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Discussion Starter · #1 ·
HELP! It's happening again! As the temperature goes up, the gas in my carburetur boils. (I've always called it "vapor lock" but people are telling me that's something different)
I've put a spacer/heatshield under the carb...nothin'! Installed a fuel radiator...zippity! Now I'm keeping my "vented" gas cap loose to keep the pressure down in the tank...diddly squat! So I'm now carrying a HALON extinguisher while I beg you kind people for a solution!
 

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Are you running a mechanical or electric fuel pump? If you're running a electrical pump then a return line to the tank should solve your problem as it will circulate the unused fuel back to your tank instead of dead-heading it.

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68 468 700R4, and here it is;
mutert23
 

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Discussion Starter · #4 ·
I'm running a stock mechanical fuel pump. I don't know what pressure. Current carb is a stock Rochester Quadrajet...same thing was happening before with a Carter AFB Competition Series.
 

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Discussion Starter · #6 ·
Stock routing until firewall where it now rises up onto the inner fender, goes through an clear in-line filter (where I can watch the pretty bubbles) then on to a fuel radiator that's mounted behind the grill on the passenger side (about 4-5 inches in front of the radiator support), fuel line then exits out of the bottom and straight to the fuel pump.

Trash in seat? Certainly possible. But the identicle thing happening to 2 different carbs? Plus it's never been run without a fuel filter. Thanks, I appreciate your input it's making me consider the alternatives.

I'm replacing the engine and going through the whole engine bay (wiring, etc.) within the next two weeks. This is obviously a great time to add return lines or whatever modifications to the fuel system so it can be reliable here in Las Vegas, in the heat of the summer.
 

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What makes you think that the fuel is boiling in your carb? What problems are you encountering? I searched for your other posts but couldn't locate any.

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67 Camaro LS6 454/TH400/12bolt 3.73
1989 TransAm 5.7L WS6 W/all the options
 

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You might want to insulate the fuel line after the pump.
And if you install a fuel filter from an air conditioned car that has a fuel bleed jet with a return line back to the tank it will circulate the fuel and allow it to cool.

When hot fuel is released by the needle and seat it sees a pressure drop. And it will be more vulnerable to boiling, especially if the carb is hot. It's kinda like champagne being released from pressure in a bottle, it wants to fizz.

When exactly is your engine doing this. Running, stopped, day, night?

Also check your exhaust system for leaks around the fuel tank. I've seen hot exhaust heat up the gas tank and make the fuel boil!

Not a good thing.
David

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The older I get, the faster I was!
 

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Might try different gas. My son did a science project once comparing different brands. One brand, BP I think, had a very low boiling point or vapor pressure or whatever you call it. Really a lot of difference in brands in this regard. Might put a cowl hood on that machine to cool it off under there. Lower the float level?
 

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Just a couple more thoughts...

From what I understand from your description of your fuel line routing, your filter and cooler are before your fuel pump. This isn't a good idea if you have problems with your fuel boiling since fluid flow restrictions will only compound your problem. You do not want any more pressure drops on the suction side of the pump than those that preexisted. For any given temperature, your gas will vaporize more readily at a lower pressure. Any of these restrictions will add to the pressure drop between the tank and the fuel pump inlet: a filter, additional pipe length, small tube diameter, clogged gas tank pickup screen, and pipe bends. Any filters, etc. should always be placed after the pump.

Since you see the bubbles in your filter first, your problem is probably between your tank and your filter. Check for loose connections at your filter fittings in the unlikely case that your pump is drawing air into the line.

You must eliminate the vaporization (bubble formation) you see before any carb will ever work properly on your car. The bubbles you see are likely percolating the fuel through the carb inlet line and causing your needle/float all kinds of havoc. This frequently leads to flooding.

Let us know what you find. Good luck.
 

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Discussion Starter · #11 ·
The problem comes up in ambient temperatures above 100F (pretty common in Vegas come summer) after the car has been at operating temperature for maybe 15 to 30 minutes. It almost feels like I'm running OUT of gas when it dies, but when pop the hood I hear gurgling sounds at carburetor and gas percolates up around what I believe is the acclerator pump rod. I can also see a stream of fuel vapor rising and condensing on the choke plate and other surfaces of the air horn.

