Ford Repair: 69 shelby gt500 428 cobra jet, fuel stabilizer, cobra jet

Question
car trys to starts and sounds like it will, but then it dies out and backfires through the carb.it appears to be getting too much gas.  took out 4 vsparkplugs and replaced them but still have the same problem.  i was told that the plugs were fouled.  i tend to believe it is a carb problem.  fire shoots out of the carb while trying to start.
i dont drive it.  i usually start it up and let it run for 15 to 20 minutes a month.  try to use the gas in the carb.  i let it run till it warms up.
any suggestions would be appreciated.

Answer
R G-

  First of all, I bow to your car, sir.  The GT500s are argueably the finest incarnation of the Mustang ever.  As for your problem...it's hard to pin down.  There are a couple factors here I can imagine giving you the problem you mentioned.  First off is obviously the carb.  Since carbs have the seals and gaskets just like an engine does, it's possible for them to dry up or wear out.  You could also have a float problem or a problem with the jets or needles.  The second issue I could see is sort of carb related and that's the gas itself.  You didn't mention how often you fill the car up with gas or if you use a fuel stabilizer, but without a stabilizer gas goes bad within a few months and becomes thick and gummy.  It can clog up carb parts causing them to remain shut or get stuck open.  Lastly I would simply check your timing.  If the timing chain is worn out and the timing is off, it can cause fuel to back up into the intake.  That would help explain the backfires through the carb (backfires can also blow floats which will cause the carb to run super rich).  Hope this helps.

Steve