Engine sputters and chokes at part throttle

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giantpune

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I'm looking for some input to chase down a gremlin. Car is a 1973 H code 351C, C6 transmission. Previous owner has installed a holley single plane intake thats supposedly made for the 2V heads, headers, and a pretty new looking holley vacuum secondary carb. I put on a new distributor yesterday, hooked up to ported vacuum.

The Issue I'm having is the engine stumbles if you're driving and let almost all the way off the gas. It idles fine, both in gear and in park. It accelerates ok. If you let off the gas completely, it'll coast. But if you try to give it part throttle while you're coasting, or if you accelerate up to like 45MPH and then let off the throttle partially to try and maintain that speed, the motor will sputter and hiccup.

I have tried tuning it with initial timing at 8, 12 and 14 degrees. Each time, I went by the book setting timing and adjusting the idle mix screws with a vacuum gauge. Vacuum gauge stays stable in the high teens. And after setting the idle speed, adjusting the accelerator pump so it activates right when you hit the throttle. After a while, I figured maybe it was trash in the carb or something. I replaced it with a brand new edelbrock AVS2 carb. And damn if it doesn't do the same thing after I got that bolted up and went for a test drive.

Does anybody have any guesses what to try next?

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It does sound like a fuel problem. The vacuum readings seem to indicate you do not have a vacuum leak. But, you changed the carb, but not the fuel filter? If you suspected trash, that’s got to go. I’d change it and see if I have any improvement. If not, try the full throttle test. Get it up to like 35, then WOT, and see what happens. If you get same, I would try and either test the fuel pump output and or pressure. Let us know what you discover.
 
It does sound like a fuel problem. The vacuum readings seem to indicate you do not have a vacuum leak. But, you changed the carb, but not the fuel filter? If you suspected trash, that’s got to go. I’d change it and see if I have any improvement. If not, try the full throttle test. Get it up to like 35, then WOT, and see what happens. If you get same, I would try and either test the fuel pump output and or pressure. Let us know what you discover.
For 35 to WOT, that depends. If I'm at 35 and already giving it more than half throttle and then put my foot down, it'll pick up and go. If I'm at 35 and foot off the throttle, then when I stomp on it, it kinda bogs down a bit like it wants to die and then powers through.

What makes you say its a fuel issue? If I start from a stop and give it the gas hard, It'll go all the way up as fast as I want to go without stumbling. It seems like the fuel pump is able to put out as much as the engine can take.
 
If you didn’t think it was a fuel issue, why change the carb? While we all try and explain our car issues, mine has a few, what we do and communicate, tells the others where we think the problem is. Since you swapped the carb, not just check the other for junk, I suspected since the replacement didn’t make a difference, that maybe a fuel issue, but not clogged, just fuel starved. That was my thinking.
 
If you didn’t think it was a fuel issue, why change the carb? While we all try and explain our car issues, mine has a few, what we do and communicate, tells the others where we think the problem is. Since you swapped the carb, not just check the other for junk, I suspected since the replacement didn’t make a difference, that maybe a fuel issue, but not clogged, just fuel starved. That was my thinking.
It seemed to me like something was up with the transition circuit, or maybe a bad power valve. Swapping the entire carb out was easier than tearing into the old one and was a way to fix whatever unknown issue there was with the carb.
 
Two carbs - same problem. Have you looked at the distributor? Condition of points? Any wires rubbing? Unhook the vacuum advance and see if it does the same thing.

The WOT bog is probably your carb needing to be tuned to the combination.
 
Two carbs - same problem. Have you looked at the distributor? Condition of points? Any wires rubbing? Unhook the vacuum advance and see if it does the same thing.

The WOT bog is probably your carb needing to be tuned to the combination.
The whole distributor is new. I replaced the old one a couple days ago. The old one had lose parts rattling around inside it.

Just now, I unhooked the distributor advance and capped that port at the carb. The car runs worse now under acceleration. But the stumble at part throttle is gone. If I had to quantify it, the spitting and sputtering under part throttle with the advance connected is like an 8/10. And the spitting and sputtering under acceleration with the advance disconnected is like a 2/10.

The vacuum line between the carb and the distributor is a straight shot of new line, with no Ts or branches in it. So there shouldn't be a leak involved there.

Edit -
Based on the engine running more better with the vacuum advance disconnected, I went out and started playing with that. Its the adjustable type. I have initial timing set at 8 degrees. When I connect the advance line to manifold vacuum at idle, it made the engine sputter and the 0 degree mark on the balancer wasn't even visible anymore when the light flashed. So is like the distributor was giving an insane amount of advance. I stuck an allen wrench in the hole and adjusted it while connecting it to and from manifold vacuum. After a bunch of counterclockwise turns, I got it so when it gets vacuum, it gives 10 degrees of advance. Then I connected it back to the ported connection and drove around. It runs a bit better. Still not great, but its a step in the right direction.
 
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The whole distributor is new. I replaced the old one a couple days ago. The old one had lose parts rattling around inside it.

Just now, I unhooked the distributor advance and capped that port at the carb. The car runs worse now under acceleration. But the stumble at part throttle is gone. If I had to quantify it, the spitting and sputtering under part throttle with the advance connected is like an 8/10. And the spitting and sputtering under acceleration with the advance disconnected is like a 2/10.

