Going dead at 80 mph

[Tommyt]

I’ve got my Mustang running great, very dependable. Starts runs fine ,no miss, long distance no problem. At 80 mph she goes dead, comes right back if I let off.
I installed a cam and maybe have a miss match. Electricity is good even gave the Petronix its 12 v same failure. Perhaps gas slowing raising float due for test drive.
351 c my question if the engine had been built strong and I installed a base cam will that cause going dead at 80? Worn springs?

 

[droptop73]

What's the symptom when it "goes dead"? Does it miss or pop? Is it like shutting the key off? Does it run smooth right up to that point?

 

[rvrtrash]

I had a '72 Mach 1 that did that at highway speeds once because when I installed the new engine I forgot to put the ground strap between the engine and the firewall back on.

Steve

 

[Stanglover]

Just throwing this into the mix. 80 is just a speed. What are the rpm's? Will it "die" at the same rpm's sitting in the garage or just while under load on the road?
It's either fuel, sparks, or lack thereof, or grounding in my unqualified opinion.
You say you have a Pertronix module, which one do you have? Did you ground that module in the distributor and what distributor do you have? (I'm also suspecting a bad Pertronix module)
The more information given the better to help the qualified guys answer your questions.
By the way, don't hit anything at 80, or you'll be the one to "die".

 

[71ProjectJunk]

Cam probably has nothing to do with it, in fact I would state that 100% for certain it has nothing to do with it, except that I don't know all the facts, but I have never seen a cam turn off a running engine at a certain speed or RPM like that. As stated above your issue is either fuel or spark. One or the other is not getting to the cylinders when you get to 80 mph. Need more info to attempt to diagnose.

 

[Lazarus]

If it is stumbling/stalling out around 80 but when you let off it is catching again I’d rule out fuel first. I’d look for a kink in the fuel line... not definite of course but easy to rule out. Check really well, run your hand over every inch, particularly around the rear end. We had one member with a similar issue chase their tail for a while before finding a pinched fuel line in a hard to see spot near the wheel well I think. Do you have a fuel pressure regulator that is too restrictive? Something gumming a fuel line?

Can certainly be spark also. I had this happen once in a camaro where the engine harness connection was sorta rattling apart at a certain resonant frequency/ speed. Could maybe be a timing issue under load also but I don’t know much about Pertronix’s. Sounds like from above a bad engine ground can do it as well... but again, I personally would rule out fuel first.

 

[boilermaster]

Tommyt,
As stanglover suggests, what rpm is this happening ?
This does sound like a fuel issue, but could be a loss of spark because of a loose connection .
Have you checked your float level and float drop specifications yet?.
If you have a stock fuel pump, you sgould be looking for 5-7 PSI at idle and volume should be 1 pint in 20 seconds at idle, the shop manual lists the idle speed spec. as 500 rpms for this test.
Besides checking for kinks in the fuel line, do not rule out the fuel line sucking in air, especially where the portions of the line are rubber, and those rubber lines can collapse too, especially if one has a restricted fuel filter on the inlet side of the pump, also remember there is a strainer on the pick-up tube in the tank.
All the fuel lines should be inspected BEFORE doing a fuel pump test.

 

[71coop]

I agree with Geoff. I had what might be a similar problem a few years ago - see this thread:
https://7173mustangs.com/threads/power-loss-around-3000-rpm.35961/page-3#post-403016
It was a hard fuel line that I accidentally pinched while doing a trans swap. At 3000 RPM it just cut off the fuel supply enough to make the motor die.

 

[vintageman]

I think Lazarus' fuel line advice is spot on, there have been a few here with very similar issues that led to a kink in the fuel line. Good luck and let us know what you find out.

Tom

 

[TommyK]

If by "going dead" you mean it falls on its face and loses power and you didn't upgrade the valve springs when you changed the cam, it is probably valve float. If by "going dead" you mean it turns off like a light switch, it is probably a fuel or spark issue. Fuel is usually the easiest to troubleshoot. Start by pulling the fuel sender out of the tank and inspect the condition of the sock on the end of the pick up tube.

 

[Tommyt]

If by "going dead" you mean it falls on its face and loses power and you didn't upgrade the valve springs when you changed the cam, it is probably valve float. If by "going dead" you mean it turns off like a light switch, it is probably a fuel or spark issue. Fuel is usually the easiest to troubleshoot. Start by pulling the fuel sender out of the tank and inspect the condition of the sock on the end of the pick up tube.

