Well crap, - I had a brand new Ames 4bbl throttle cable laying around and after installing it I got the idle down to about 800RPM. I'll never be able to get it to 500 so I can get the solenoid set to idle at 650RPM. The search for clues continues. Bad thing is I'm checking my dizzy for mech advance and am sure I'll have to kick the timing up slightly after I'm done, which will bring the RPM higher again.