Vapor lock and options
 

Hi,

I have a vapor lock issue with my Firebird 67 400. I have just a simple braided line from the pump to the carb with AN fittings (see attached pictures).

Is it best to add a return line just at the output of the mechanical pump runing under front engine or to split the fuel line just before the q-jet inlet ? (or change the pump to add one with a separated return output line ?)

Any other options are welcome ! Thanks to post pictures of your setup !

Pierre

What are your symptoms? Hot or cold? Is that a fuel filter on the inner fender?
You have a lot of rubber hose, how old is the hose? Also, If that's your fuel filter, I think you should have it after the pump.

I battled vapor lock for a number of years and although I was able to make it better, or curtail it for longer driving periods, I was never able to solve it myself until I went to a throttle body EFI setup and put the fuel under pressure.

If you are in a high altitude region that uses gasoline blended for high altitude use (Colorado, WY, upper mesa of AZ etc.) it may be hard, if not impossible to completely remedy the issue because of the temperatures that fuel boils at in these regions.

Based on the pictures you've provided I'd do a couple things.

1. Definitely run a return line from the fuel pump
2. Add a phenolic fuel pump insulator
3. Change the braided steel feed hose to a standard rubber fuel hose and use a heat shield on any lines in the engine bay

I see you're already running a phenolic/plastic spacer under the carb.

The ultimate goal is to keep the fuel as cool as you can. Anything that is sinking heat is putting that heat into the fuel. If your fuel system is up to the task, running the bowl in the carburetor with slightly less fuel will help, as it's not sinking heat for as long before it's consumed. The problem there of course is if your fuel system doesn't have the capacity to feed the engine everything it wants, all the time, without the reserve of the carb bowl, that creates it's own issues.

If you have the cooling capacity to do it, running the engine on a cooler thermostat, like a 160 would help as well. For that matter, if you can't keep the engine running on the thermostat at all and regularly run in the 200+ range, fixing that will also help the situation with vapor lock.

I have a fuel filter near the thank and in the carb inlet. Hose is not so old.

When the engine temperature climbs near 195-200 deg (never above), it tends to stall when I brake hard at low speed. It's hard to restart too.

I have all the good stuff (Cliff carb, KRE heads, 30 deg. timing, alum. radiator, electric fan, 160 thermostat, phenolic spacer, new alum. water pump, water wetter, 40/60 mix, etc..)

(I'm thinking of going with an electric pump setup to solve the problem ... but would really like to stay with mechanical)

There are some mechanical pumps that have a built-in provision for a return line. Usually used with AC which had more tendency for vapor lock. Before that was the factory way there were fuel filters placed in the vertical fuel pipe area between the pump and carb. The filter had two outlets that allowed the vapor to separate from the liquid and be pumped back to the tank. I used the filter way and have had no vapor lock since.

I'm not dismissing everybodys suggestions, but stalling when braking, I would check the float level? Hard to restart, as said could be the fuel boiled out from heat, but also the bowl might be empty due to a low float level.

There's a handy chart on page two of this PDF, discussing vapor to liquid ratios of various fuel types.

https://www.grabner-instruments.com/...5-e640bdc7e338

It's important to note that we are talking about temperatures specific to the liquid fuel in use. At around 150 degrees the temperature common e10 pump gas will be about 80% vapor. What's occurring at this point is that your fuel pump is compressing the vapor and is no longer able to supply liquid fuel to the carb bowl. As engine speed slows and fuel consumption falls, the force of the vapor will stop liquid fuel from entering the venturis and the engine will stall.

The chart in that PDF also doesn't take into account fuel system pressure. If you read through the full work you might ask yourself how anything, even modern vehicles avoid vapor lock. A big reason is the fact that the systems of modern vehicles are under a good bit of pressure. 43psi for most early EFI automobiles while direct injected vehicles see fuel pressures at the nozzles of up to 1500psi. Just like your cooling system, the fuel under pressure will resist boiling.

