I'll post some pics tomorrow if you really want them... but in brief, for each adjustment (my carb has 2, but some have 4) there is a boss cast into the baseplate. Each has a welch plug about 5/16" in diameter, installed backwards (lip toward the carb) so I can't just grab it with a screw extractor. Behind the plug is the idle mixture screw, which has a special head, although I've read that the right size Allen wrench will work.

This carb originally came off a '86 or '87 Ford truck, with either a 351W or 460. The primary bowl actually has cast into it, "Manufactured by Holley for Ford". They apparently really went out of their way to make it difficult to change the idle mixture, and I think the plug is actually hardened steel too!