85 Gt carb and intake question

[cabbage45]

i have a question. I changed out my carb to a holley 4160c (already had a aftermarket carb on it), the problem now is that after hard acceleration when you let off the gas it back fires through the exhaust. I want to know if you have to change the intake also when using a different carb. I think it's pulling vacuum under acceleration. If I need to change the intake what is a good option, the engine is original and still stock.Thanks

[84GT350CONV]

I have a 4160 on my car with the original intake. After the rebuild of that carburator, it works fine. I did have to rig up a dashpot and the microswitch so the car would not stall when running the air conditioning. The microswitch is for turning off the AC at full throttle. Your car was originally equipped with a 4180 Holley that originally had the dashpot/microswitch setup. You may have a timing issue or you need to look at that replacement carb. Did you adjust the idle mixture screws with a vacuum gauge and check your float levels? Holley has YouTube videos on how to do this. Was your gasket under the carb in good condition?
JA Cook is our Guru on here for carbs and helped me as well as many others.

[cabbage45]

I didn't put it on, a shop that builds mustangs did the work. The carb was brand new, they said that they adjusted it. i'm at a loss at this point. i have spent a lot of money trying to get this thing running right, but nothing seems to work and the mustang shop hasn't been much help. They want me to figure out whats wrong with it, then get the parts and bring it back so they can fix it. If I knew all that I would have fixed it myself to start with... LOL

As a side note: when running at about 1/4 throttle in any gear (1-4) at about 2100 rpms the car will surge back and forth.

[Ethyl Cat]

Sounds like it is too rich

[WilliamCapri]

Hi Cab,

Google drop throttle backfire. Most responses say that it is due to a lean mixture. It's counter intuitive but that's the way it is. A lean mixture, when the throttle is closed suddenly lets a poorly combusting mix get into the exhaust system where it ignites.

Find out what primary jets are in your carb. If they are less than 63's you are probably lean. The 4160 is similar to the 4180 in that it has 4 circuits to feed fuel.

Idle, transition, venturi, and power valve. You say you are surging at partial throttle. This is when you are running on the idle and transition circuits. The amount of fuel for these is controlled by the jet size. My experience has been to increase the jet size to solve this.

When under heavy load, wide open throttle, the venturis and power valve are in play. The power valve is determined by your vacuum at just off idle. Worry about that later.

Also, check the timing. Is your harmonic balancer original? These things are aging and known to turn resulting in false timing setup.

As 84GT says, JA Cook is the master, do whatever he says when he adds in.

[TopGear85]

I'd just find a 4180c and be done with it. Then you won't have to add any switches or a dashpot and the car will have the carb it originally came with (since your engine is all stock anyway).

[cabbage45]

I found my problem, the vacuum advance was plugged ino the wrong port on the carb. We changed it to the correct spot and reset the timing, runs much better now.