing screw, torn gasket, carb o-ring not seated. The EVOs have been good engines, so crack not likely. But a few years ago we saw a few of the Tower engines give people fits. After looking and asking, I found out each had a crack near the engine lug... thin casting and crappy mounting.

2) Fuel foam - usually, but not always, caused by vibration. Suction on needle valve systems (remote ones usually) causes microbubbles to come out of solution. So even with a good installation, if the engine is sucking hard - it will vapor out bubbles

3) Fuel line restriction some ARFs come with lousey clunks and fuel tubing. Verify on that.

4) Make sure the pressure line is clear too. That can get restricted.

And yes, a restriction on the exhuast causes backpressure and a rich situation.

Let me know how things worked out for you?