The vacuum line between the carb and the distributor is a straight shot of new line, with no Ts or branches in it. So there shouldn't be a leak involved there.

Edit -
Based on the engine running more better with the vacuum advance disconnected, I went out and started playing with that. Its the adjustable type. I have initial timing set at 8 degrees. When I connect the advance line to manifold vacuum at idle, it made the engine sputter and the 0 degree mark on the balancer wasn't even visible anymore when the light flashed. So is like the distributor was giving an insane amount of advance. I stuck an allen wrench in the hole and adjusted it while connecting it to and from manifold vacuum. After a bunch of counterclockwise turns, I got it so when it gets vacuum, it gives 10 degrees of advance. Then I connected it back to the ported connection and drove around. It runs a bit better. Still not great, but its a step in the right direction.
Prior to changing to my Petronix billet distributor, I had to do the same adjustment on my distributor. It was new points type distributor from rock auto, the adjustment Allen screw was almost all the way in, after a bunch of turns out, the idle was much better.


Good luck
Tom
 
So now its running better. My current issue is the engine randomly wants to die. I can start it up and just let it idle and like 5 minutes later, it'll spit and sputter. If i fiddle with the throttle, i can keep it running and after like 10 seconds, its back to running smoothly.

It doesn't seem to matter if i'm idling or cruising around. It still does it. I also waited for it to act up and i pulled the vacuum lines and plugged some with my finger and jiggled the wires to the coil and all the plug wires. None of the stuff i jiggled seemed like it had any effect.

Does anybody have any guesses? Do anything like the coil or fuel pump act like this when they're starting to go out?
 
While I don’t suggest you use a glass fuel filter, for testing purposes, it might help to isolate if it’s not getting a consistent flow of fuel. While not a great idea, it’s something to try.
 
While I don’t suggest you use a glass fuel filter, for testing purposes, it might help to isolate if it’s not getting a consistent flow of fuel. While not a great idea, it’s something to try.
For debugging purposes, I rigged up a temporary fuel regulator, pressure gauge, and clear filter right before the carb. The pressure gauge stays steady at 6.5psi and fuel flow does not appear to be interrupted. The thing still will sit there and idle a while and then out of nowhere start acting up.

I did have the radiator cap off and was watching inside. It did seem like the car started out cold and the engine crapping itself coincided with the coolant level in the radiator dropping. Like engine gets up to temp, thermostat opened, water pump started pumping water from the radiator to the engine and right about then is when the engine did the spit and sputter thing.

I tried to keep watching to see if it would do it again. But it seemed like the water just kept flowing. Like the thermostat stayed open and the water was constantly cycling through the radiator and engine. Next time I go out and mess with it, I will try to see if it happens around the same time.
 
I wonder if some temperature control, possibly of vacuum hoses, may be the cause of your problem.
I was thinking that. The vacuum and intake are definitely not in factory condition. The intake manifold is a holley. The vacuum tree up by the thermostat is present, but has nothing connected to any of its ports. There is one large vacuum tree sticking out of the manifold in the rear that has the brake booster, transmission, and vacuum canister connected. The EGR valve is present in the intake manifold, but the vacuum port is capped. And the PCV valve is connected directly to the carb. I'm not sure where else any of these things may be.
 
Is the EGR valve a working unit. In other words, would it hold vacuum if you applied it? I ask because the info you’re providing is seeming to be more an electrical issue of some sort than a likely fuel condition. My Ford mechanic neighbor of 40 years when I was a kid use to say, stead problem is fuel, intermediate is spark. Now, not always true, but is a good general indicator. And you’ve done grade A testing without a conclusive resolution.

If the EGR valve was stuck in the open position, what symptoms would it generate?
 
What's the status on the battery cables? I see an aftermarket positive, has the factory negative/ground cable been swapped? Is there a solid battery->fender apron->engine block ground path?

It wouldn't hurt to add one from apron to engine to see if it changes anything.
 
Is the EGR valve a working unit. In other words, would it hold vacuum if you applied it? I ask because the info you’re providing is seeming to be more an electrical issue of some sort than a likely fuel condition. My Ford mechanic neighbor of 40 years when I was a kid use to say, stead problem is fuel, intermediate is spark. Now, not always true, but is a good general indicator. And you’ve done grade A testing without a conclusive resolution.

If the EGR valve was stuck in the open position, what symptoms would it generate?
I tried a vacuum pump just now and the EGR valve will not hold a vacuum. I'm not very well versed on their behavior or how to test them.
But I can say that the thing has been disconnected and capped off the whole time I've had the car, and when I try to pull a vacuum on it, the gauge drops back to zero very quickly.

I call it the EGR valve, because thats what it looks like and thats what I got when I searched the part number. But based on what @Hemikiller posted here, its not in the correct spot. https://7173mustangs.com/threads/egr-block-of-plate.32120/#post-330062
I do not have a carb spacer. This doodad is mounted on the back of the intake manifold.

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I’m not really sure what a stuck open EGR valve would do to an engine either. I don’t even know how to test that. But, you could remove it and see if it stuck in the open position. If it is, you might be able to loosen it and get it to seat, so the exhaust gases are not continuing to mix with the incoming fuel charge.

Maybe others here can chime in on the expected performance issues of a stuck open EGR valve.
 

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