Vapor Lock 70 Dart Swinger 340 Car

[Mattax]

After a drive and parking car for just a few minutes I can see fuel bubbling in the clear fuel filter just before the carb

That is normal.

Car thus far has restarted only when accelerator is floored when cranking it over. Previous owner said it died during a longer drive and they could only get it to start after letting it cool down overnight.

This part is not normal. It should start hot with thottle closed and no choke or throttle just cracked open a bit.
Would love to see some photos of the engine bay.

Next time you take it for a short drive, when you get back, remove the air cleaner and look down the carb throats. Choke should still be open so you can do this without touching the throttle. Is it visibly wet? Close the hood and let it heat soak and look again. Is it wet?
If it is then the fuel level in the bowl is too high. Next will be to figure out why.
If its not, then maybe the idle is set too rich - which will be ok for a cold start but not a hot restart.
or maybe its something else but lets start here with what you suspect - which is flooding.

I have an electric fuel pump in my shop that works with existing mechanical pump.

If there was vapor lock in the suction line, then one could say that pressuring fuel from inside or near the fuel tank will make it harder for vapor to form. But your observation is bubbles in the pressure side of the fuel pump, and you suspect flooding. The flooding is either expanding fuel or pressure overcoming the needle and seat. As discussed a vapor return line could help, or a even fuel pump with the relief pinhole in it (factory around 1972). But as stated above, I think the first thing to do is physically confirm what is going on and you may be able ot address it directly.

 

[Mattax]

1963 master Tech on setting filter to allow vapor out.

Carburetion and Performance Diagnosis (Session 188) from the Master Technician's Service Conference

Hot start issues in '72 that came in part with fuel reformulation and using retarded initial timing to put more heat into the engines at idle.

1972 Imperial & Chrysler Engine Performance Facts & Fixes Service Book (Session 291)

 

[matthon]

Clear fuel filter can lead to issues.
Although I've never seen a plastic one melt, they do get brittle over time.
If it's the cheapo glass with the chrome ends, take it off, break into small pieces, discard, imo.

 

[1MeanA]

I would make sure that it is a vapor lock issue. I chased a similar hot start issue last summer only to find that the it was a distributor reluctor gap issue (long story).

 

[maldridgeks]

That is normal.

This part is not normal. It should start hot with thottle closed and no choke or throttle just cracked open a bit.
Would love to see some photos of the engine bay.

Next time you take it for a short drive, when you get back, remove the air cleaner and look down the carb throats. Choke should still be open so you can do this without touching the throttle. Is it visibly wet? Close the hood and let it heat soak and look again. Is it wet?
If it is then the fuel level in the bowl is too high. Next will be to figure out why.
If its not, then maybe the idle is set too rich - which will be ok for a cold start but not a hot restart.
or maybe its something else but lets start here with what you suspect - which is flooding.


If there was vapor lock in the suction line, then one could say that pressuring fuel from inside or near the fuel tank will make it harder for vapor to form. But your observation is bubbles in the pressure side of the fuel pump, and you suspect flooding. The flooding is either expanding fuel or pressure overcoming the needle and seat. As discussed a vapor return line could help, or an even fuel pump with the relief pinhole in it (factory around 1972). But as stated above, I think the first thing to do is physically confirm what is going on and you may be able ot address it directly.

I’ve attempted to re-create the problem twice, with one day being slightly cooler outside but the other was plenty hot. Came home and checked down the carb throats - they were dry. Buttoned car all back up and allowed to heat soak 20min or so, and still nothing in the carb throats. After checking the carb I went ahead and tried starting the car and it started just fine. After thinking on it, the only thing else that has changed is that I did refill the gas tank with non-ethanol fuel. Right after i filled it up, the problem did recur, but after running it a while (and possibly further diluting the older fuel that may have had ethanol), the problem appears to have disappeared. Thoughts?

 

[Mattax]

the problem appears to have disappeared. Thoughts?

It could be the fuel recently bought has lower RVP (Reid Vapor Pressure). Lower RVP is mandated Federally for summer fuel (normally shipping 100% by May 1). However I recently found an company report on line that suggests to me that the manufactures don't just switch summer to winter gasoline. The way I read it is they ramp up and then ramp down the high volatile components (like benzene) so in January they are producing the highest RVP fuel, and by March they have already begun to reduce the RVP.

 

[maldridgeks]

It could be the fuel recently bought has lower RVP (Reid Vapor Pressure). Lower RVP is mandated Federally for summer fuel (normally shipping 100% by May 1). However I recently found a company report on line that suggests to me that the manufactures don't just switch summer to winter gasoline. The way I read it is they ramp up and then ramp down the high volatile components (like benzene) so in January they are producing the highest RVP fuel, and by March they have already begun to reduce the RVP.

Interesting. Should I learn more as we work with the car further, I will add to this post. Sure appreciate everyone’s input.

