Rising Idle

[Hard Luck]

Out driving the CST and as the day pressed on, idle speed increased. I will try to be as specific as possible.

327 with a slight cam, turbo 350, HEI, Edelbrock manifold with their Q-jet, and long tube headers. Outside temps were steady at about 40. Ran a bunch of errands, stop and start with the average speeds around 30 to 40mph. Total time was about 2 hours. Engine temps at a constant 180 degrees.

Fresh tune up, recent oil change, new belts and hoses, coolant system flush with new thermostat.

Started the day with an idle of 800 in neutral and in gear about 500. Little bit of lope, does not seem like it can go any lower in gear. After a few stops the unladen idle went up to 1000 but in gear was still 500. Runs and drives just fine. A few more stops and idle rises to around 1200 but in gear is still 500 and still runs fine. No visual clues as to what is going on.

Any ideas?