My guess would be the powervalve since it won't idle and you tried to lean it out with the air fuel screws and couldn't.
I have rebuilt one carb in my life and i could stumble through another, but I'm not sure. I think you remove the front bowl, then a plate (primary metering valve) that is about half inch wide. That plate is where the powervalve is.
If you are comfortable doing the work yourself, talk with Vince at discount inboard marine (skidim.com) and he will be able to tell you more. Go ahead and plan on replacing the gaskets as you remove them.
I'm sorry I can't give you any more help, but I could stumble through it, but I don't know it well enough off the top of my head. Check this site out. Its a car holleycarb, but it is very similar. http://www.chevyhiperformance.com/howto/45798/photo_20.html