Running great one day and next it stalls every time I hit the gas too hard

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Bl1zzard

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1974 Dodge Dart Swinger with a fresh 360 LA, Holley 750DP, Edlebrock intake, stock ignition.

So on Saturday, I went for a boot on the back roads and had a great time doing burnouts at every intersection. The car was responsive and doing exactly what I wanted it to. I had a blast.

On Sunday my son comes over and I say lets go have some fun. First set of lights there's a new Mustang Shelby right in front of us. He's giving the look so I know it's on. Mopar vs. Ford. At the green I punched it, knowing he was going to and I kept up for the first couple of hundred feet, then he really hit and left us. Oh well, we continued out to the back roads for some more fun. First intersection, hit the gas and whump it stalled. From then on the rest of the drive, anytime you get aggressive with the gas pedal it stalls, even a highway speed it has a huge stumble if you hit the gas.

So my friends, where should I go with this. The vacuum is good, choke is manual and wide open, plugs are a little black for my liking, but I haven't really done anything except set the timing to 14 degrees at idle and 32 degrees all in. I haven't touched the carb, it came from my cousins 340 Dart he said he's never rebuilt it. Could something in the accelerator pump shot be dumping too much gas all of a sudden?
 
If it was me...

1. check fuel pump pressure
2. gas tank dirty ? ,sock filter on sending unit disintegrated and blocking the feed tube
3. water in fuel ?
 
Pump diaphragm is shot would be one of the 1st things I'd think ...and it's a used carb, so yeah... that or a big vac leak.
Fuel pumps have always just gone out, not really weakened with slow decline in psi. In fact many have just pissed fuel into the crankcase on me.
 
Could be a tank full of fuel sock bits clogging it, possible.. but every time he stomps on it? Had that happen, at least a tank full of net lookin bits. and the symptom was internment.
My cuda acted the same as the condition he's experiencing. Pulled the filter, cut it in half and it was full of sediment. Quick simple check.
 
Had a similar situation after having the carb rebuilt and installed using the 'thicker' gasket in the kit. Turned out the gasket compressed a bit and allowed a vacuum leak under open throttle conditions. snugged the carb bolts & all is/has been good! Simple enough to check.
 
If it was me...

1. check fuel pump pressure
2. gas tank dirty ? ,sock filter on sending unit disintegrated and blocking the feed tube
3. water in fuel ?

Thanks, further information. When we rebuilt the engine this winter we put in a new tank, sending unit, fuel pump, etc. It's always had a full tank since then, I don't believe there is any water in the system. The car live in a heated garage.
 
Pump diaphragm is shot would be one of the 1st things I'd think ...and it's a used carb, so yeah... that or a big vac leak.
Fuel pumps have always just gone out, not really weakened with slow decline in psi. In fact many have just pissed fuel into the crankcase on me.

You mean the pump diaphragm on the carb right? I have a new fuel pump and no vacuum leaks, it sits at 14inches at idle with my vacuum guage. Fuel pressure is good, sits at around 5.5psi.
 
My cuda acted the same as the condition he's experiencing. Pulled the filter, cut it in half and it was full of sediment. Quick simple check.
I'll check that but the whole fuel system is new from that tank to the engine. Changing a fuel filter is easy though.
 
What exactly would be shorting when the engine torques over?
"Could be" is the question. Costs you nothing to verify.
A tach wire comes to mind. A positive battery cable.
Anywhere a connection may be affected by engine movement. Even loose or poor connections. Maybe bulk head connections can be an issue?
 
I'll check that but the whole fuel system is new from that tank to the engine. Changing a fuel filter is easy though.
Correct, easy to replace, even though it''s new, it as well as the carb. inlet screen are "down stream" from the tank and will collect the crap.
 
Thanks, further information. When we rebuilt the engine this winter we put in a new tank, sending unit, fuel pump, etc. It's always had a full tank since then, I don't believe there is any water in the system. The car live in a heated garage.


So that eliminates a lot of fuel related issues as long as you feel confident in the parts you installed . I found out the hard way that new doesn't necessarily mean good. Dave69 brings up a good point as well. If you are losing a ground, specifically to the Ignition module or if you have a broken or loose wire to the coil, the ignition could be losing power when the car is under acceleration and the vibrations may cause contact loss.

I'm not certain of any of this...just clutching at straws!! LOL !!
 
This is likely a lean spot caused by an accelerator pump shot gasket. I would start with the easy stuff. Set the fuel pressure at 7psi with a Holley. Make sure the float levels are set correct and pull the air cleaner with the engine running at idle, for two reasons,
1. Look for fuel dribbling from the boosters. If it is either the float level is too high or the needles have trash in em.
2. Check the pump shot. It should start to spray instantly when the throttle is cracked and you should get a nice spray out of the nozzles when rapidly opening the throttle by hand.
Both quick easy checks that can tell you a lot.
 
