Alright guys I am really stumped on this one.


1979 F150
4x4 D44/9"
400 cam, intake, carb, headers, recent(ish) rebuilt
New 9mm Ford Racing wires, Accel cap and rotor, 0.45 gapped new plugs, MSD box.
NP435/205


I have been troubleshooting this problem for more than a week now and still cannot figure it out. One day while driving to work the truck started to sputter like it was running out of fuel- with half a tank. The problem quickly worsened as the truck reached operating temp and the miles went on. After about 10 miles the truck was barely able to limp its self down the road. I had to switch into 3rd to make it up any decent grade!


Its like something switches and makes the truck run like crap. It idles great warm or cold. When its cold the engine runs just fine and has very little hesitation. However when warm, any throttle input makes the truck want to die. And when it does catch, the truck jumps forward, pops, cackles, and then wants to die again.


So far I have replaced some fuel lines, fuel filter, and a fuel pump. None of which have changed a thing. I also took all plug wires off, put them back on, and triple checked firing order.




I did a compression check a few months ago and got great pressure. No oil burns when it runs. No coolant burns and it pressure tested just fine. About a month ago I tuned the carb with a drag racing friend and got 12lbs vacuum at idle. Even after tuning the engine would break up after 3,000 rpm.


What do I check next?

 

Drop your fuel tank. I bet you have a blockage at the fuel pickup/sender assembly. The filter at the end of it deteriorates.

David

 

oh sorry, I forgot to include that I replaced it with a new stainless steel sending unit. The old one was riddled with rust holes and did not have any filter at the end. The fuel tank is new as well.

 

Rotor and cap.
Been there

 

That honestly sounds more like an electrical problem than a fuel problem. You checked the fire it is getting when it starts having issues? I see you have an MSD box. Does that mean you have an MSD distributor also?

 

you didn't mention the age/condition of your coil.
how is your vacuum advance connected? does the problem still occur with it disconnected and plugged?
is your choke opening properly as the engine warms up?

 

Old rubber fuel lines between pump and tank can cause that problem... they collapse and get sucked shut or bleed in air. Worth check/replace if not already done.

 

good advice blue and white. if the lines are crap i would recommed replacing them with fuel injection hose. hold up alot better todays newer gas. had the same problem on my 78 when it was pulling the stock trailer.

 

I'm leaning more towards a spark issue. Put the stock set up back on and see how it runs. You could have a bad MSD box. ....it does happen from time to time.

 

what do the plugs look like ?, sounds like a fuel issue . mite be running lean .

 

Fuel lines were all checked and replaced as needed when I dropped the tank for a new sending unit.


The MSD box is accompanied by an MSD coil. I did not put those parts on the truck so I am not sure how to even go back to stock. Could the coil be bad? I think the box is a 6al or something like that. I have no idea how old the coil and box are since they came with the truck.


I do need to go run down the miles of vacuum lines. That is one thing I have not replaced. Perhaps as the engine gets warm, so do the hoses. Maybe warm and old enough to collapse and prevent vacuum?


The distributor its self looks stock, but im not sure how to tell. Its blue and not one of the HEI units.


I have not checked the choke function. Its an electric choke that kicks off with the bip of the throttle. Just have someone watch the carb as I kick down the idle?


The vacuum advance might be something. What would I do to check it?

 

All good things to check above.
Just to relate a story.
My T bird would do the same thing.
Ran great cold then a little later down the road it would just stop.
Repeated many times.
Turned out to be the carb (original 4100).
I pulled it off to send it to Holley for a re color and rebuild. In the interim I replaced it with a Holley 600.
No more stall problem.
Still don't know what it was but something happened when then carb got hot.
May not be your problem but if you're going nuts it's worth a look (maybe rebuild).

 

I had basically the same situation a few years ago. It ended up being the fuel line. I know you said you changed it but the damage is most likely already done. Ethanol eats the older rubber and makes like little black speckles that come loose and end up in the metering block of the carb. Theyare small enough that they go right passed most inline filters. I ended up cleaning my carb a couple times I figured it out. I also had pretty good luck with the glass inline filter with the replaceable filter. I think it was a Mr. gasket brand plus you can see what your catching.

 

You're probably right about that.
We don't do ethanol in Cali but there are additives (maybe ethanol) as well as age that could do that.
Curious about the heat part of the equation though.
Mine and 61s problem were both heat related.

 

If it's like mine was I think as the engine heats the mixture is not sufficient. I remember when I would clean the carb it would run great for a while or sometimes minutes. I know the one time I oulled in the driveway and it was like a flip of a switch instantly lazy. Sometimes it missed but most times just lazy. Once I swicthed to the type filter I was talking about that was the last time I cleaned the carb. I also used fuel inj hose like others said and my tank is spotless. It was all in that hose and it sure was a pain.