I have been fllying a quick 50 with an os50 engine for about a year now. my heli was set up fine, but now all of a sudden when I take off straight up with power the heli twist to the nose left pretty forcfull. I use the futaba 401/9253 gyro and previously when I pulled straight up with power the tail staid perfect right where it started. I can't figure out whats making the tail twist in avc mode. help

There could be a problem with the servo. Just a guess.

Belt driven tail. Tighten it up a little. They can loosen over time. Also check pulleys and belt for damage.

Had a similar problem... Had a hard time trimming it out and wagged quite a bit... Also would turn when applying full + or negative.... Tried tweaking the gyro settings for about 2 weeks before I noticed the belt was loose... Luckily never lost the heli due to the fact. Check the belt