distortion on a Hollis Ultra Flanger?

[brokenstarguitar]

I've built a couple of the Hollis Ultra Flanger using the GEOFEX layout/build doc. The first 2 I had some issues with the 3.9v Zener/led combo and the last one I had no issues till now. (Maybe the use of a different amp then what it was tested on) Anyway, it seems there's a lot of distortion in the circuit. Does anyone have any ideas how to lower the distortion? I was thinking about putting g a trimmer on 2 (D1 & D2) diodes that limit the signal going into U2. Would that work or is there a different part of the circuit that controls the distortion and/or gain? Any advice would help and be appreciated. Thanks!