why will my mustang not idle

Sounds like the carb may need cleaning. How long has it set not being drivin?

As per our pm: Check for vacuum leaks. Older cars tend to do more setting in the garage than being drivin. How long had it been parked before you drove it to store? How old is the gas in it? If gas been in there for a month or more you probably fighting a condensation issue. Try adding some fuel cleaner to your gas if so. If gas older than that then I would remove all the gas from tank, clean the tank and then add new gas.

Also if it still has the old points style ignition might try filing them with an Emory board. Condensation plays hell on them too. If it does have old points ignition I would look into replacing them with the electronic ign conversion kit. They are fairly cheap and work great.