It will track straight, then as soon as the tail lifts, goes left, and even the smallest right rudder, it veers right, ease off and its back to the left. Even with a lot of expo dialed in, it still does it, so going to put a 3axis in it once I figure out where to put it.
Expo and AS3X won't help for the transition from wheels to wings. If the model veers when the tail-wheel lifts it is likely because the tail wheel and the rudder are not "running" in the same direction. The roll test along the ground with a centered rudder will show this.
Many models need right rudder to run straight on take-off. Just a part of life and flying.
Adding a few degrees or toe in to both main wheels makes a model tend to self-correct for straight rolling.