This is likely a lean spot caused by an accelerator pump shot gasket. I would start with the easy stuff. Set the fuel pressure at 7psi with a Holley. Make sure the float levels are set correct and pull the air cleaner with the engine running at idle, for two reasons,
1. Look for fuel dribbling from the boosters. If it is either the float level is too high or the needles have trash in em.
2. Check the pump shot. It should start to spray instantly when the throttle is cracked and you should get a nice spray out of the nozzles when rapidly opening the throttle by hand.
Both quick easy checks that can tell you a lot.
I can't set the fuel pressure on a mechanical pump, but I really don't think it has anything to do with fuel pressure. Remember it only stalls when I wack the gas. If I go easy on it, and slowly press it to the floor the secondary's open up and away she goes. If there was any kind of fuel starvation problem it would happen all the way up the revs not just from the wack on the pedal.
I'll look for the fuel dribbling out of the boosters. Maybe all my fun on Saturday knocked some crap loose somewhere and jammed up a needle, but you would think that would bugger up the idle too.
I'm gonna focus on the pump shot, though I'm not sure it would stall. Wouldn't it just bog if there was no extra fuel squirted? It almost seems like it's dumping too much fuel and stalling it.
 
I can't set the fuel pressure on a mechanical pump, but I really don't think it has anything to do with fuel pressure. Remember it only stalls when I wack the gas. If I go easy on it, and slowly press it to the floor the secondary's open up and away she goes. If there was any kind of fuel starvation problem it would happen all the way up the revs not just from the wack on the pedal.
I'll look for the fuel dribbling out of the boosters. Maybe all my fun on Saturday knocked some crap loose somewhere and jammed up a needle, but you would think that would bugger up the idle too.
I'm gonna focus on the pump shot, though I'm not sure it would stall. Wouldn't it just bog if there was no extra fuel squirted? It almost seems like it's dumping too much fuel and stalling it.
Nailing the throttle brings the acc pump shot on hard. Since it works fine slowly getting into it, then when the secondaries come in it runs... sounds like the accel pump should get your attention!
 
I can't set the fuel pressure on a mechanical pump, but I really don't think it has anything to do with fuel pressure. Remember it only stalls when I wack the gas. If I go easy on it, and slowly press it to the floor the secondary's open up and away she goes. If there was any kind of fuel starvation problem it would happen all the way up the revs not just from the wack on the pedal.
I'll look for the fuel dribbling out of the boosters. Maybe all my fun on Saturday knocked some crap loose somewhere and jammed up a needle, but you would think that would bugger up the idle too.
I'm gonna focus on the pump shot, though I'm not sure it would stall. Wouldn't it just bog if there was no extra fuel squirted? It almost seems like it's dumping too much fuel and stalling it.
I went through it all again tonight. Warmed it up and checked timing (14 degrees at idle), vacuum (14inches at idle), looked for fuel dribbling (none), accelerator pump shot (nice clean spray of fuel), no vacuum leaks, no shorts that I could see. No hesitation or stalling when I was hitting the gas in park so I decided to bump up the timing a little to 18 degrees (plugs were slightly black @ 14 degrees) and took it for a spin. Now it didn't stall when I whacked the gas, just a small stumble.

So do you think I'm on to something with the timing or should I keep focus on the carburetor? I did notice a little dried gas right around the base plate to the body of the carb. Maybe it's time for a rebuild. After all my cousin probably ran it for 10 years, then it sat for another 15 until I got it from him. I just plunked it on and set it up. I don't think it's ever been rebuilt.
 
I went through it all again tonight. Warmed it up and checked timing (14 degrees at idle), vacuum (14inches at idle), looked for fuel dribbling (none), accelerator pump shot (nice clean spray of fuel), no vacuum leaks, no shorts that I could see. No hesitation or stalling when I was hitting the gas in park so I decided to bump up the timing a little to 18 degrees (plugs were slightly black @ 14 degrees) and took it for a spin. Now it didn't stall when I whacked the gas, just a small stumble.

So do you think I'm on to something with the timing or should I keep focus on the carburetor? I did notice a little dried gas right around the base plate to the body of the carb. Maybe it's time for a rebuild. After all my cousin probably ran it for 10 years, then it sat for another 15 until I got it from him. I just plunked it on and set it up. I don't think it's ever been rebuilt.
Yep, go through it.
Black is obviously rich..
... and at 14 initial with the idle mix correct.. 'even if not optimal timing'.. the plugs shouldn't be black.
So you're fouling plugs- its rich
It also seems to like more initial timing, good. You're onto something.
 
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Well, I can't see it being timing; My Idle-Timing is also set to 14*. But I can vary that Idle-Timing from the cab, with my dash-mounted dial-back timing retard box, any where from 5* to 20*, without touching the carb. And I have never experienced a bog, anywhere in that range.
IF
the bog occurs from some pedal-smash, not from idle, then some to all of your primary pumpshot may already have been used up.
Perhaps
the bog is occurring when the secondaries open, and the back bowl is NOT full, or more likely; the rear pump is faulty. Maybe the link-rod fell out.
 
It probably wants more timing, as increasing initial timing by 4* has helped. It may not be the whole fix, but is part of it. What happened is this: the extra 4* allowed the engine to make a little more HP; not quite enough HP as you still have a slight stumble, but enough extra HP to almost carry the load.
Your story is another great example of how important initial/idle timing is to carb tuning & why timing needs to be dialled in FIRST before touching the carb.

In D. Vizard's Holley book, idle timing is mentioned numerous times. But this a book on carbs, right, not ign? Above is the answer.
 
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