/wwwthreads_images/icons/tongue.gif The automakers battled this problem for YEARS. It first showed up in the 57-58 Pontiacs and Olds when they used the "Tri-Power" triple two barrel carbs. Their answer was 3/8" fuel line and a fuel pump that would bail out the Titanic. The problem continued, however, AND vapor locking was worse with that big line idling in traffic on a hot day. PLUS....there was a problem of washing fuel right by the needle valves from too much fuel pressure at off-peak power settings. ANSWER.....get this.....they took a page from the DIESEL handbook. Diesels always run fuel back to the tank for several reasons, not the least of which is to keep the fuel injection pump cool and everything the same size internally. In a gasser, having a return line(slightly restricted) keeps colder fuel flowing to prevent vapor lock, AND enables you to run a monster pump and still not blow gas by the needle valves. I had a 454 Suburban once that the previous owner had plugged the return line because they could not figure out how to use it with extra fuel tanks. Till I discovered that little glitch, it was miserable. Wouldn't start hot without a long cranking spell and then a cloud of unburned fuel(it was blowing past the needles when the engine stopped) and VAPOR LOCK like you would NOT believe. I even overhauled the carb thinking it was dripping past the screws in the base. What you need is a two-pipe system like the CJs had from the fact tore eeee./wwwthreads_images/icons/crazy.gif

CJDave

 

Now that you brought up the subject of a return line Dave, I've got a question. Can the return line be tee'd into the supply side of the mechanical fuel pump, cause my original return line is rotted, and I'm looking for a simpler solution than running a new line.
hope you say it can be done CTjeepnut

 

/wwwthreads_images/icons/tongue.gif Absolutely, positively not. Where it needs to originate from is the side-outlet port on the three-port filter, as close to the carb as is humanly possible to get. From there, it must go to the tank, OR....TONS of vapor lock will be the result. Our '79 304 CJ has flexible fuel line from the filter port to the hard line on the frame about where the fuel pump is located, then a home run to the tank. You want to be MOVING fuel in a circle, taking advantage of the cooler fuel in the tank to stave off vapor lock. I see posts from time to time about vapor lock with Jeeps where the return has been disconnected.
/wwwthreads_images/icons/crazy.gif

CJDave

 

Where did you mount the pump? Your "draining back to the tank" comment sounds like you may have
mounted it near the engine. It goes back as close to the tank as you can get it. Electric pumps don't
pull fuel very well, they can only push it.

 

What causes vapor lock anyway? From these posts, it sounds like it is fuel warming, but wouldn't warmer fuel be better - allow it to vaporize more completely in the carb?

Chuck
1995 YJ 4.0 5spd, 3" body lift, 32" Bridgestones (w/damn 3.07 gears), Durabak

 

/wwwthreads_images/icons/tongue.gif Vapor lock is just as you suspected, warming of the fuel, which causes the entrapped air to expand and form bubbles which block the line. Carbs can only deal with liquid, so the engine begins to starve out. It's surprising how fast it happens on certain vehicles if the fuel line is not routed correctly./wwwthreads_images/icons/crazy.gif

CJDave

 

It's mounted on the frame, right behind the crossmember. It's as close to the tank as I could get it, with it being as low as I could get it.

The first pump drained back into the tank within minutes, but not all the time. This one does it sometimes but it takes a lot longer. I've got a clear fuel filter at the engine, so I can see it very easily when it does drain back. I guess the check valves in these pumps are defective, which would explain why it doesn't pump as much as it should. I may bring this one back too, and see if the new one will be any better. They are under warrenty, so I can do this as much as I want, I guess.

CJTaz, what kind of fuel pump did you use? And where did you mount it?

80 CJ7 350, SM465, and lifted 4"
98 KX250

 

In reply to:

Vapor lock is just as you suspected, warming of the fuel, which causes the entrapped air to expand and form bubbles which block the line.

Actually, it's not entrapped air expanding, but the gasoline itself which is boiling, creating the vapor.

-Dana

Before you swallow facts, find out what liberal made them up.

 

/wwwthreads_images/icons/tongue.gif Sounds good to me!/wwwthreads_images/icons/crazy.gif It IS more prevalent at high altitudes by the way, since liquids boil at a lower temperature up here in the rarified air of the mountains/wwwthreads_images/icons/crazy.gif/wwwthreads_images/icons/wink.gif.

CJDave

 

/wwwthreads_images/icons/tongue.gif i have two from NAPA auto parts that work good./wwwthreads_images/icons/crazy.gif

CJDave
I never believe any statistics unless I've made 'em up myself.

 

Swayse,

I'm port fuel injected so I have a 50 psi pump, way too much for your system. I used to have an
electric with the 258 4-barrel, but that was four years ago and I forgot what kind it was.

 

CJDave, how do you have yours hooked up? I was assuming that you have two electric fuel pumps from NAPA.

80 CJ7 350, SM465, and lifted 4"
98 KX250

 

/wwwthreads_images/icons/tongue.gif Er...ah, no. We used those pumps on our welders to nurse them from the truck fuel tanks./wwwthreads_images/icons/crazy.gif

CJDave
I never believe any statistics unless I've made 'em up myself.

 

Opps, sorry. I guess that's what I get for assuming. I'm going to stick with this pump for now. I'm getting used to having power, so I don't use it as much as I used to. But, come summer time, I will have to hook up that return line, and get a bigger pump. I guess I could always use this pump for a spare on my Lee Spider spray rig, it has an electric inline fuel pump.

80 CJ7 350, SM465, and lifted 4"
98 KX250

 

I just happened to be browsing Summit's web page and they have a pump and adjustable regulator advertised on their front page for $79.
Heres a link:
http://www.summitracing.com/index.htm

'79 CJ-7

 

With the return line how do you keep any pressure going to the carb? Should the pressure be regulated before or after the return line heads back?

 

/wwwthreads_images/icons/tongue.gif The regulator would be after the return line IF YOU USED IT. In the return line setup, you would probably not need the regulator as the three-port filter has enough back pressure to adjust the fuel flow and pressure. Just my thoughts on it./wwwthreads_images/icons/crazy.gif

CJDave
I never believe any statistics unless I've made 'em up myself.