Fine Tuning a 360 Magnum with a Brawler "Double Pumper"

I just checked it about an hour ago. Unhooked the throttle cable so there was nothing touching the throttle other than the return spring. If I lightly open the throttle and then slowly let it go back down (like I would just driving normally down the road for example) it sticks slightly open, increasing idle by about 150rpm. If I touch the linkage and push it back so its touching the adjustment screw it idles as normal. It does the same if I let the throttle snap closed like blipping the throttle. I did also check to make sure the accelerator pump wasn't binding on the cable bracket or anything, no interference.

Could that just be a tuning issue? If so what would said issue likely be?
if that's all it is, I would think even a single return spring should close it. But that does seem to confirm its mechanical.
Double return springs are mandatory by a lot of sanctioning bodies.
Having had a throttle cable fray and stick (on a public highway no less!) I can understand the requirement.