I ran across an interesting article on Sonerai.net. The author described a mod to the GPAS intake manifolds that he claimed improved idle and gained him 25-50 RPM at WOT. What he did was remove the divider inside the manifolds that separates the front and rear intake ports. He has a slide valve carb so he doesn't know how it would work with the Zenith. I have a Zenith but I might give it a try. Mine doesn't idle as smooth as I would like when its cold but it idles decent when it is hot. I wouldn't do it just to improve idle. The additional 25-50 RPM at WOT throttle has my attention though. I would do it for 25-50 RPM!
The article is at: https://cloud-data.nyc3.cdn.digitaloceanspaces.com/sonerai/data/articles/vw_tips_part 1.pdf
The article is at: https://cloud-data.nyc3.cdn.digitaloceanspaces.com/sonerai/data/articles/vw_tips_part 1.pdf