rpm loss of acceleration

Im trying to get mustang that i have for sale running as smoothley as possible before i sell it and im having a bit of trouble with the acceleration. It has the normal low end acceleration fine but once i hit 4000rpm it begins to flatten out. It was never like this before, first i assumed it was carberator problems and began taking a closer look and all of the flaps are opening good (4 barrel carb.) So i thought it would be the spark plugs, i changed all my spark plugs today and retimed them. Nothing...
I dont know if anyone on this site can help me out with advice on an older domestic car but any suggestions would be appreciated.. i try to keep that car as mint as possible so this is really bugging me.
Thx.

What year of mustang is it? What model is it?

Hmm.. it could be your O2 sensor. That is what happened with my Dads '03 Cobra. Obvioulsy it was replaced under warrenty. :)

Yay for free repairs!!

Clogged fuel filter? fuel pump heading out the door?

carb out of tune?

Quote:

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Originally posted by dream_out_loud
What year of mustang is it? What model is it?

Hmm.. it could be your O2 sensor. That is what happened with my Dads '03 Cobra. Obvioulsy it was replaced under warrenty. :)

Yay for free repairs!!

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Cars without Fuel Injection...ie his Mustang...do not have O2 Sensors...but good troubleshooting nonetheless...

did you re-time the plugs, or re-gap them??, your post wasnt super clear to me...

...check the timing make sure it hasn't changed, or is within spec...also...just check basic things...your throttle linkage, fuel filter, air filter...make sure those are good and clean...

hey sorry i should have put that a bit more clear. It is a 1985 mustang gt.
I retimed and replaced the plugs...
Im pretty sure its not my throttle linkage cause off the line it is as good as ever when it is fully depressed.
The air filter and and fuel filter are pretty clean but I didnt think of the fuel pump though, im gonna take a look at that.
Thx for the suggestions everyone
:thumbsup:
:also i should specify what is wrong with it a bit better. When i start off the acceleration is normal untill i hit mid 3000 or 4000 and then i loose all acceleration.

Hey this might help when i put the rpm auto meter gauge on my civic the ignition coil plug was not in propley an i loss most power when the car went ova 3500 rpm but if i went ova the car would turn off so in the car park of macca's i just got some pliers an made the - coil plug tighter (rpm gaude goes to - ). After i did this i never had a prob agian antil now after i run someone up the ass:nut: bit im rebuilt the car now so im happy, Im not sure if this will work but just give it ago