‘57 312 hesitation/stuttering under acceleration and exhaust fumes

[Chambers777]

Hi there,

Looking for help and advice on this, under acceleration the car seems to stutter and splutter for a second or two and then finally finds itself and kicks on. It is definitely more pronounced when going uphill, I’m not talking of heavy acceleration or anything just general driving around town and the suburbs etc. I’m also getting quite large drafts of burnt gas/exhaust smell when I have the windows down and driving which I haven’t noticed before, I’m wondering if these two issues could be connected. It’s a rebuilt motor with around 2500 miles on it, The car starts hot and cold with no issues, one turn of the key and she goes, I’m running a electronic ignition and Holley 600CFM carb. In drive she idles around the 500rpm mark, in neutral between the 700-800rpm range. I have noticed that on start up and backing out the garage she spits out some black soot on the ground, I thought that was pretty normal as I think on cold idle these things run pretty rich.

Because of the lockdown over in Scotland I haven’t managed to drive her properly for best part of a year, she has been started every month, brought up to operating temp and shut off. Last summer the spark plugs were changed out to some good premium ones sold from one of the T Bird suppliers, that’s all that I have changed ignition wise, never had any issues with accelerating or excess exhaust fumes in the past.

Any ideas where I could start with this, running too rich?? Needle/float issue?? other than this the car does run very very well

Appreciate any advice or help, many thanks for your time.

Chris Chambers
Prestwick, Scotland

[klogsdonjr]

Chris, I'm sure others will chime in here with more knowledge than I. I have the same carb and when it was being dialed in after running for a bit I experienced the same stuttering accelerating uphill. For me it was a simple leveling of the gas inside the carb chambers on each side so that it was up to the side overflow ports, equal on each side. Didn't have the exhaust issue you mention. Hope my experience helps some.

[T-Bird Bob]

Hello Chambers,
I am not familiar with your carburetor model in particular, but that behavior can be observed on any carb with the following problems:
A) Acceleration pump not working
B) Choke not opening fully or overly rich by some other means (idle screws or even jets)

In fact I think it is two separate phenomenons: the stutter on acceleration is from the non functioning accelerator pump and the soot and fuel fumes are from an over rich condition.

Here is what you check:
1) Choke:
-get your engine to fully operating temp
-remove air cleaner
-Now you should see the choke flap and it should be fully open (almost or fully vertical)
If that is not the case, adjust the choke mechanism (sually the plastic pot needs to be loosened and turned until the flap is oen - but check your carbs manual)
Here is a nice video from Holley:
https://www.youtube.com/watch?v=0H5IBSFgYJE
2) Accelerator pump:
-Once you have done the choke adjustment or if the choke is fine, look into the primary venturies of your carb and operate the throttle.
-For each stroke of throttle (from fully closed to fully open) you should see a small stream of gas being squirted in the primaries (on both sides).
-you need to pump multiple times, because sometimes the checkball on the pump reservoir refill line is stuck and the accelerator pump doesn't refill correctly after the first stroke.
-If that is the case, it might not be enough and you have to adjust the accelerator pump linked to increase the volume.
Here is a nice video:
https://www.enginelabs.com/news/video-h ... arburetor/
They also talk about other tuning ways, but if it worked before I doubt you need a different cam or nozzle.

3) General Over-rich condition (not created by partially closed choke)
If you have done all the other things, this is where you might get rid of some of the richness and fumes you are smelling.
Air-fuel ratio is difficult to measure. I have a wide-band oxygen sensor and gauge with a bung I welded to a 1.5" pipe approximately 1.5 ft long. I can stick that in any exhaust and measure AFM directly. If you don't have such a thing, you have to tune idle mixture by vacuum:
-get your engine to idle at operating temperature (fix choke before as above if necessary)
-connect a vacuum gauge to the vacuum line of the wipers (make sure you are not measuring the fuel pump output - ask me how I know)
-now measure vacuum at idle and turn BOTH idle mixture screws in our out 1/4 turn at a time.
-Adjust to maximum vacuum.

Here are some other considerations:
-Always change only one thing you find lacking and test drive after each step (Choke first: see what changes. Accelerator pump next: see what changes. etc.)
-Gas smell can come from a fuel leak somewhere in the engine bay and the gas evaporates in the hot engine bay. You could for example have a ruptured accelerator pump diaphragm. That would explain both, the acceleration stumble and the fuel smell when driving.
In that case, you have to start the engine up from cold and see if you have a leak under the accelerator pump before the engine gets hot (once it is warm, you wont see it because it evaporates).

Good luck and please report back what you found...

Have fun!
Bob

[Chambers777]

Many thanks for your informative replies, great info. I am just back in from the garage just now and was going to try a few things out, before I started I pulled the plugs to see if their was any fouling, when running my hand underneath the manifold I found an HT lead not connected up to one of the plugs!

The weather here just now isn’t great so I can’t take it out to see if this is the problem but it started and ran fine. Could this be the source of the issues I was having?? I’ll take a test run next time weather allows and see how she performs.

[ICON 1956]

Hello Chris, What you just described did exactly what my 65 mustang did, I found that the petro in the carb bowl is flooding. I too had black soot coming from my tail pipe...All the above are great suggestions to follow through.

[thessler]

You didn't mention if it did this hesitation before you laid it up. If it ran fine before you laid it up it will run fine again.
I know you started it every now and then, that's good but no where near the same as driving it. Just as an example, warming it up to temp doesn't even use the accelerator pump, never mind the carbon just building up in the engine. Not saying what you did was wrong. If it were mine I would give it some fresh gas and drive it for a week then evaluate.
Just some thoughts, Tom

[JimCunn]

Check the hose connections on your vacuum advance.