The old man is running out of gas at highway speeds. Any ideas?

[car10107]

PO installed 235/75/15 tires on the truck. I like the them and they help with a little bit of "rubber overdrive" so I will most likely stay with the size. The taller tires do cause my speedometer to read slower than I'm really going and I wanted to know how much it was off. Added a free GPS speedometer app on my phone and layed it on the edge of the cluster so my wife could snap a picture of the speedometer reading, GPS speed and tachometer at highway speed. Figured I would use this info to make some calculations and get a different gear or one of those convertor boxes so the in dash speedo would read accurately.

Got the old man on the freeway with the hammer down. My wife snapped a pic at 75 and another at 80mph. She said the pics were clear so I backed down to 75 again and we were just cruising along until the engine stumbled and lost power. I let off the gas and it came back to life after a second or two. If I kept the speed around 65 or less, it was fine. If I tried to accelerate above that, the engine would cut out until I backed off again. We got off at the next exit and drove the side streets back home with no issues. Tried to rule out secondary ignition by revving the engine to the same or higher RPM that it was at before in a lower gear this time and had no issues. It feels like the float bowl is running dry as the demand for fuel increases.

This truck doesn't see much highway use and I usually just drive it around town. When I first bought it last year, it was doing a similar thing on a big hill we have by the house. I would be on it at the bottom of the hill and it would do fine for about 10 seconds off full throttle, then fall on it's face until I let out. The fuel pump had a slight seep from the weep hole anyway so I replaced it with a new Carter pump. The issue on the hill seemed to go away so I thought the problem was solved by the new pump but now I'm confused. I rebuilt the 2GC when I did the fuel pump because it had no pump shot which was no fun. I double and triple checked the float level setting at that time.
The truck drives great except if it's at full throttle for over 10/15 seconds. Just feels like the float bowl is running dry under heavy load. I took the sending unit out of the tank a couple weeks back to inspect the sock and it was actually clean as can be. Fuel line goes under the cab then to a simple inline filter (filter is new and old one was clear, not clogged). From the outlet of the filter, I have a 10" section of rubber to the line on the frame. Line on the frame goes all the way forward then has another 10" section of rubber to connect it to the pump inlet. Pump outlet is the OEM steel one piece line up to the 2GC. It's 73 here now and was probably closer to 68ish at 10am when this transpired so I don't know if I should be thinking vapor lock or ? I know the new pump could be bad I guess but I don't feel it's likely. Forgot to mention the brass inlet screen thingy at the inlet to the 2GC is clean and clear too.
Any ideas other than installing an electric pump which I don't want to do? Thanks for your time guys.

[AcampoDave]

Have you set the dwell and checked the timing? What about the mechanical advance and vacuum advance? You're winding pretty high at speed and a lack of timing in the distributor curve may play into your symtoms. Besides, I got nothing else as it seems as though you've pretty much checked the whole fuel system.

[geezer#99]

Could be float level is too high. Floods a tiny bit at higher loads.
Normally higher elevations need less level.

When I lived at 3500 feet I set the level 1/8 inch lower.

[64shortbox]

There might be one other thing BUT I've only seen this ONCE in my lifetime - the eccentric on the front of the cam that activates the fuel pump might be worn enough that at freeway speeds the stroke on the fuel pump arm might not be enough to pump the required fuel to the carb. Again - I've only seen this once in my lifetime & I'm 67 years old. Just a thought.

[car10107]

Thanks for the input guys. Dwell was set last month to 28 and base timing is 10BTDC. I am going to make sure the dwell is still where it should be as well as what the timing does with centrifugal advance at higher engine speeds.
Also very good suggestion to check the stroke of the fuel pump to verify the fuel pump lobe on the cam is good. Gonna dig deeper this weekend.

[The Rocknrod]

Originally Posted by 64shortbox

There might be one other thing BUT I've only seen this ONCE in my lifetime - the eccentric on the front of the cam that activates the fuel pump might be worn enough that at freeway speeds the stroke on the fuel pump arm might not be enough to pump the required fuel to the carb. Again - I've only seen this once in my lifetime & I'm 67 years old. Just a thought.

That might be why many have to go to an electrical pump and dont realize it. I never thought of that.

[SkinnyG]

I went through three (3!) Carter mechanical pumps on my '77 squarebody in the space of two years - same issues: it would run out of gas under power, and often left me stranded in traffic. I finally went to a AC Delco electric (EP12S) and haven't had an issue since.

[car10107]

Originally Posted by SkinnyG

I went through three (3!) Carter mechanical pumps on my '77 squarebody in the space of two years - same issues: it would run out of gas under power, and often left me stranded in traffic. I finally went to a AC Delco electric (EP12S) and haven't had an issue since.

That's unfortunate but very helpful news. I thought I was doing myself a favor by selecting the Carter vs one of the made in prc low dollar models. I really want to stick with a mechanical pump so if the lobe on the cam is ok, I will try an AC Delco mechanical pump.