The two screws on the front control the idle mixture. They should be set with the engine at operating temperature. Start by turning each screw in until it bottoms lightly (engine off), counting the number of turns for each and comparing. They should be within a 1/4 turn. Then set them back to where they were, averaging the number of turns so you have a baseline. Next, adjust the screws.
Two ways, one is to hook a vacuum gauge up and set the screws to give the highest vacuum at idle; the other is to adjust to give the smoothest sounding idle if you don't have a vacuum gauge. I turn one screw 1/4 turn in and listen for about 3-5 seconds to hear if the engine liked the change or not, then continue accordingly, same method 1/4 turn then listen (or read the vacuum gauge) until I like what I see or hear. As a quick rule of thumb, start with both screws out 2 turns then using the above method, turn the screw in until the engine starts to stumble. Back the screw out until it is smooth again. Now adjust the other screw using the same method. After getting both screws close, you can now fine tune by going back and forth between the screws moving both in the same direction 1/8 turn. Again look for the highest vacuum reading or listen for the smoothest idle. Once you and the engine are happy, shut off the engine and again count how many turns it takes to bottom the screws lightly. They should be very close to the same, and now you know where the screws should be set if some one plays with them.
If you don't get a response on one or both screws as you turn it in with the engine running, it's time for a good cleaning/rebuild. Also, a large difference in the number of turns would indicate the same thing.
Another issue can cause an unstable idle in gear. If your distributor's mechanical advance is already adjusting the timing at an idle, you can have a good idle with no load but when the engine is slowed down by shifting into drive, the timing will be retarded and will cause the idle to drop off enough to require the two foot method.
To test for this you'll need a timing light and an extra body. Pull the vacuum line to the distributor off and plug it with a golf tee. Set the idle speed screw to get a slow but steady idle, then turn the idle speed screw in a 1/2 turn to get a slightly higher idle. Check the timing with the car in neutral, then watch what happens to the timing when your helper puts the car in gear. Note: make sure they have their foot on the brake first, and don't stand in front of the car while doing this test! You may have to look fast to see what's happening before the car quits! If you have a good helper, they may be able to vary the idle speed while in gear using the two foot method so you can see if the timing is changing. If it is, that could be causing the unstable idle.
