Last fall, I had a running issue with my 1979 400. It would start and run great when cold (with a small off-idle stumble). As it warmed up, it would seem to be going just fine, until all of a sudden the engine would just go flat when you pressed on the gas. I'd mostly notice this first when trying to go up a hill; I'd roll into the accelerator, and all of a sudden the engine would sound different, lose power, and then start misfiring. I could limp it along for a while, but it would get progressively worse until eventually I could only move with the throttle blades completely closed. Things I tried to fix it:

1) New Duraspark ignition box
2) New distributor pickup
3) Confirmed ignition timing set to spec

Finally, it got too late in the season, so I just put it away. Fast forward to last weekend. I dropped a new ignition coil in it, as the engine had suffered a fire two years ago and I suspected the old coil may have gotten damaged by the heat. I took it out for a cruise and it ran for a full hour with no apparent problems. I took it out again the next day and ran it for 15 minutes, again with no problems. Earlier this morning, I took it out, and I barely had it going for 5 minutes when I started pulling uphill and the engine went flat again. I have verified that the ignition coil sees about 7 volts, key on, engine off, so the primary coil circuit seems to be working. The one thing I didn't check is the coil voltage after the problem occurs, which would eliminate the resistor wire as a potential cause; but then, wouldn't it have had a problem during the previous two runs, as well? I'm completely stumped on this one.

 

Oh, yeah, one other tidbit: If I let the car sit for an hour, it'll start up and drive just fine for a while. It's definitely something heat-related, I just can't figure out what. It's fully stock, EGR, thermactor, the works.

 

It could well be a fuel issue. Vapor lock, clogged filter or carb inlet jets or even a weak pump.

 

It's a brand-new pump, but since I've never checked the fuel pressure on it, that doesn't necessarily eliminate it as a potential cause. Fuel filter has about 500 miles on it as well. It just seems weird that everything worked fine for those two runs, and then today it decided to misbehave. I doubt vapor lock simply because today, the engine didn't get very warm at all before the problem appeared.

 

I can think of a couple of things to check. First, I'd check that the choke is coming off. A rich mix can cause an engine to fall flat, but it usually won't miss, so maybe this isn't it.

But a lean mix can cause both a loss of power and missing, and a failing fuel pump can do that. But, so can a bad hose in the fuel system ahead of the pump. It allows air in and that kills the pump's ability to move fuel. And the old rubber in the hoses isn't up to ethanol, so I'd check every hose and connection.

Also, you didn't mention plugs and plug wires. I wouldn't think that they would cause the power to go flat, but they will certainly cause a miss.

 

I've double-checked the choke, and in fact, just got finished re-readjusting it after having some troubles getting her started in the cold. As for the plugs and wires - the wires, cap, and rotor were replaced last spring as part of the repairs needed after the engine fire; they all have less than 500 miles on them. The plugs were replaced 2 years ago when I first got the car, and they have around 1000 miles. At this point, I'm reasonably certain that the issue is in fuel delivery. I never cleaned the tank, and the car had been sitting for something like 20 years before I pulled it out to work on it. At this point, I have 3 guesses:

1) Fine particulates clogging the topside fuel filter at the carb,
2) The sending unit filter sock is clogged inside the tank,
3) The hose connecting the tank to the metal line is sucking air.

As for the other rubber hoses in the fuel delivery system, I've already replaced those; only the one at the tank itself is still original. I'll change the topside filter and the tank hose and see if that works; if not, it's time to drop the tank and pull the sending unit. I have to do that anyway to repair the fuel gauge functionality, so I may as well clean the tank and seal it while I've got everything apart.

 

I doubt it is the sock in the tank as they are pretty coarse. But some have blown them off by putting air pressure to them. Anyway, I'd bet on the top hose or the fuel filter getting clogged.

But, how old is the gas? Ethanol doesn't last long.

 

Maybe six months, but I put stabilizer in it and this gas doesn't have ethanol. A lot of the gas stations around here sell non-oxygenated 91 octane for use in boats, but it's legal to use in collector vehicles as well, so I fill her up on that. I pulled the carb filter and there was some black crud of vague description sitting inside the intake port, so I'm assuming that 1) the filter was plugged, and 2) the black crud may well be that bottom hose disintegrating from the inside out. I'll replace that tomorrow, fire her up, and take her for a spin to see if that resolves the issue.

 

Good luck!

 

To close the loop on this one, I pulled the old hose and it basically fell apart on me as I was taking it off. I put a fresh piece of rubber on it, started it up, and drove it for a half hour with no issues. (Aside from the aforementioned off-idle stumble, but even that was improved after an accelerator pump adjustment, and may just need some fine-tuning.) Thanks for pointing me toward fuel delivery! I was so convinced it was a heat problem because it ran fine for a few minutes, but my guess is that it just took time for the accumulated effects of the plugged filter and leaking fuel line to slowly starve the carburetor of fuel.

 

Yippee! I love it when there's a happy ending. Especially a cheap happy ending. Enjoy!

 

Yeah, too bad the ending isn't quite so happy. I had her out this afternoon for a cruise, trying to build confidence and get her some fresh air and exercise. After about half an hour, cruising at 60 mph, all of a sudden the engine went flat. I pulled off to the side of the road. It idled okay, but if I stabbed the gas, it would sputter, then rev up. After I did that twice, it stopped sputtering when I stabbed the gas and would rev right up. I put it in gear and it started to drive okay, but after about 15 seconds under load, it fell flat again. Eventually I killed it, so I let it sit and started playing with moving the ignition coil to different places in the engine compartment to get it better cooling. After trying four different places and stalling it out four times, I came to the conclusion that coil heat soak is not the problem. I don't suspect the ignition module because it barely got above ambient temperature during my driving, and I've tried two different modules (a cheap Chinese unit and a NOS Motorcraft one). I don't suspect the distributor because I've been through two pickup units and both the pickup and the reluctor wheel are newly replaced. I'm still suspecting fuel delivery, but it seems to only be happening when the engine is at full operating temperature. Could it be the fuel pump? It's newly replaced, but that doesn't mean I didn't get a defective one.

Edit to add: I'm also wondering if it could be fuel boiling in the carb when the engine gets hot. I've mentioned before that it's not ethanol, but does that necessarily mean it can't happen?

Edit 2: It restarts instantly after it stalls out, but if I do that, as soon as I put it into gear, it dies. Revving the engine might let me get it into gear, but eventually after multiple restarts it reaches a point where it's simply unable to run until I let it sit and cool off for 5-10 minutes.

 

I don't think it is boiling the gas in the carb or it wouldn't start right back up. More likely it is fuel delivery ahead of the carb.

Remind me, do you have a clear fuel filter on it at a place where you can tell if if there is gas being delivered? If not, that's what I'd do next.

 

Yeah, the other thing about that is that the fuel pump is the only real variable in the fuel system, and I only first noticed the symptoms appear after I changed it out. I will put a clear filter on and see if I can see problems in the fuel flow.

 

You may have gotten a bad one. But the filter should tell the tale.