If your engine runs at 200 degrees and your fuel flow through the system is slow enough due to low engine speeds, eventually with an e10 mixture, you're going to start seeing issues with vapor lock as components soak up all that heat and transfer it to the liquid fuel. Using a heat insulator between the carb and the fuel pump which are in direct contact with the engine block slows the heat build up in those components, but they will eventually soak from the ambient under hood temperatures.

Using gasoline that has no ethanol blending is your best choice. It is unfortunately becoming harder and harder to find at local pumps however. You can of course buy your own fuel and pump it at home, but then you limit your travel distance. This is why I'm an advocate for modernization of these components to deal with standard pump fuel.

Here's a link to find ethanol free gas near you. https://www.pure-gas.org/

Gee, nobody suggested a clothespin in the fuel line! ;)

First off ditch that fuel filter at the tank since it’s acting more like a restriction then anything else!

I've seen it quite often. I've heard from owners that swear it works.

I'm not an authority on thermodynamics but I wouldn't think attaching an object made of material that is not really heat conductive would do much of anything.

Also stalling when braking hard has nothing to due with a vapor lock condition.

Drive somewhere that when you get this condition to take place you can pull off the road, remove your air filter and see if your accelerator pump is working .

If it does then your fuel level is too high either due to float adjustment or a bad float .

If you get no pump shot then your running out of fuel which could be because of that filter at your tank and or the fuel is boiling off.

Never had luck getting a mechanical pump ( even with a vapor return ) to work 100% of the time without some kind of stumble in very hot and slow conditions with todays gas….. in cool conditions and little idling it had no issues

Since my driving is almost always relegated to very hot conditions I tired of wondering when it was going to cough up a fur ball after a long light on a 90 degree day…

An electric pump cured that in short order….never looked back

My brother struggled for years with vapor lock. He tried everything to fix it: carb heat shield, heat sleeves for the fuel line, upgrading the cooling system, etc. He also has an electric fuel pump. None of it helped. What finally made the issue go away was using ethanol free gasoline. Presto, problem solved.

Same experience as above. I tried everything I could using a mechanical pump and was never successful with vapor lock on hot days. Now run an electric pump with return style regulator and no more issue even on 100+ degree days using crap E10 gas.

Also bear in mind the OEM cooling systems with a big fan running all the time blows air right over the fuel line. Back in the day when these cars were made a lot of the materials they used weren't up to today's standards and would degrade quickly with high under hood temps. The big OEM fans weren't just to cool the water passing through the radiator, they also helped keep under hood temps to a tolerable level. The OEM systems provided quite bit of air flow down around the fuel pump, and the metal line going to the carb.

Most likely not your problem ... but something to think about.

Lots of good suggestions and ideas in the thread. Ethanol Free gasoline would probably be the biggest single, biggest improvement if available easily for you. Removing the fuel filter from the suction side of the system is a good idea. Modern cars have the filter in the line line that because the pump is in the tank so it's under pressure. Adding a low pressure electric pump will keep the entire line under pressure. For the past 10 years or so, I have just added 1 gallon of race gasoline to a full tank of 93 octane pump gas. Vapor lock gone instantly. The crap pump gas we have available vaporizes at a very low temperature compared to the old days.

I solved my vapor lock problem using an in line fuel filter with a 1/4 inch return line.

I had problems with my Camaro with a mild 350 having vapor lock issues for years. I changed to a three core radiator, shielded fuel lines from headers, wrapped fuel lines, spacer under carb. I could see my see through fuel filter not being full and having bubbles. I would shut down, pull air cleaner and watch hissing fuel push out and onto the butterfly plates of carb. Scary. I went to non-ethanol premium and never an issue again! I run the same in all my classics for that reason, plus the crap that happens to your carb with the ethanol crap.

This is exactly what happened to my 68 GTO prior to selling the car. I would recommend the non-ethanol fuel at this time, having the least amount of effort and expense at this time.