l have a 1973 mercury cougar xr7 with a 351 c. has a edlebrock intake manifold, street avenger 670 cfm holley, msd 6 ignition system, msd pro billet disributor, msd master blaster 2, msd 8.5 spark plug wires, duel flowmaster 40 is and a k&n air filter. l get about 7 miles to the gallon l used to get 12 with all same parts this all happened over a years period its been tuned up and still no dice ive been driving it like a baby and still getting low mileage any suggestions on what it could be
i have a 1973 mercury cougar xr7 with a 351 c. has a edlebrock intake manifold, street avenger 670 cfm holley carberator , msd 6 ignition system, msd pro billet disributor, msd master blaster 2, msd 8.5 spark plug wires, duel flowmaster 40 is and a k&n air filter. l get about 7 miles to the gallon l used to get 12 with all same parts this all happened over a years period its been tuned up and still no dice ive been driving it like a baby and still getting low mileage any suggestions on what it could be

just go ahead and trade your car in .For a brand new car

It is hard to pinpoint a problem like this. It could be the choke not opening fully, the float sticking, dirt in the needle valve not allowing it to seat properly, incorrect mixture adjustment, restricted air filter, ignition timing or advance, transmission problem, dragging brakes, a trunk full of dead weight, or todays' gasoline. E-85 probably has a little less energy than yesterday is gas. You will need to remove the sparkplugs to see if an overrich condition exists. Chances r the problem is coming from the carb.

check the fuel pump, it could be leaking internally. or try an electric one. Are you running any kind of vacumm advance?

CHECK THE COMPRESSION
my guess is the valves are burned and leaking..
because of lead free gas

See also:
http://verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm

Do you keep the carb adjusted regularly? A carb needs to be regularly adjusted to keep in tune, unlike today is fuel injection systems. If your carb is running overly rich, it could cause the loss in fuel mileage. Check the mixture on the carb. l have owned one of these cars in the past, and l must admit, while a great driving and riding car, it did get terrible fuel mileage for the power output.

Maybe its time you got one of these: http://forbesautos.com/gallery/2008/mercury/mariner_hybrid/?partner=a

Well since u said it used to get 12mpg with the parts then maybe u should check out your fuel flow, and sometimes it can take time for those parts to start effecting your fueling. Also another things did u happen change your tire size at all because changing your tire size to a bigger size or profile it tends to take up more gas because the distance it takes to make a complete rotation has been increased so it will need more gas to shorten the time of that distance.

It is probably ur Hydrocalator but my 2002 Ram 4.7 got 13-14mpg American when l got it.
Cold air intake gained 3 mpg
Dual flow-master cat back 4 mpg.
Teflon axle grease 2 mpg.
throttle body spacer & tornado 4 mpg.
slick 50 2 mpg.
electric fan, lose the clutch fan 3 mpg.
sparkrite plugs 2 mpg.
computer chip 6 mpg.

l now get 40 mpg imperial ''driving hard''
382 HP
495 foot pounds of torque at 1200rpm .
l can not believe those stupid engineers at Dodge could not think of this. I've increased the efficiency of an internally combustion engine to 60%. It is been around 30% for the last 50 years. To think, l thought, these were all scams