QuadraJet Idle Problem
from corvetteforum.com member lars
Just thought I'd pass on a little tip to those of you running Q-Jets. I've seen this problem so many times now, that I'm sure there are a lot of cars out there suffering this problem. This weekend, I was once again working on a guy's car that was suffering from eratic idle once warmed up, and the idle mixture screw on the passenger side was unresponsive. This was the 6th time this summer I had seen this problem, so I was pretty sure I knew what the issue was....
A Q-Jet has several vacuum passages/plenums on the bottom of the throttle plate ("base plate"). Some of these passages run very close to the outer edge of the throttle plate. Aftermarket base gaskets are cut/trimmed VERY close around their perimeter, and MOST base gaskets (especially the 4-hole gaskets) are actually smaller at their perimeter than the actual perimeter of the Q-Jet throttle plate. What happens is that one of the vacuum passages on the bottom of the carb will actually "hang over" the outer edge of the base gasket - very slightly - giving you an almost undetectable vacuum leak directly into the primary side throttle. This will mess up your idle and your idle mixture capability.
To check it out, simply unbolt the carb and pull yhe gasket out. Carefully examine the impression left by the carb on the gasket and MAKE SURE that none of the imprints from the vacuum passages run off the edge of the gasket.