Thanks Bob and BW. So here is what I have discovered...The vents in the fuel cap (M38) and in the vent in the fuel pick-up in the top of the tank both work only for quite high pressure. Neither vent was opening easily for make up air to get into the tank as I used fuel, so a significant vacuum was being created. Now it never really seemed to affect fuel delivery, but I can't help but believe that it must have made the pump work harder. I also found that the slightest bit of air getting in through a small crack in the (in tank) filter, or entering through a not-so-secure- hose clamp will indeed cut out the engine. The 1/4" tube from the top of the pick-up is merely a cut 1/4" (cut to about 4") brake or fuel line and then the flared end screws in to the fitting at the top of the pick-up. Part of the problem is that the cut end that the hose slips over is not flared. So, a rubber fuel line slipping over the metal line relies solely on the hose clamps to keep air out. A guy could take the line off and flare it, but they don't come that way from the mfg.
Now, here is another big improvement I made. On the vent that is also located in the top of the pick-up, I removed the small metal flap that is supposed to open as you use fuel letting air into the tank. This has the effect of letting air in easily and also letting air out easily. To keep fuel from sloshing out, I installed a 1/4" line to the outside from the vent, ran it along the top of the tank, and then turned it up about 10" at the backside left of the seat. So the pressure in the tank is now always the same as the outside air pressure. I closed the vent in the cap as it is now no longer necessary. This eliminates the vacuum inside the tank and also lets me get an additional 2+ gallons of fuel into the tank! Finally, having the tank fully vented has solved several issues I was having.
