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Vapor lock?

Cj5dale

Cj5dale

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Aug 19, 2021
I have a 1970 cj5 with dauntless. Mostly original. Rochester carb, prestolite distributor.

I have an issue where I start the Jeep, drive about 20 minutes to a restaurant, etc.

Then when I come out and try to start it doesn’t start.

If I keep trying and trying and trying it will eventually start.

I get home, let it set for a day, week, or month a try to start, it starts right up.

Is this vapor lock? I have the heat shield, etc.
 
Heat soak. Fuel in the carb is boiling and flooding the engine. Hold the pedal to the floor and crank it, that should get enough air in there for it to start.
Believe it or not, you should try a different gas station. Some brands are more susceptible to this than others.
 
Sounds like vapor lock to me.

Uncle tony did a video recently that explains why our old cars get vapor lock so easily now but worked fine 50 years ago.

 
Thank you. I only use ethonol free gas if that makes a difference
 
I’ll try that next time.

The video above was good. I’ll see if I can get a fuel filter with a return outlet as well
 
Not a V6, which have a rep for really suffering from this, but illustrative anyhow.

Howard Eisenhauer

Thread 'E10 and Vapour Lock'

We've had a few threads touching on this subject, here's what happened with me.-

I had a heck of a time last summer getting Tonk running properly, a good part of that was carb issues from a miss-matched needle and jet but there was also an issue with vapour lock that I never had with non-ethanol gas.

I added a separator, here's a video on what was happening before & after...

  • Like
 
My '71 Dauntless has the 3-line fuel pump. I did not replace that heat shield when I installed the remanufactured engine ~10 years ago.
I have never experienced vapor lock even when it's 100° degrees outside or at 13,000' above sea level.
I'm not bragging; just sharing facts. I always thought that third line back to the fuel tank was to eliminate vapor lock.
 
dnb71R2 said:
My '71 Dauntless has the 3-line fuel pump. I did not replace that heat shield when I installed the remanufactured engine ~10 years ago.
I have never experienced vapor lock even when it's 100° degrees outside or at 13,000' above sea level.
I'm not bragging; just sharing facts. I always thought that third line back to the fuel tank was to eliminate vapor lock.
Click to expand...
This has been my experience as well.
 
Maybe it’s something else?

I did verify that I have the 3 line fuel pump

I have one other gas station brand that I can try out to see if I experience the same problem with their gas.
 
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I can pull the top and check it out to see if I can see anything obvious going on.
 
I have a 55’ with a Dauntless V6. Mine died on me the other day leaving the grocery store…it was 110 degrees in NM. I have a new radiator and electric fan with adjustable controller. Once I opened my gas cap it fired right up. I set the controller to continuously run the fan and haven’t had any more problems.
 
Still working through trying to solve this issue.

Here’s what I’ve tried and-or verified so far….

1. I only run ethanol free gas. I have tried gas from 2 different brands and still experience the issue.

2. I verified that my carb does have the clip on the needle valve as mentioned above.

3. I didn’t see anything obviously wrong in the bowl.

4. I verified that I do have a 3 line mechanical fuel pump.

The most recent time it did this, temps were in low 90’s. I drove about 5 miles to store. Shopped for about 15 minutes and Jeep failed to start. I held pedal to the floor and it eventually started.

Drove same distance back home. Pulled in garage. Shut it off. Immediately tried to restart and it started right up.

After I shut it off, I decided to take a dowel rod and hold one end to the fuel line and one end to my ear. I could hear gurgling in the line. Both fuel lines run above the engine close to the valve cover, which I think is how they came from the factory.

Not sure if that is normal or if it’s another data point.

I also failed to mention I have a poly tank and am using the gas cap that originally came with Jeep. Pics below.

I’m assuming this is a vented cap??

1782930933463.jpeg

1782930963386.jpeg

Any additional thoughts or things to help track down?

The Jeep runs and idles great with no apparent issues. Just has trouble starting back up sometimes.

It can set for a month or so without being started and will start right up with no issues.
 
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Have you verified that the return line from the pump to the tank is clear?

I'd suggest for trouble shooting purposes putting some clear sections in the lines & measuring engine temps to see what's happening when as I did in my video.
 
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