Need some ideas

I replaced the Magnaflow mufflers with dumps on my 72 with Dynomax Super Turbo mufflers and tail pipes. Since the change my car will start, run for a couple minutes, then stall out like it's fuel starved. I can try to start it again but it will fire and die. If I let it sit for an hour it will start, run, and die. I've checked the fuel, fuel pump, fuel tank vented, points, plugs, vacuum hoses, etc.

Would changing to more restrictive mufflers cause this issue? Any ideas of what to try next?

I don’t know how you made the exhaust system swap, but I would check anything that you removed to install it that remotely could be a culprit. Normally, the last upgrade is the cause of the next trouble.

Steve McMahan said:

I don’t know how you made the exhaust system swap, but I would check anything that you removed to install it that remotely could be a culprit. Normally, the last upgrade is the cause of the next trouble.

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Yep, that's what I thought, too. I cut the old mufflers off, fitted new intermediate pipes, new mufflers, and tailpipes. I welded the tailpipes where they fit into the muffler. The other connections forward of the muffler used band clamps. I examined the area I did the work in and don't see anything that would cause problems. Scratchin' my noggin here.

Edit: I disconnected the battery when welding and connected my welder's ground at the end of the tailpipes.

If you have any electronics like ignition modules on the car you have to disconnect them. At least the ground wire but I disconnect both. Disconnecting the battery won't protect them when welding. Sometimes welding can partially damage a module and cause the symptoms you have. If you still had the exhaust pipes connected to the engine you could be putting weld voltage throughout the chassis. Just a stab in the dark.
After reading your post again I see you use points. Perhaps the condenser has gone kaput?
A real head scratcher for sure. Interested in what you find, hopefully sooner than later!

Are the new tailpipes interfering with the fuel line outlet on the tank's driver's side?

MAD IRISHMAN said:

If you have any electronics like ignition modules on the car you have to disconnect them. At least the ground wire but I disconnect both. Disconnecting the battery won't protect them when welding. Sometimes welding can partially damage a module and cause the symptoms you have. If you still had the exhaust pipes connected to the engine you could be putting weld voltage throughout the chassis. Just a stab in the dark.
After reading your post again I see you use points. Perhaps the condenser has gone kaput?
A real head scratcher for sure. Interested in what you find, hopefully sooner than later!

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Yep, the electronics are pretty much as it left the factory. That's a good thought about the condenser. I think I have a spare around to try. Thanks for idea.

Cleveland Crush said:

Are the new tailpipes interfering with the fuel line outlet on the tank's driver's side?

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Nope, all well clear. They're Precision Exhaust items and made on a jig for 71-73 Mustangs with staggered rear shocks. They fit very nicely.

It sure sounds like vapor lock. How are you confirming fuel pressure? When the car won't start try putting a small amount of fuel down the carb and see if it runs for a second or two.

My guess would be fuel line damage during the installation process. If one of your exhaust pipes dropped on the drivers side near the fuel tank outlet you may have pinched the steel line coming out of the tank or pinched the fuel line elsewhere during the install. If there is just a small passage then fuel could pass by the blockage in a small amount that would let you start the car but more demand after running for a few second would starve the fuel line.

The difference in restriction between those two mufflers should not be enough to cause the problem described.

When the engine dies, have you confirmed that the fuel bowl is full? What carb are you running? Where is the fuel filter located? If the bowl isn't full, starvation, if it is full, make sure choke is working properly.....a choke pull-off issue will run when cold but die as soon as engine warms up a little. You'd be able to repeat the sequence after fully cooling down, if that's the culprit.

I'm in agreement with others, in that changing mufflers and/or pipes shouldn't, in and of it itself, be the issue. Your engine dying issue happening at the same time is either just a coincidence of something non-related, or the installing of exhaust components has interfered, melted, or otherwise altered fuel delivery physically, but, with either a completely stock pipe and muffler set-up ,or absolutely no exhaust system at all, your engine should start and idle. Fear not, you'll find it, your car is not purposely trying to mess with you. Start systematically from the sender-forward, you'll sleep better knowing you've omitted this and that from the list of possibilities.

I had a muffler that was made wrong years ago that would build up back pressure and cause the engine to run badly and try to die out. Can you open the exhaust up before the mufflers or did you weld it? Unlikely that it is that, but it is worth a try if it is just a case of loosening some exhaust clamps and separating the mufflers from the intermediate pipes. You might upset the neighbors a bit, running it without mufflers though, LOL.

