Trial and error, unless you have access to a chassis dynamometer and an exhaust gas analyzer. If you haven't taken the carburetor apart yet, close the jet all the way, count the turns required to close it from its present position, then when you put the carburetor together, set the adjustment at the same place. From there, watch the color of your spark plugs after running a short time- 25-50 miles or so, doesn't necessarily have to be non-stop: Dark brown/dry black color indicates the mixture is too rich, lean the jet slightly; white tip indicates the mixture is too lean, enrich the jet slightly. A good color is slightly tan/off white. Also, when you are test-driving, listen for detonation/pinging/spark knock under both light and heavier acceleration. If the ignition timing is set correctly, detonation may indicate that the mixture is too lean- enrich the jet a little. The spark plugs are the best indicator of fuel mixture, but it takes a little experience (and miles) to get comfortable with what they are telling you. Regardless, clean, white is not the best condition. Also, burned electrodes or broken insulators are bad conditions, although those won't normally occur in a short drive.
Also, the spark plug color will be affected by the spark plug heat range you have. I'm not sure about your GMC, but AC42s or 43s probably are all right. AC44s may be all right for light duty- no heavy loads, but may be a little too hot if the original specification is for 42s. I'm sure someone else here can give the proper spark plug type.
Harvester
