75 Truck - Engine dies when hot

[Greybeard]

New tank, bad fuel cap?
New tank, stuff got in it (leaves, a rag) and temporarily clogs pickup- not likely but strange things have been known to happen.

You already changed module with known good module it seems, so the next likely spot if electrical, would be ignition switch. It doesn't stall when sitting still, only when moving? Not carb then. Especially not if it's instant off, that needs to be electrical. Fuel problem would cause an out-of-gas condition every time. Does the ignition go through a an oil safety switch (an oil pressure switch, I have mine set up this way) if so, is oil pressure good? Wiggle key switch at idle to see if you can induce problem. Switch out ignition circuit breaker if it has one. Double check steering column wiring connector under dash, make certain it's tight and corrosion free. If nothing pans out run a jumper straight from the battery positive to the HEI bat connection and drive the truck normally, this will isolate the problem to the HEI coil, the module, the rotor contact point under the cap, or simple loose connections on the HEI. If it does not die then the fuel has been eliminated, and the HEI itself has been eliminated. That leaves the wiring from the switch or the switch itself.  If the truck has a tach of any kind unplug it, it may be shorting out. These trucks are so simple there is no need to overthink it like one would with a fuel injected computer controlled truck. 

[Alanc]

I have tried a jumper straight from the battery to the HEI battery connection without success that plus duplicating the issue on 2 different distributors leads me to think it is not electrical. 

Today pulled plugs and they look line engine is running rich, I swapped on a different carb since a friend had one handy and drove quite a few miles until it finally acted up, this time though it just stumbled a couple of times and then went back to running normally.  Today was not as hot - *only* 87 degrees and partly cloudy and it took a fair amount of driving plus some stops to let it heat soak to get it to act up. 

So, now thinking it is fuel related, some mild form of vapor or other issue that heat brings on, thinking it might make sense to try to tee off a fuel pressure gauge right before the carb so that I can monitor and see if pressure is dropping??? 

[Irish_Alley]

does you gauges/ights or anything act up when its doing this? i know you covered the electrical part but just thinking of anything. the fuel pump is mechanical right?

[Alanc]

Have not noticed any indication or issue with gauges/lights when either dieing or stumbling. 

Yes, mechanical pump, new pump and tank were recently installed by previous owner (of unknown quality...)

[Irish_Alley]

im confused as to why it starts back up right after. im grasping at straws right now but hows the vent line for the tank?have you tried removing the gas cap to see if it acts up?

[Alanc]

Update:  Since I had the carb off with a spare one on the truck I tore it down, put a carb kit in and reinstalled it.  Dialed the carb in and reset timing, went for a drive and hit the issue again, this time symptoms were pretty classic vapor lock.  Infrared temp gun pointed problem right at fuel pump.  A friend had an electric fuel pump (cheapie Autobest) on the shelf so we plumbed that in on the frame back by the tank, bypassed and removed the mechanical pump.  Will see how this does... 

If this works then I would like recommendations on a solid electric fuel pump (setup for carbs) not sure I want to rely on the cheapie model we threw on.

[Irish_Alley]

you dont want too much pressure. 5-7 psi on the carb

[jaredts]

By far the cheapest and easiest option is a mechanical pump replacement.  I think they last longer than inline electric pumps as well.

[Greybeard]

My first guess is an Orielly's lifetime fuel pump- as close to garbage as a fuel pump can get (comes really close to how bad their lifetime Chevy starters are). If you decide on a fuel pump get a good stock one at Napa. I have not changed dozens of fuel pumps but when the need arose and after the first three from Orielly's went bad within the first month I got one from Napa and not a problem since. The biggest problem I face is the truck sits doing nothing for such long periods the mice get up and chew holes in my fuel lines at the tank.

As an aside, another wild guess is that the pump itself is not the culprit for the vaporlock. The fuel is getting hot before that and the fuel pump is finishing it off but it is not the cause. If you never gave any thought to it while mounting the electric pump now might be the time to check the routing from tank to pump, especially if headers where recently installed, it might be much cheaper and simpler to just wrap the line with tin foil in the area near the exhaust. And if you do end up getting a new mechanical pump be certain to use a cork gasket instead of just silicone or a rubber gasket. Cork is a better insulator in my humble opinion. Be certain your cam hasn't gotten a flattened fuel pump lobe too.

[rich weyand]

On another tack, which carb is it?  Is there a heat-shield gasket between the carb and the intake manifold?

The Edelbrock carbs in particular are sensitive to overheating.

[Greybeard]

By far the cheapest and easiest option is a mechanical pump replacement.  I think they last longer than inline electric pumps as well.

2nd that. And unless they make them much better these days, don't even think of getting mud or too much water on them.

[BBM3]

Check to make sure the rubber lines are not kinked any.  I have seen this a lot especially if the work is done under the truck without removing the bed.  Usually the lines are left too long and get a kink in them.

^This is very good advice.^
Even what may appear to be a minor kink will result in the problem described by the OP.
I know firsthand. I have been trying to solve this exact problem. The other day I dropped the LH fuel tank (again) and replaced the rubber section of fuel line with new. The original was really soft and probably collapsing. The original rubber line looked okay but when I removed it it was as soft as overcooked spaghetti. I'm guessing years of 10% ethanol fuel has been breaking down the original rubber.  Since I replaced it with ethanol tolerant fuel line I have had no problems and can now drive my truck more than 7 miles, stop for 20 minutes, and drive back home for the first time this summer.