1992 GTS rumbling/shaking at idle

[seadoodle]

So here's the situation. My GTS shakes at idle like it is cavitating or engine is running rough, but becomes smooth when I tap the throttle to raise the rpm. Sometimes it will idle for a short time without the the rumbling.At full throttle, it runs great. When i adjust the idle to make it run smooth, my idle is too fast for the no wake zone. I believe my issue lies with the wear ring, but upon visual inspection, I don't see any defects on the impeller or wear ring. I also messed with the lkow speed adjustment by putting it 1/8 leaner thinking that a richand mixture was making the engine run poorly, but that didn't help so I turned it back. Does anyone have any idea what to try next, maybe I should turning in richer? My intake grate is clear also.

 

[mikidymac]

THe pump will have nothing to do with idle. These engines do shake quite a bit at idle.