Yes, the filter and cooler ARE on the suction side of the pump. I can/will remedy that...THANKS! Someone had mentioned an electric fuel pump and return line. Would that be a better fix? Am I just going to continue chasing gremlins with the mechanical pump? Would I just create new problems with an electric system? What would YOU folks do?
 

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You might want to try what Tom suggested.I had a GTO that got vapor lock often.When I started putting higher quality gas in the tank,I didn't have the problem any more.Your case sounds pretty severe,but it's worth a try.
Dan and David are right,a fluid under pressure has a higher boiling point,like your cooling system.An electric fuel pump,mounted as close to the tank as possible,should do the trick.But I would do the easier/cheaper fixes first,like putting the filter after the mechanical pump,to see if this solves the problem.Just my .02

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Discussion Starter · #13 ·
Thanks everyone! I reconfigured my plumbing last night. We'll see how it goes today. If I were to go the electric route, is there a site that would give me the "how to's"? You said pump as close to the tank as possible. Electric source? Fuel line size? Return line? And if yes, what size and how do I get it back INTO the tank? Would all this be explained in the fuel pumps instructions?

Thanks for everything! This is a great resource! I can't wait to be able to actually contribute something instead of asking all these questions.
 

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Just something else to try is get your fuel line down low off the fender well, it probaly rises above the carb or very close to the same level, this can cause your vapour lock as any air in the lines rise to the highest point, if it above the carb no more flow, if the line out let to the carb is the highest point then any air will bleed off the lines into the float bowl.
 

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Just another thought on this subject, if a liquid that is under pressure raises the boiling point, then a liquid that is subject to a vacumm lowers the boiling point, The mechanicasl fuel pump is a great vacumm pump, it pulls and pulls or creates a lot of vacum.
If your tank is not vented then the whole system from the cap to the pump is under a vacum, but since you remove the gas cap, and still have the same problem then the vacum has to be within the fuel line, is it possible the pick up sock inside the tank is torn and plugging the lines, Check the fuel line carfully from the tank to the pump, has it been crushed or kinked, is a rubber hose colapsing into itself, I think you will find your problem in the line some place.
Let us know what you find, we are all interested in the soultion.
Rob
 

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If you are running stock exhaust manifolds, make shure the heat riser is not stuck shut. That would route hot exhaust thru the intake manifold heat crossover.
If your intake manifold is aluminum, I'd block the heat crossover.

If there are any leaks on the suction side of your pump, it could cause trouble.
It sounds like that carb is still getting really hot. What is the engine temp?
David

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Discussion Starter · #18 ·
Thanks everyone! As I said earlier, I rerouted the filter and fuel cooler to the pressure side of the fuel pump. The car ran without incident although it wasn't as hot out yesterday as it has been so the jury's still out on whether that fixed it. There are no collapsed/kinked fuel lines but I'm not really sure how I can check "the Sock" or for other debris in the lines. Any suggestions there?

While I currently have no gauges, it doesn't seem to be an overheating problem. There've been no boilovers or any such thing.

Again thanks for your help. It looks like I'll be keeping the mechanical fuel pump when I install my new engine. (I'm seeing the machine shop folks today!) I'll let you know how things are going after this "cold snap" blows past and we hit 110F again.
 

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Tom3 - just curious the results your son got on his science experiment? I was having a bear of a time with one of my cars and a carb shop here told me that some of the "new" gas will boil at just 110 degrees. Creates lots of problems in our old cars. Did you get these kind of results?
 

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Dug out the old science fair projects. Did you know that GE light bulbs burn out fastest, Duracell alk. batteries last longest, Rayovac Renewal batteries don't last nearly as long as standard alk. and after five recharges go dead 40% faster even. And so on. Anyway the gasoline comparison came up with vaporization temps as:

BP at 120 degrees
Citgo at 205
Marathon at 167
SuperAmerica/Ashland 135
(None of these fuels had any ethanol or other listed additive. So. Ohio gas with no oxy. additives were required.)

If anyone is having trouble with vapor lock, etc. it would be a good idea to check out the gas first I'd say.
 
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