I posted on the chevy site but thought it might get more looks here.
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.

It sure sounds like you didn't use a thick carb base gasket. For 6-8 bucks you can get a 1/4" to 3/8" thick carb gasket. Aluminum intakes are known to boil the carbs dry with out the thick gasket , especially if you still run the heat cross over.

Do you have a heat riser?
If you do, a stuck or slow in opening opening one you will get the same problems as you describe.

old scout,
Could you please explain what you mean by the heat riser. It also sounds like boiling the carb dry makes sense to me. If that is the case then the fact that my fuel runs back into my tank at night seems to be an unrelated problem. I can park it for 1- 6 hours during the day and it will fire right up on the first crank, but if I park it overnight then it takes 20 seconds of cranking before it gets any fuel. It also seems odd to me that it appears to be more related to slow wheeling then to daily driving around sacramento, with the exception of the one time in Santa Rosa when it was 104 degrees outside and I stopped for about 10-15 minutes. Thanks for your input.
Michael

Heat riser:
http://www.thorleyheaders.com/tech_faq2.html

So how thick is your carb gasket?

Thanks Old Scout,
Yes, I have a heat riser. Does it open slowly? Got me. When I start my truck cold in the morning steamy exhaust comes out of both tailpipes, It does appear to be more from the drivers side then the passenger side(heat riser side) at first, but seems to equal out after it is warm. I think you may be right about the boiling the carb dry. My carb flange gasket is no more than 1/4 inch thick, maybe only 3/16. I will need to find a 3/8 inch gasket. Do the plastic or layered carb spacers do any thing to diminish the heat transfer to the carb as they advertise, like if I put two gaskets in place? Would removing my heat riser help, or would this just make cold morning starts that much more fun? The engine builder told me something about blocking off the heat transfer tube in the intake manifold when I installed the manifold, but at that time I wanted to keep the motor as close to stock as possible to make sure it passed smog. Though as I think about it, the truck should be warm to test it and blocking the manifold heat tube that worms the carburator should not make that much difference? Thanks for your advice. Any more on why my fuel runs back into my tank would be appreciated, maybe it is a related problem, who knows.
Michael.

I have the same problem with my fuel flowing back at night into my tank. If you figure out why it's happening and how to fix it please tell me cause it a PITA cranking my truck for 30-45 seconds.

Thanks

My only question with the "Boiling the carb dry" theory is why then would it run for the 5-10 seconds?

It seems to me it wouldn't run at all again until the bowls filled back up, which can take up to 20 seconds depending weather or not the fuel lines turned to vapor also (IE: if they run near the intake, heads, exhaust, etc. which all can be very hot)

My thought was that maybe the problem is the fuel line is turning to vapor due to it's placement in conjuntion with (or maybe just) the float being set too low.

Example: So the float level is too low, and your driving/wheeling in high temps. You shut off the vehicle, for whatever reason, and you still have your small amount of gas in the bowls and passages. Then you go to start it up, but your hot fuel line has no gas in it (not any that will travel to the engine before exhausting out the top of the carb as vapor) so you drain your bowl before more gas can get to the carb and fill it back up. And the problem is Multiplied if you actually PUMP the gas pedal in an attempt to get it going, because the little gas that is filling the bowl at this point isn't enough to get the engine turning, but it's still draining the bowl more.


Anyway, not that I'm a Q-Jet expert, but I've never ran anything else.

PS: I've never had a problem caused by Too short of a carb-to-intake gasket. That's not saying that OLD SCOUT doesn't know what he's talking about, but that's never happened to me.

Hey cliffhanger,
What you say makes some sense, and I am certainly not a q-jet expert. So you would recommend shielding my fuel lines better and raising the float level in my carb? Maybe I'll do it all, shield the fuel lines, space the carb, raise the float level, and change the fuel pump out just in case the diaphragm has a small leak in it and is the culprit that is causing my gas to drain back into my tank at night. I don't know but I appreciate the responses they give me ideas, keep them coming.

If you had a bad diaphragm in your pump your oil would smell like gas. The reason you get drain back is bad check valves in the pump or an air leak.

What I thinking is you heat riser is not opening all the way and your getting heat cross over all the time . The manifold will become super heated and aluminum is 10x better at conductor of heat than cast iron. The use of an aluminum intake with out a thick carb gasket and or blocked heat cross over is asking for trouble.

My recommendation is just sell that pos and put the money into some new mods for the yota...:D