[Pierre1967]

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

[HoovDaddy]

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.

[JLMounce]

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.

[Pierre1967]

Thanks all for your help,

JLMounce, why change the braided steel feed hose to a standard rubber fuel hose ?

[lucky1]

Originally Posted by JLMounce

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 use a mechanical pump, where can I get a phenolic insulator for it ?
Thanks.

[TRADERMIKE 2012]

Originally Posted by lucky1

I use a mechanical pump, where can I get a phenolic insulator for it?
Thanks.

Mikes Reply:
The gasket is a type of Phenolic, enough to make a nominal effect only, listen to what the man said, loose the manual pump all together and go to the Elec. Fuel Pump. The Elec. Fuel Pump must be installed lower than the Gas Tanks lowest point. The Pressure involved, putting the Fuel under, might be enough to resolve your issues, if not move on to another one of our suggestions. Mike out.

[Pierre1967]

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)

[pfilean]

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.

[TRADERMIKE 2012]

Originally Posted by Pierre1967

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)

Mike here,
Try removing the blockage fuel filter at the Carb., run one inline. Get the special 3-hole Thermostat in the Temp. range your Engine requires. Get a Heavy-Duty Truck Clutch Temp. activated, Clutch Fan. Use factory 19" steel Fan blades with a Shroud. Seal the Radiator completely around the perimeter. Let us know what happens then. Don't forget the Electric Fuel Pump with the return line, some use an adjustable Regulator up front where you can adjust it, that has the bypass built in. Run a second Fuel line forward for twice the Fuel Volume.

[HoovDaddy]

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.

[Schurkey]

Originally Posted by Pierre1967

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

Originally Posted by Pierre1967

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)

Originally Posted by HoovDaddy

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.

Half the problem in this thread is that folks are using the term "vapor lock" incorrectly.

"Vapor lock" is NOT "fuel boiling in the carburetor".
"Vapor lock" is NOT "fuel evaporating out of the carburetor".
"Vapor lock" is NOT "fuel boiling in the pressurized plumbing between pump and carb".

"Vapor lock" is the inability of an engine-driven (or, potentially, an inline electric) fuel pump to pump vaporized fuel. The fuel may boil (vaporize) IN the fuel pump, or it may boil in the suction plumbing between tank and pump inlet. At any rate, the diaphragm-and-check-valves of the typical engine-driven fuel pump are inefficient at pumping vapor rather than liquid fuel. Therefore, little-or-no fuel pressure, little-or-no fuel volume, the engine stalls or has reduced power because the fuel pump is "sucking air".

The fuel boils in the pump or the inlet plumbing due to engine- or engine-compartment heat; exhaust heat radiating from manifolds or headers, or the pipes under the vehicle, and it's compounded by the lower boiling point of a liquid at less-than-atmospheric pressure ('cause it's on the suction side of the fuel pump.) A lower-temp thermostat is of marginal usefulness since that doesn't change the temperature of the exhaust much. It can change the temp of the engine block, and the air coming off the radiator, though. So in some cases, it can help. Others, maybe most--no. He has a 160 thermostat, but the engine runs at ~200 degrees. Finding out why the thermostat has LOST CONTROL of engine temp would be high on my list of priorities--but fixing it may/may not help the stalling-when-braking problem.

A restriction in the inlet-side plumbing makes the pressure-drop worse, which makes the boiling point lower. This is why fuel filters belong on the pressure side, not the suction side. If a filter is used on the suction side, it's typically a fairly "coarse" "primary" filter, with a fine, "secondary" (smaller micron rating) filter on the pressure side.

^^^And all of that is probably moot, since the actual problem the guy is having is NOT typical "vapor lock" symptoms. I'm with HoovDaddy. Until the carb is proven good for float level, with proper float-bowl baffles installed correctly, float bowl vents in-place and not plugged, and the whole carb proven to not be overheated, vapor lock is not something I'd be considering. Assure that the brake booster isn't bleeding excessive air into the manifold when the pedal is pushed, too.

Adding a vapor return system (1/4" tubing) is most unlikely to cause a problem, I've done that to most of my carbureted vehicles. So even though this is unlikely to be a vapor-lock problem, a vapor return is still something I'd recommend on general principles. Re-plumbing for a return-style regulator and full-size return plumbing (because at idle, almost all of the fuel being pumped is getting returned to the tank) would be far more complex, more expensive, and--maybe--not any more effective than a plain vapor return.

A friend of mine had his daily-driver hot-rod burn due to rubber hose on the pressure side of the fuel pump--which rotted from the inside, under the steel braid. He had to throw his transmission into "neutral" then push his flaming car out of the driveway and into the street or it'd have taken the house with it. So I'll say it again: Get rid of that steel-braided junk from fuel pump to carb. Install a proper, double-wall seamless steel tube that's not going to rupture when you least expect it. If you HAVE to use one or more short sections of quality hose on the pressure side of the pump, inspect it regularly, DON'T use cheap-junk unshielded worm-gear clamps, and replace it at the first sign of trouble. I use the steel-braid-over-Teflon (PTFE) liner hose rather than steel-braid-over-rubber, or plain rubber.

[Brentco]

Originally Posted by Schurkey

^^^And all of that is probably moot, since the actual problem the guy is having is NOT typical "vapor lock" symptoms.

Stalling and difficult hot restarts are not symptoms of vapor lock? Maybe we could use some clarification from the OP, but it certainly sounds like vapor lock to me.

It’s good to keep an open mind, and there could be multiple things going on, but if the problem was a leaking brake booster or incorrect float level I don’t see how that could cause hard restarts.

[JLMounce]

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.

[HoovDaddy]

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

[Bill Hanlon]

Gee, nobody suggested a clothespin in the fuel line!

[JLMounce]

Originally Posted by Bill Hanlon

Gee, nobody suggested a clothespin in the fuel line!

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.

[25stevem]

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

[25stevem]

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.

[mchell]

Originally Posted by Pierre1967

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

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

[ID67goat]

Originally Posted by mchell

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

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.