This might be lengthy so be patient. I have a '79 K5 with a mildly built 350 (crane cam, edelbrock intake) that was just rebuilt to stock specs by a ruputable engine builder in Sacramento. It is running the stock q-jet that has also just been rebuilt. It runs just fine 99% of the time. Shortly after installing the motor I wheeled the Rubicon in june in 80 degree weather with no problems, and initial starts were easy with just a few cranks. My first problem was later last summer I was driving through Santa Rosa on a 104 degree day and when I stopped at a grocery store it started fine and then just idled down and died after running about 6-10 seconds. I could not restart the truck until I had someone spray starting fluid down the carb and spin the motor fast for a while to re- prime my fuel pump/lines. I had no further probs until in august I went to wheel the fordyce and had it seem to vapor lock on me 4-5 times in a couple of hours. If I turn it off it will start easily then die after 6-10 seconds and then not start unless I let it sit for 15-20 minutes, them it starts fine after a little cranking. In between the Santa Rosa incident and the fordyce it ran and started beautifully the rest of the summer in Sacramento. Another problem that I think might be related is that when I park it for the night now it seems to lose all its prime, the fuel runs all the way back into the tank and I have to crank it for like 15 seconds to refill the pump and bowels in the carb befor it will start. Another interesting thing is that I have had the most trouble out on the trail were I am just crawling, kind of like without enough airflow down the framerails the fuel lines are getting too hot and the fuel is vaporizing. This theory is ok until yesterday I went snow wheeling and it was probably 40 degrees outside and I was just wheeling up a snowy road, I was pushing some snow but did not appear to be working the motor that hard. The truck seemed to vapor lock again. This time I just put it in park and left it idling. Got out to take a leak and it idled down and died after about 20 seconds of idling and would not restart. After going for a quick hike and throwing some snowballs with friends for about 30 minutes it started fine after 10 seconds of cranking. The truck has a brand new modine radiator(new with engine rebuild about 10 mo\5,000 miles ago), water pump and hoses, thermostat, and never runs hot or even warm in the summer. (Temp gauge works well) The fuel system has no leaks in it, no damp spots on any of the lines, no smell of gas at all and I have a sensitive nose. It does run duel exhast and an exhaust pipe runs down the Right framerail where the fuel lines run, but it is still 6-12 inches away from the fuel lines and especially in cold weather would doubt the lines get all that hot, I have felt them and they don't burn my hans at all. Can a fuel pump (also new) malfunction in this way by losing prime when you let the truck sit longer then about 10 hours or by vapor locking even when cool. How about a canister or what? I have thought about sleeving all of the fuel lines with a heat sheild like jegs or summit sells for that but thought I would get some advice before shelling out the money. Prior to the engine rebuild the truck ran the duel exhaust for about five years in the same place with no vaporlocking events. I also ran new rubber fuel hose at all the tank attachments and at the fuel pump at the time of the engine/tranny rebuild. New fuel filters also. Any thoughts would be appreciated before I throw the carb away and get a tbi setup. Also would an inline electric fuel pump be of any benefit?
Thanks.
Michael.

Carb? What the hell is that? Sorry I dont even remember much about them. I used to be great about 12 years ago. I would guess the carb is the problem.

Hey wild1, I have this EXACT PROBLEM.
I used to think it was the fuel pump, but that's brand new and no luck. I thought maybe it was leaking out somewhere even where I couldn't see it, so I re-wrapped all the threaded connections with teflon tape. I'm still at a loss.
It's like all the freaking gas drains right back to the tank. My in-line filter will be dry in the morning.
I keep thinking I need a one-way valve plumbed into the fuel line.
If you figure out anything, post it!
-- Mike

Your carb needs to be rebuilt AGAIN because its "loosing" it's "prime" when sitting still. It usually is cause by the "accelerator pump diaphram" being worn/torn/wore out! this would help the carb to "drain" dry ....IE: empty fuel filter. When the motor is running and shuts off suddenly is because it is receiving to much fuel and "choking" it to cut off motor. When you try to "restart" it after choking out it floods the motor so then you gotta wait for the excessive gas to "drain" and when it does start you end up with black smoke out of exhaust (extra fuel) then it smooths out.....rebuild carb and you'll be fine guys..... :D :beer: :p

Either the accelorator pump diaphram, or you may have a torn or worn diaphram in the fuel pump. Some of the fuel pumps purchased new are defective from sitting on the shelf too long. The diaphram is bad.:nuke: