The air gap is in the distributor, between the pick up and the reluctor. Look under distributor, it is in my Haynes manual.
1. remove distributor cap
2. align one reluctor tooth with the pickup coil tooth by rotating engine.
3. loosen pickup coil adjustment screw.
4. insert non magnetic (brass) feeler gauge (0.006") between teeth.
5. move pick up coil until light contact is made with feeler guage. NO force should be necessary to remove feeler.
6. using a 0.008 feeler guage, check the air gap. feeler guage should not fit between teeth (DO NOT FORCE).
7. replace cap.
If this doesn't fix your problem, you need to set your timing.
1. Disconnect vacuum advance line. If you have a vacuum guage, attach that to the vacuum port on the carburator. If not then disregard this part of the instructions.
2. Loosen the distributor hold down bolt and bracket enough to allow rotating of the distributor.
3. Start the engine. If you have the vacuum guage, move the distributor until peak vacuum is attained, then back the timing down slightly (clockwise). If you do not have a vacuum gauge, then listen to the engine. When peak rpm is attained, back the timing down slightly (clockwise).
Hope this helps.