Help! Has this happened to you?
 

My '68 T337C was fitted with normally aspirated IO-360's a while back. The front one has approx. 150 hrs. since factory re-man.

After about 45 mins. flight time, I'll lose fuel pressure on the front engine, and it will start to run rough. I'll apply low boost pump, engine performance will smooth out and return to normal, and I'm good to go for another 30-45 mins, at which time the anomally re-occurs. Continental doesn't think it's the engine-driven fuel pump, but rather a fuel starvation problem or vapor return line blockage (between the engine-driven fuel pump to the fuel tanks).

Both fuel cap vents work, and the wing tip over-board vent is fine. I'm not sure how to test for proper vapor return line functioning.

Any ideas, anyone?

my back engine would always vapor lock while running 70 to65 %.
insulated all the fuel line and the problem went away.

I had the same thing. Insulation of hoses, baffle on the front near the fuel pump, and overall baffle conditions all effect the vapor lock issue.

That sounds like a classic case of non-vented fuel tanks to me! If your 100% that your caps are good then this is a waist of time, but if not then try swapping the caps from wing to wing! See what happens!

_travis