I'm experiencing what seems to be a fuel vapor lock issue. My car is a stock 68 Charger R/T 440 with OE style fuel lines, vapor separator w/ return line, mechanical pump etc. I have the intake heat cross over blocked and a 1/2" wood spacer under carb.
I get what feels like fuel starvation at high RPM only after it has been run for a good 30-40 minutes and everything is good and heat soaked. It runs great when cold or during cool temps (like 75deg and under). I also don't have any problems just cruising or idling. Only under WOT at 4K RPM and above. Water temps are always 180-190ish. Also, if i jet the carb rich (like mid 11's on A/F guage) it seems to do better, but the problem will still show up after a while of running it hard.
Could it be that the vap sep/return line is adequate and slow engine speeds but at high RPM it can't get rid of all the vapor?? Just looking for opinions as i'm running out of ideas. I also want to keep factory equipment so no elec pumps or the like. It needs to look completely stock. Thanks for any replies.

Worn fuel pump pushrod?

high rpm it'd b less likely to vapor lock as the most fuel is being consumed in that state so it's moving fast & least likely to stagnate & absorb heat. I'm thinking starvation, I'd get a vac gauge w a psi side (most have it) & tape it to your windshield & run a neoprene line to a T at the CARB INLET & see what you have for pressure at the offending rpm EDIT that's 4000+ rpm at speed not in your driveway

I had the exact same problem on my old 69 GTX.

It had one of the old crappy repop fuel vapor seperators on it and the return orifice was to big. Thus allowing to much fuel to return and it would not keep up with higher rpm thrashings.

To finally fix it, I put a dab of solder in the return line, and using a hand drill put maybe a 1/8" hole in it. Never had a problem again

That's what has me the puzzled. If it was vapor lock i'd think i would see it at low speeds as well. But it is definetly temperature dependant. I forgot to mention that i can hear bubbling in the gas tank from the return line after i shut it down and pop the cap. Is this normal?

A fuel pressure test would be a good check. My fuel pump is a brand new edelbrock mechanical but perhaps its not functioning properly after it gets heat soaked??

Interesting info on the vap sep. It is a brand new unit from year one. I have an old one lying around i could swap and try.

Should have #54 hole in it.

I had a similar issue with my vapor separator. They came with a .060 hole but I had heard that it would be better at .030. I soldered it up and redrilled. My fuel pressure increased by 1.5psi...

Yep, this seems to be the issue. Took off the new one i got from year one and the return orifice is closer to .100. Put on my old one that has the correct .055-.060 orifice and its definetly better.

I still get a little starvation at high rpm 2nd gear though after about 30minutes of run time. Runs great when its cold. I need to do like RR said and see whats going on with the fuel pressure.

Ok, so i hooked up a fuel pressure gauge to see what the heck is going on and here's what i found:
-When the engine is 'cold' (not heat soaked) fuel pressure is good. Stays between 4-6psi throughout a WOT run.
-After 15-30 minutes (depending on ambient temp and how hard i flog it) fuel pressure will slowly drop as i make a WOT run. Starts at 6 and drops to less than 1 and thats when i get the starvation. A/F guage confirms this by going lean.

I swapped my new eddy pump for a new napa pump and tryed the test again and get the same results. At this point i pulled the gas tank and removed the sending unit to check it out. Sock is clean. Sender lines are clean. I also blew through the fuel line to check for blockage and it is clear as well. I pulled the fuel pump pushrod and it measures approx. 3.22". All seems fine. So my next step is to replace all rubber hoses at the connections in case i'm sucking air some place. Any suggestions for what else i could check? Again, it runs good and holds pressure until is gets good and heat soaked. Thanks!

sock clean/lines clear/diff pump, iirc you checked tank venting. I wonder if it is vapor locking. Not running E10 correct? Lines not close to any exhaust? dont know if this could b easily done but just brainstormin, could you bundle up/pack the mech pump w dry ice for a quick run?

I am running E-10...thats all there is in my area. I do have 5 gal of race fuel i could mix in to see if that would help..?

If is was vapor i would think i would see it at low engine speeds too. It will sit and idle all day without a hickup. Maybe i'll take it out at 4am when its 65 degrees out.

It's the E10. confirm not by mixing some race gas w it but run it completely empty then use the race gas. Plus the E10's vapor pressure is going to b worse w your higher altitude.

I'll give that a try. Thanks for the help.

Holler how it turns out

Well i put the straight race gas in and it ran good. When it was good and hot fuel pressure would drop to 2-3psi during WOT run, but it would keep pulling and A/F ratio stayed constant.
So i guess i must have been boiling the gasahol just enough to cause starvation at high RPM but not enough to cause a total vapor lock situation. Wonderful stuff that E-10 is

Now, i need to see how much race gas i need to mix with the gasahol and still have it run good. I wish i could get non-ethanol premium close to home...

Sorry you had to go thru all the checkout of the fuel system for naught. I should a known from your descrip of it being "temp dependant" that it was vapor locking but didn't think it'd b at high rpm as it's typically a heat soak issue but I didn't know about the E10. We're spoiled a bit here w both pumps avail since it's intro 25+ yrs ago

It runs good now so i'm happy.

Now i get to go to the Mopars @ Thunder Mountain Show/Race at Bandimere this Sunday!!

Haha...you should have seen me a couple days ago Optimist wouldn't have been the right word. Thanks for helpin me get it sorted out.