 

[Tommyt]

Falls on face is correct comes right back if you let off.

 

[Tommyt]

I’ve got my Mustang running great, very dependable. Starts runs fine ,no miss, long distance no problem. At 80 mph she goes dead, comes right back if I let off.
I installed a cam and maybe have a miss match. Electricity is good even gave the Petronix its 12 v same failure. Perhaps gas slowing raising float due for test drive.
351 c my question if the engine had been built strong and I installed a base cam will that cause going dead at 80? Worn springs?

 

[Tommyt]

Thanks for inputs. I installed all new fuel system and I made my own line so I’ll inspect all that. I’ll check all my grounds. The failure is the engine just dies but not like turning off. Let off and it immediately starts pulling and running fine. I’ve got a tach to install I’ll try to duplicate the failure in the garage.
One more thing is a boo-boo I did. I welded on the steering wheel and forgot to disconnect the battery. Yeah I did. It is stripped on one side and I tried to attach a bolt. Didn’t work but the heat broke the wheel lose and I got it removed. I may have damaged there the entire dash has to come out just haven’t got that far yet. Again thanks for all the help and ideas. I’ll update in the near future.

 

[Stanglover]

Thanks for inputs. I installed all new fuel system and I made my own line so I’ll inspect all that. I’ll check all my grounds. The failure is the engine just dies but not like turning off. Let off and it immediately starts pulling and running fine. I’ve got a tach to install I’ll try to duplicate the failure in the garage.
One more thing is a boo-boo I did. I welded on the steering wheel and forgot to disconnect the battery. Yeah I did. It is stripped on one side and I tried to attach a bolt. Didn’t work but the heat broke the wheel lose and I got it removed. I may have damaged there the entire dash has to come out just haven’t got that far yet. Again thanks for all the help and ideas. I’ll update in the near future.

When you installed all new fuel lines, I hope you used proper fuel line clamps on the rubber pieces and NOT gear clamps. I'm sure you know that is my absolute pet peeve. Gear clamps could let air past. As for the rubber tube, it ought to be Fuel Injector grade to withstand ethanol. Ethanol will almost certainly eat the rubber in an ordinary line and could cause blockages. That's my theory and I'm sticking to it!
Good luck moving forward, frustrating for sure, but you'll get there.

 

[Superbond]

Just throwing this out there.
If it is a original stock carburetor setup and it has a spacer plate under the carburetor. Check the spacer plate for holes.
I had a 76 F250 4x4 that sounded like it had the same issue and it was a worn out spacer plate! And yes I was speeding also when it would happen to me. Just a thought

 

[Tommyt]

When you installed all new fuel lines, I hope you used proper fuel line clamps on the rubber pieces and NOT gear clamps. I'm sure you know that is my absolute pet peeve. Gear clamps could let air past. As for the rubber tube, it ought to be Fuel Injector grade to withstand ethanol. Ethanol will almost certainly eat the rubber in an ordinary line and could cause blockages. That's my theory and I'm sticking to it!
Good luck moving forward, frustrating for sure, but you'll get there.

I used the standard ole clamps. Is that a gear clamp? Also I just used regular hose I so far haven’t used corn gas.

 

[Tommyt]

Just throwing this out there.
If it is a original stock carburetor setup and it has a spacer plate under the carburetor. Check the spacer plate for holes.
I had a 76 F250 4x4 that sounded like it had the same issue and it was a worn out spacer plate! And yes I was speeding also when it would happen to me. Just a thought

I have a spacer I’ll take a look see thanks

 

[swice]

Up to ‘73 the ignition runs through the tach, then back to motor, if original, might check as any short will stop the engine

 

[James561]

I’ve got my Mustang running great, very dependable. Starts runs fine ,no miss, long distance no problem. At 80 mph she goes dead, comes right back if I let off.
I installed a cam and maybe have a miss match. Electricity is good even gave the Petronix its 12 v same failure. Perhaps gas slowing raising float due for test drive.
351 c my question if the engine had been built strong and I installed a base cam will that cause going dead at 80? Worn springs?

Happened to me and it turned out to be the cotter pin in the distributor gear sheared!