Fuel line size question

[dave6672]

I have a 67 C10 that originally had a 283, 2 barrel, so it has a 5/16” fuel line. I have swapped in a stock 72 350, 2 barrel that was in a Monte Carlo, and it had a 3/8” line originally. Both carbs are Rochester 2 barrels, but the 72 version on the 350 is a larger diameter carb at the air horn. So I have a 5/16” sending unit and original 5/16” line running all the way to the fuel pump. At the pump inlet it goes to 3/8” and is 3/8” to the carb.

The truck idles fine and will take throttle at idle, but stumbles badly under load, when trying to drive it. Could that amount of 5/16” line on an engine that originally ran a 3/8” line, cause that kind of issue?

The plugs are new so they are hard to read, but maybe a bit lean looking?