18" I think is OK at idle, though you'd like it higher - should be higher if the engine is recently rebuilt, I think, but it's still a healthy #. You may get higher if you tweek the timing, and check the idle mixture screw?
Most important thing would be to check carefully for any vacuum leaks around the base. "Common Wisdom" way to do this, is to spray carb. cleaner or some such around the base of the carb - if there's a leak, it will get sucked in, richen the mixture and the engine will speed up noticeably for a few seconds.
Personally, I don't like spraying anything on my engine if I can help it. I use an unlit propane bottle or acetylene torch for the same test. Use just enough flow so the engine speeds up some when you hold it near the air intake. Then, slowly move it around all sides of the base of the carb to see if there are leaks in the gaskets.
While you're at it, you can check for leaks around the intake manifold as well, not-to-mention the vacuum line to the vac. advance, and to the wipers and/or the hydrovac, if applicable.
My thoughts, anyway . . .
-Michael
