A few "old" Mustang Tech Tips
Wolfgang Kohrn 1992 

Fuel

differential

FB rinse hose

ignition timing

ammeter

fuse link

plug gap

trans fluid

Fuel
Before kicking your Mustang: Never drive a pony without fuel, you won't have much fun and you can save a lot of time of troubleshooting. This is free of charge for beginners, sorry to experts. Now to the more serious ones.

Differential oil
Check your differential oil occasionally. You can bet, that also your previous owner forgot. So this is a must. A change should be in order as well, as it usually never gets a change, if nothing went wrong.

Humid interior
In Fastbacks check the water hose going down from the rear vent windows. They very often crack or get loose, so water drips into the hidden corners and causes bad damage.

Ignition
If you have trouble with correct ignition timing and nothing else works, check your balancer. If the rubber has detoriated, the timing ring (and marks with it) will wander around the core, thus making the setting ridiculous. Do not rely on your tinimg light. Rather use your ears and common sense to finetune your engine or go to a professional shop.

Ammeter
If you have problems with your ammeter on classic Mustangs in that it shows no real movement, when turning on the headlights (engine running),check the small voltage regulator and wiring connections behind the dash first. If it is o.k. and the ammeter itself has not yet lost "its life", then you may improve its performance by extending or cutting the wire to the alternator. This increases or decreases the resistance. Be careful and go by 1/2 inches and check the direction, where the needle goes after this modification. Otherwise you will loose it. If you use a transitor ignition module, the ammeter is affected, as well.

Fuse link
From April 69 Ford used Fuse Links in Mustangs. This is a short isolated cable between the battery connector of the starter relay and the alternator. The slightly smaller diameter cable was supposed to burn,if there is an abnormal extreme load. On this calbe Fuse-Link is usually printed. (C9AZ-14256 is the replacement cable). You may detect a defect cable by the bubbled isolation, open wires or similar. Sometimes nothing can be seen. In that case disconnect the terminal at the starter relay and put a ohmmeter to the end of the cable and the starter relays. A fuse link that is o.k. does not show any resistance. If your headlights do not light on and the battery is o.k. the fuse link cable might be defect as well.

390GT spark plug gap
Ford advised in June 67 for owners of 67 390GT owners to enlarge the plugs gap from 0,028-00,32" up to 0,034". This should calm your rough idling, that is typical for those engines.

Trans fluid reading
Reading the automatic transmission fluid level can be very difficult. If you drill small holes in the dipstick lower part at the Full or Add mark, you have an easier reading. In 1969 Ford changed (beginning with the 240cid engine) the FULL mark to a SAFE mark. The reason was, that due to temperature changes the fluid level could be still accepted above the former FULL mark at higher temperatures. If you are below the ADD1 mark, a quart (0,9 Litre) is the maximum, you should add.