 

[Mike69cuda]

A phenolic carb spacer, about 3/8 of an inch, worked like magic for me after I tried a lot of other things. I think it is a great thing to do anyway, if you have the hood clearance.

 

[maldridgeks]

A phenolic carb spacer, about 3/8 of an inch, worked like magic for me after I tried a lot of other things. I think it is a great thing to do anyway, if you have the hood clearance.

Thank you. I will look into this too. Hood clearance is my concern.

 

[Swinger 340]

I’ve attempted to re-create the problem twice, with one day being slightly cooler outside but the other was plenty hot. Came home and checked down the carb throats - they were dry. Buttoned car all back up and allowed to heat soak 20min or so, and still nothing in the carb throats. After checking the carb I went ahead and tried starting the car and it started just fine. After thinking on it, the only thing else that has changed is that I did refill the gas tank with non-ethanol fuel. Right after i filled it up, the problem did recur, but after running it a while (and possibly further diluting the older fuel that may have had ethanol), the problem appears to have disappeared. Thoughts?

Winterize ethanol fuel make a "vaper lock" worst. Try to stick with non-ethanol fuel in the winter. If your carb is working correctly, the motor should start and once the bowl in empty, the motor will stop, in a vapor lock. I have a Dodge van with a 340 and installed a electric pump at the tank, to push the fluid to the mechanical pump. Problem solved. Vapor look is a carburetor issue. For those who don't know how it works, here is my explanation. When fuel heats up in the line, as near headers or exhaust manifolds, the fuel turns into an air pocket and will not let fuel be drawn from its source. Once the fuel is cooled to be drawn, or pushed, it will function correctly. The reason injected autos as of today do not have this issue, due to the fuel pump in in the tank.

 

[matthon]

I have aftermarket efi and once the in tank pump is off the fuel drains back out of the tbs, nice.

I put a new Holley vacuum secondary carb on my Dart and took it for a rip, everything works well.

I did notice fuel vapors coming out of the bowl vents, more so in the front.

If I put my finger over it and let off, fuel spits out. Only on the front bowl.

It doesn't drain the bowl in anyway if left to sit hot or until cold.

My brain started to consider a spacer or a return filter, but it's 100* here now, car isn't overheating, fuel line wrapped from before it enters the engine compartment to the carb line, and no problems restarting when hot, or cold.

 

[dr dodge]

the 440 in my challenger perk'd.
I made a "deflector plate" it went on top of the manifold, below the carb spacer
used roof flashing and it stacked up with the carb gaskets, 1 extra gasket.

slightly wider than the carb, but long in the front and back
angled to push heat out from under the carb (I think it gets stuck under there)
you could see it, but not by much
worked great, no more perk issues (780 vac sec holley)

 

[toolmanmike]

Thank you. I will look into this too. Hood clearance is my concern.

Also something to take into consideration is choke linkage. If you add a spacer and you have a divorced auto choke with the coil in the intake, You will need to somehow lengthen the actuator rod the thickness of the spacer. Electric choke, no problem.

 

[Johnny Dart]

Ede #9266
Thank me later.

 

[Demonx2]

It could be the fuel recently bought has lower RVP (Reid Vapor Pressure). Lower RVP is mandated Federally for summer fuel (normally shipping 100% by May 1). However I recently found an company report on line that suggests to me that the manufactures don't just switch summer to winter gasoline. The way I read it is they ramp up and then ramp down the high volatile components (like benzene) so in January they are producing the highest RVP fuel, and by March they have already begun to reduce the RVP.

This is true and how the industry works. I can also tell you we (the OEM's) had a rash of restart issues flate up one summer in Texas. Turned out Valero had a bunch of light ends (high RVP) and wanted to get rid of it so they used it up so to speak and produced fuel outside the summer RVP limits. The customers found it alright!!

 

[plymouth4onthefloor]

Do you have AC? Then you can ad this AC fuel cooler from Mercedes, this was for hot countrys factory installed.
https://www.ebay.com/itm/267144100509
Big connectors are AC the small ones for the fuel.

 

[Illahe]

Hello everyone. I've read through the numerous threads on vapor lock, but couldn't find the answers I'm after. Working on a 70 dart swinger 340 car with a 1979 360 in it with edelbrock intake and carb. Car exhibits signs of vaporlock. I'm weighing either an electric fuel pump or a wix fuel filter with the return line connector. Leaning toward the return line and the wix fuel filter. My gas tank doesn't have a return line adapter so I have been looking at a new sending unit that includes the return line connection. Has anyone solved their issue with this approach and if so, which sending unit did you use?

Also, has anyone tried the hot rod sleeves with success? Wondering if this is worth trying first.

Thank you.

If at all possible run a return line to the tank with a quality electric fuel pump.

See: 440 GPH Fuel Pump with Regulator - Pro Systems Racing Carburetors

You don't need this much pump, but this design gets it done in the staging lanes of the Florida sunshine.