I agree with both the above. Just buy a vacuum gauge (any auto parts store, inexpensive). An inexpensive tune-up tachometer would also be nice.
The vacuum gauges usually have a bunch of fittings to adapt it into the manifold vacuum somewhere. Typical spot is to splice a hose into the distributor vacuum advance line with a Tee fitting.
Once the engine is warmed up, set the idle speed with the tach to spec. This is done with an adjustment screw on the throttle arm that moves the throttle shaft very slightly to prop open the throttle plates. If the tune is way off, you may need to adjust idle screws before setting the speed, adjust speed, set screws, and so on.
To adjust the idle screws, simlpy slowly turn each idle screw until you have maximized vacuum reading. If that doesn't give you a smooth idle, you have some other problem such as a vacuum leak in a manifold gasket or carburetor gasket.
If your carb is being flodded with too much gas from a mis-adjusted float or bad needle/seat, the engine usually is running VERY rough and black smoke is pouring out the tail pipe. Very dramatic. Idle adjustment won't fix that. You must dis-assemble the carb and do internal repairs (set float to spec, replace needle and seat, etc.). If your problem is that you aren't getting the gas mileage you expect, the problem isn't in your idle adjustment. That is a primary jetting or engine condition problem.
[ October 15, 2002: Message edited by: firstname.lastname@example.org ]</p>