66 chevelle 396, a/t. rebuilt engine with about 150 miles on it. Car runs great, but I notice that when the car is not pulling, i.e coasting down a hill or leveling off at desired speed the engine has a miss. it has a 270 comp cam. one of my buddies says that its the cam, but I don't think thats it. is it timing, dwell, carb or what? I just need some direction as to what I should look for first. thanks

If the car/engine runs great what's the problem??A little miss when the engine is on full vacum (eg: coasting)is not unusual.
Remember these are Chevy's not Lexus.

I agree that it may not be all that unusual but it may be sucking vaccuum and causing the miss, either intake manifold or carb.

Probably too much advance (I assume you have your vacuum advance hooked up)

If you have 36 total advance, you could easily have over 50 degrees advance at cruise.

Had the same problem (vacuum would put about 16 degress in ) and my vacuum advance was not adjustable. Like yours, pulled very well when accelerating, but let off and it seemed to miss.

I just unhooked my vacuum advance and solved the problem.

Interesting, what is the vacuum source manifold, ported? I am not sure but don't a lot of guys disconnect the vac advance and build that into the base timing?

Yeah like coyote said, make sure your vacuum advance is hooked up to the vacuum port above the butterfly blades on the carburetor not the one below it. Try adjusting your low speed mixture screws also. It could lean out with your throttle closed and a forced higher rpm i.e. going down hill.