Had to tow the 69 home yesterday.

[Syco67]

Got a call from my sister, the 69 GMC had died. She said it started hesitating, and backfiring out the tailpipe, had no power, then it just died. It was pretty hot out, but the temp gauge stayed on the low side the entire time. The truck sat for an hour before I got there with my trailer. The truck started and ran fine to get it on, then I towed it home. What can I look at here, we had the same problem last year. I was thinking vaporlock. WE replaced the carb, fuel pump, filters, replaced and rerouted the fuel line, adjusted the points/dwell, Cap/rotor/wires/plugs are still pretty new, 4 core radiator, water pump, t-stat. When it's cool out, the truck runs awesome, but when sitting in hot traffic, this always happens.

[LUV2XCLR8]

Sounds kinda like vaporlock 2 me also, how is your fuel line routed 2 the carb

[jhow66]

Tank may not be vented enough in hot weather--pulling a vacuum in tank. After driving it hot weather stop and remove gas cap(caution). If it "whooses" you have found your problem. Install a vented cap.

[LUV2XCLR8]

^ After reading his response, check your coil, if it's a points dizzy, had this
happen to GQ a while back, drove us trying to figure out the issue

[Syco67]

The fuel line come out from the pump and straight up the front of the motor behind the fan, but in front on the lower radiator hose, then it is kind of suspended above the motor until it gets to the carb.

The coil was replaced last year with an ACCEL superstock.

I will definitely check on the gas cap. It was replaced a while back because it was leaking, but I am not sure if it is vented or not.
I really sounds like it is running out of gas, I hope it is as simple as the cap. I'll have to play with it this week.