...and I'm clearly missing something.

Replaced fuel tank, cleaned all fuel lines and there was a crap load of sediment and rust flakes. I did NOT install the Gross fuel needles, but made sure that area was free of any contamination.

I ran a hose off the spigot on the filler neck, ran it as far away as I could...the spigot is too small to make any bigger imo. The non- vented cap was used for the first run.

I drove it around 20 miles thru stop and go traffic, and save for some very slight whiffs of fuel it ran fine...

...it's always run fine.

Get home and now the test - will it run rough after a short break, as it has been doing.

After 20 min. or so, now it's time to check it...no fuel has escaped under the carbs, but now something new -I see a pie plate size spot of fuel has escaped directly under where the fuel cap lives, and upon removing said cap, a rush of air enters the tank.

I go to start the car, and it's stumbling like it always has AFTER running it, and I go up front, look under the car and I see it's clearly blown fuel out of both carbs.

The only thing I can say after replacing the tank, and ridding the fuel system of all contamination, is that it seems like the fuel gauge isn't racing towards empty as quickly as it did before.
​​​​​​
I gotta say, I'm pretty demoralized...

So is any of this connected to the fuel cap, or is it all about the needle and seat?...or is it something else that I am missing?

Please, if anyone has any knowledge regarding what's causing this, I'd really, really appreciate it, as it just doesn't make any sense that eliminating all the contamination of the fuel system, has made no improvement.

As always, thanks guys

 

i'd run a test without the cap. and might want to get one of laser thermometer guns and test engine bay heat a few times after a test drive to see what temps you're getting.

and if your are creating a vacuum in the tank there's no telling where the gasoline may go. it may be storing itself somewhere and leaks out slowly while the car sits. may want to verify, if you didn't, whether the pie plate stuff was truly gasoline, just to be sure.

as a last resort you may want to hook up a gravity fed "test" fuel supply directly to the input to the fuel pump (if possible) so as to eliminate or confirm that the problem is or is not associated with the tank and/or line to the fuel pump. this will take some work...

 

I would go immediately to a gravity feed direct to the carbs even cutting out the fuel filter. You should on no account draw a vacuum in the tank. The breather has to be blocked.

This is the only way you are going to prove whether you have a carb issue or fuel feed issue or both.

 

Just another thought. Was the car parked in the sun on hot tarmac? Are you sure it was "upon removing said cap, a rush of air enters the tank." that it was not a rush of fumes exiting the tank? In such cases vapour pressure will build up above the liquid fuel level. In both cases it means the breather was not operating as designed to. Please provide some pictures of your breather set up on your non standard Stainless Steel tank. The cap could be sealing below the breather off-take. This would be a real blunder but such issues can occur on non standard components.

 

I go with Glyn on this one. Gravity feed direct to the carbs missing out the pumps and filter bowl and double check it is not pressure in the tanks due to heat rather than vacuum caused by a blocked vent.

 

I think I'll let someone else deal with it...taking out the tank to check, enlarge or whatever the breather orifice, bypass the system etc is too much bs to deal with by myself.....my mate's not available to help now so I'll just wait until the holidays are over and let a shop deal with it...

The " high quality" fuel tank needed fettling to even mount up, and I still had to remove materiel from the aperture the feed line goes through to be able to connect the pump connection...really?

Sorry for a bit of a rant...

Thanks for all comments, Happy Holidays!

 

understood... there's a limit.