Just to rule out a restricted muffler, you could do the rev engine and hand test the exhaust pipe outflow, to see if there is a noticeable difference. It’s a long shot, but something else to check off the list.

I had a fuel line collapse on the suction side of the fuel pump. Just about the same symptoms you are describing. Car would start and run then act like it was starving for fuel after it warmed up a bit.

tpj71mach said:

I had a muffler that was made wrong years ago that would build up back pressure and cause the engine to run badly and try to die out. Can you open the exhaust up before the mufflers or did you weld it? Unlikely that it is that, but it is worth a try if it is just a case of loosening some exhaust clamps and separating the mufflers from the intermediate pipes. You might upset the neighbors a bit, running it without mufflers though, LOL.

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I too, had a brand new "performance oriented" muffler develop a dull, almost imperceptible rattle when first starting, figured it was a defect in manufacturing. The sound went away after a minute, so I left it alone. It never altered how the car ran, to my memory, but, like you, I had a defect right out of the box. Eventually I swapped those out for another new pair of same. Another fine quality product from " Bastards Inc", so it's always a possibility. On another note, the main fuel line runs from the tank , along the driver's side rocker panel underneath, and may have been crushed a bit if the muffler shop lifted the car along the "crimp moulding" along the rocker panel. When my car was used in the Bounty Killer movie, they lifted the car up by the rocker "crimp moulding", and put a squashed area in the fuel line, the car still ran OK, but that is how I know about that particular malady. I replaced it eventually.

will e said:

It sure sounds like vapor lock. How are you confirming fuel pressure? When the car won't start try putting a small amount of fuel down the carb and see if it runs for a second or two.

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Thanks. I associate vapor lock with a hot engine. Fuel flow seems unimpeded, the carb bowls are full, and cycling the throttle squirts fuel into the intake. I have some starter fluid so I'll give the carb a shot and see what happens.

AcesArneson said:

My guess would be fuel line damage during the installation process. If one of your exhaust pipes dropped on the drivers side near the fuel tank outlet you may have pinched the steel line coming out of the tank or pinched the fuel line elsewhere during the install. If there is just a small passage then fuel could pass by the blockage in a small amount that would let you start the car but more demand after running for a few second would starve the fuel line.

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I went back under the car and the fuel line and hoses are all in good condition, no kinks or bends. Fuel flow is unimpeded even after the car stalls.

basstrix said:

The difference in restriction between those two mufflers should not be enough to cause the problem described.

When the engine dies, have you confirmed that the fuel bowl is full? What carb are you running? Where is the fuel filter located? If the bowl isn't full, starvation, if it is full, make sure choke is working properly.....a choke pull-off issue will run when cold but die as soon as engine warms up a little. You'd be able to repeat the sequence after fully cooling down, if that's the culprit.

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Yeah, I didn't think the mufflers would make that much difference but figured I should ask.
There is fuel in the bowls. It's a Holley 4160 750cfm I've been running for a few years. Easy to check the fuel level in the bowls. It does have an electric choke - could welding the pipes caused a problem with the choke? I checked the choke and it seems to be functioning properly.

Spike Morelli said:

I'm in agreement with others, in that changing mufflers and/or pipes shouldn't, in and of it itself, be the issue. Your engine dying issue happening at the same time is either just a coincidence of something non-related, or the installing of exhaust components has interfered, melted, or otherwise altered fuel delivery physically, but, with either a completely stock pipe and muffler set-up ,or absolutely no exhaust system at all, your engine should start and idle. Fear not, you'll find it, your car is not purposely trying to mess with you. Start systematically from the sender-forward, you'll sleep better knowing you've omitted this and that from the list of possibilities.

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Oh, I've had this car for 47 years. It has been known to purposely mess with me from time to time. We get along fine most of the time, though.

tpj71mach said:

I had a muffler that was made wrong years ago that would build up back pressure and cause the engine to run badly and try to die out. Can you open the exhaust up before the mufflers or did you weld it? Unlikely that it is that, but it is worth a try if it is just a case of loosening some exhaust clamps and separating the mufflers from the intermediate pipes. You might upset the neighbors a bit, running it without mufflers though, LOL.

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When it runs for those couple minutes the exhaust flows out both tailpipes and sounds good. I have an x-pipe in the system so that would reduce a single side blockage. I may have to try this, though. Thanks.