Hi All,

Well I finally replaced the exhaust on my 1985 Mustang GT. I went from the
old stock system to the following:

Flowtech headers (Equal length long tubes)
Flowtech Off-Road H pipe
American Thunder Cat-back system from Flowmaster

The sound is phenominal! Now the only problem which I have now are the
following: :cry2:

-idles at 1,000RPM instead of 800RPM
-At highway speeds of about 120KM/H it starts to chug like it's not getting
enough gas or something and I have to either downshift or slow to 100KM/H.
It then smooths out but it feels like she is hesitating.
-Seems like the engine is shaking sometimes when I am idling. Is this
normal?

So can anybody please help me with what is happening? Carb adjustment
needed or what????? Timing off? Me and my buddy are gonna check it out one night this week to see what is going on.

BTW I have brand new plugs(Accel), Edelbrock air filter(triangular), and 9mm Ford Wires and
fresh oil.

Its a 5spd.

Please help.

 

If you were efi i'd say O2 sensor problems...but since you are carbed it's a whole different ball game. sorry, won't really be able to help you out...

 

The first thing I would check would be your header bolts. Since this problem only happened after the exhaust install I would make sure they are all still there. I know this sounds simple but I have thrown 2 header bolts in my day and the symptoms you describe sound the same. Do you hear a hissing sound at idle? Make sure they are all tight and in place. Sometimes new headers have a thicker flange and the bolts don't get tight enough. If this is so invest in some stage 8 bolts, they are awesome. The back bolt on the passenger side was the one that gave me trouble. Also the header flange to the off road pipe. I was sent Ford motorsport headers one time that seemed to be fine until I started the car. The ball flange was not the right size and I had a wicked leak. The car's idle was awful and would barely drive. Hopefully it is something this simple.

 

i've been thinking on this for a little while now, and my less than useful response earlier shoudl be made up for with at least some kind of input. the syptoms you describe could also be ignition related...check your plugs and make sure you didn't crack any during the header install....

i'm not sure what other mods you've done to the car but all that exhaust just seems like way too much for me...you've lost all your back pressure. try putting your stock h pipe back on and see if the problem goes away...

 

Even if its "too much exhaust" he still shouldn't be experiencing these kind of performance issues, I'm running Flowtech E/L's (albeit, not long-tubes) and a 2.5" O/R H-Pipe into a set of Flowmasters....I'm fulie, but the car has much more top-end than it did when I bought it but a bit less down-low. The up-top pull more than makes up for the down-low loss though. BUT, that said, no drivability issues arrose due to the swap, so I doubt that is the case.

 

it actually could make some driveability issues like that on a carbed set up....there is no way for it to measure the the change in the amount of air going through the system now and therefore no way to compensate for it either. the carb may need to be rejetted...but i'm pretty stupid when it comes to carbed cars so don't listen to me.

i still think headers and an off road h/x pipe is too much exhaust until you are well into the 12s...

 

Well boys,

My mechanic buddy is gonna give me a hand tonight to have a look at her so I will let you guys know what the deal is. If I had a diagram of what does what on the carb I would try to adjust it myself..........but I don't. Anyhow hopefully it's just an adjustment.

 

I am quite sure it is not exhaust related unless you have some massive leaks. Even then the problem wouldnt be intermittend and speed related. It could have something to do with your:carb adjustments, distributor and/or timing, spark plugs and/or spark plug gap, EGR system, and possiby even the triangular air filter.
I had one of those air filters and the foam got sucked into my carb because I forgot to do something (dont remember what it was cuz it caught on fire shortly after).
You could have bought the wrong spark plugs, but I dont think it would cause the symptoms how they're being described. Your gap could be off, maybe at a certain load they dont fire right.....
The distributor could have gone wacky on its own, and possibly your distributor cap and rotor are not on tight or need replacement.
Also check all the spark plug wires to make sure they're on tight. Double check the timing and the vacuum advance.
Also, anytime you make modifications that increase air flow in and out of the motor your carb should get some adjustments. Are you still running the stock Holley 580CFM carb? In any case, have it rejetted and adjusted.
Did you hook up the EGR system again? That could cause the car to run funny if its not back the way it should be. The idle should not have increased on its own accord if all you replaced was the exhaust system, the two do not have much if anything in common. Check your vacuum lines too. It could be coincidental that you're having these problems, something could have just gone funky on its own or you could have broken something. It's hard to tell without seeing the car myself, but I hope the various things I've listed above give you something to look for.
SS

 

I would agree that the first and cheapest thing to do is check for any leaks in the intake or exhaust (including the head).
Having played with many carburetors i would think if it is not a leak then it would be your carburetor adjustments. turn down the idle to where it should be and lean it up a little bit to start with. see how it runs after that. i don't think your ehaust has any problems with it, i have had larger exhaust on smaller things then that.

Chears

 

You re-installed and plugged in, or blocked off / plugged off all the vacuum lines that went to all the smog related stuff on the pass side above the header? If left open there will be a vacuum leak or two there... could cause the higher idle?