Vapor lock, air/fuel creep - need to fix or just live with it

Is it possible to prevent fuel from boiling in the bowls of a carburetor while engine is running at low speeds (stop & go traffic) when outside air exceeds 80F?


Even if you employ an ideal return-style fuel system with electric fuel pump and regulator within 20-30" of carb, it seems like the fuel boiling in the bowls would still be the problem despite being able to keep the fuel in the feed/return lines at near ambient temp. Return-style system could keep the fuel under high pressure in the line and reasonably cool, but once the fuel's between regulator and needle/seat it would be 6 PSI (thus lower boiling point than the 40+ PSI line behind the regulator) and once it passes by the needle/seat in the bowl, it's at atmospheric pressure & regular E10 pump gas is going to vaporize around 120F +/- a few degrees maybe depending on additives / formulations blended by refineries.


Reason I ask is the measures I've implemented (listed below) failed to stabilize "air/fuel creep" of the car delivering 14.5:1 in cruise, idle & stop-n-go driving when temps are under 70F, thus the air/fuel ratio creeps into the 16-17 range when temps are above 80F.


I have a clean tank & fuel sock, clean stock routed & sized fuel lines, a Carter high-flow mechanical fuel pump, Mr G 40 micron filter feeding a 750 Mighty Demon atop a 1" phenolic spacer and GM aluminum heat shield (with thermal side blanketing added) and this is all topped off by a stock Chevy drop-down 14" open element filter.


Hood is the original flat hood (wish I could do the ZL2 hood to vent hot air, but we don't have room to store an extra hood), 160F thermostat & 4-core cooling system w/puke tank keeps temps around 170F even on the hottest days in stop-n-go (and cruises at 160F over 40 MPH).


Handheld pyrometer shows 115F at fuel bowls (which show fuel in them via sight glass & vidcam I mounted) with fuel being a little lower on the mark when warm vs. when cold or in over 40 MPH cruise speed. 135F on header-side of mech fuel pump, 115F on forward non-header side. Vic Jr manifold registers 185F on its runners & base.


Over 75F, the car stays responsive to throttle during stop-n-go traffic and will return to normal when cruising above 40 MPH, but I'm concerned that as this summer's temps rise to 90-100F outside, the vapor lock symptoms will get worse in traffic.

I'm considering going to a return-style fuel system with electric fuel pump to bring 40+ PSI fuel from tank to regulator (20-30 inches from carb) and returning to fuel tank, however, I wonder whether that would cure the air/fuel creep or if the fuel would still be boiling in the bowls no matter what I did. Should just live with the air/fuel creep in warm conditions? The car's run like this for the past 15 yrs on this build by the way - so maybe I'm just obsessing about air/fuel fine-tuning in warm weather and should just live with it.


If I go electric/return, do people have recommendations on the quietest pump (Walbro, Aeromotive, Holley) and external or in-tank layout. I figure since I'd have to plumb a return-line into the factory tank, the in-tank pump would be the quietest so I'd want to get the most reliable pump as well since I'd hope this lasts many years & miles before needing replacement.


Thanks in advance for your ideas. Pictures show the current layout. Attachment 164628Attachment 164629

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Originally Posted by stantaur

Is it possible to prevent fuel from boiling in the bowls of a carburetor while engine is running at low speeds (stop & go traffic) when outside air exceeds 80F?


Even if you employ an ideal return-style fuel system with electric fuel pump and regulator within 20-30" of carb, it seems like the fuel boiling in the bowls would still be the problem despite being able to keep the fuel in the feed/return lines at near ambient temp. Return-style system could keep the fuel under high pressure in the line and reasonably cool, but once the fuel's between regulator and needle/seat it would be 6 PSI (thus lower boiling point than the 40+ PSI line behind the regulator) and once it passes by the needle/seat in the bowl, it's at atmospheric pressure & regular E10 pump gas is going to vaporize around 120F +/- a few degrees maybe depending on additives / formulations blended by refineries.


Reason I ask is the measures I've implemented (listed below) failed to stabilize "air/fuel creep" of the car delivering 14.5:1 in cruise, idle & stop-n-go driving when temps are under 70F, thus the air/fuel ratio creeps into the 16-17 range when temps are above 80F.


I have a clean tank & fuel sock, clean stock routed & sized fuel lines, a Carter high-flow mechanical fuel pump, Mr G 40 micron filter feeding a 750 Mighty Demon atop a 1" phenolic spacer and GM aluminum heat shield (with thermal side blanketing added) and this is all topped off by a stock Chevy drop-down 14" open element filter.


Hood is the original flat hood (wish I could do the ZL2 hood to vent hot air, but we don't have room to store an extra hood), 160F thermostat & 4-core cooling system w/puke tank keeps temps around 170F even on the hottest days in stop-n-go (and cruises at 160F over 40 MPH).


Handheld pyrometer shows 115F at fuel bowls (which show fuel in them via sight glass & vidcam I mounted) with fuel being a little lower on the mark when warm vs. when cold or in over 40 MPH cruise speed. 135F on header-side of mech fuel pump, 115F on forward non-header side. Vic Jr manifold registers 185F on its runners & base.


Over 75F, the car stays responsive to throttle during stop-n-go traffic and will return to normal when cruising above 40 MPH, but I'm concerned that as this summer's temps rise to 90-100F outside, the vapor lock symptoms will get worse in traffic.

I'm considering going to a return-style fuel system with electric fuel pump to bring 40+ PSI fuel from tank to regulator (20-30 inches from carb) and returning to fuel tank, however, I wonder whether that would cure the air/fuel creep or if the fuel would still be boiling in the bowls no matter what I did. Should just live with the air/fuel creep in warm conditions? The car's run like this for the past 15 yrs on this build by the way - so maybe I'm just obsessing about air/fuel fine-tuning in warm weather and should just live with it.


If I go electric/return, do people have recommendations on the quietest pump (Walbro, Aeromotive, Holley) and external or in-tank layout. I figure since I'd have to plumb a return-line into the factory tank, the in-tank pump would be the quietest so I'd want to get the most reliable pump as well since I'd hope this lasts many years & miles before needing replacement.


Thanks in advance for your ideas. Pictures show the current layout. Attachment 164628Attachment 164629

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My 68 was doing the same thing . Like you I added a heat shield and heat sleeve on the fuel lines . I changed the mechanical high volume fuel pump to a lower pressure pump and it solved the problem on my 383 . My carb is a speed demon 750 . A slight timing change might help . I advanced my timing 2 degrees when I changed the fuel pump .

If you decide to go electric, do an internal pump,I don't hear it when the car starts.
I run a full return system with my carb, I used a tanks inc. tank and internal pump set up. the Holley billet return regulator is on the inner fender.
The fuel tank gets noticeably warmer to the touch with the returning fuel after driving around town, I've never check the ambient air vs tank fuel temp though,since I've not had any issues yet.

I have a small carter electric pump up near where my mechanical pump used to be on my 76 Dodge, it doesn't need a regulator. With that small pump and a phenolic spacer, I don't have any hot idle or restart problems anymore, 100°F+ Texas hot days.

Walbro in tank pump & return line will fix it.

I would run an in-tank pump. Since you're only need 6psi, I wouldn't run a 40psi pump stepped down. That means the pump is working harder than it needs to adding more heat to the system. If you can mount the regulator above the float bowl on the firewall, that may help.

You could also add a thermal barrier to the bottom of the fuel tank to minimize heat soak from the pavement. And if you're running a transverse muffler right in front of the tank, they make thermal shields for those as well.