[Pierre1967]

Thanks all for your help,

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

[Formulabruce]

Pierre1967 More than likely your engine temps are NOT even, and are higher than what you Indicate in the middle of the engine.
Do you use an IR gun? Where do you use it?

[Brentco]

Three key things to solve vapor lock: carb spacer, return line, and high fuel pressure. You need an electric pump to really get high pressure, but if you want to stick with a mechanical pump, Holley makes a 110 gph one that will give you more flow and maybe a bit more pressure. Robbmc also makes very nice mechanical pumps but they’re spendy for your needs. A higher flow mechanical pump, larger fuel lines (3/8” or 1/2”), and a bypass regulator (as close to the carb as possible) with a return line will solve your vapor lock issues. The pump will be moving large volumes of fuel, and the fuel will be constantly circulating through the bypass regulator and the return line back to the tank, keeping it cool. Fixed my vapor lock problems in SoCal heat with e10 fuel.

The return line is the key, so you might start with that and see if it fixes it. But it can be tricky to do a return line unless the pump is provisioned for one. And a pressure regulator is generally a good idea in any event, so IMO you may as well get a bypass regulator and run the return from there.

You’ll get varying opinions re: whether the return line should be the same size as the feed line or something smaller, like 1/4”. I like a full size return line.

[Sirrotica]

Just to give you an idea how fast E 10 evaporates. I used to drive semi, and deliver fuel to service stations. If you deliver fuel long enough, you'll have a minor fuel spill. One time I had a minor fuel spill in the summer, before I could grab the cotton diapers we carried onboard to sop up the fuel, it evaporated.

Only where there were depressions in the parking lot was there still a little bit of fuel still puddled. Any place that was flat, was dry in less than 3 minutes.

At 70 degrees it evaporates at a rate of 5.7% in 2 hours, In 35 hours at 70 degrees your carb will be completely evaporated.

Years ago when we used to wash parts in gasoline, it would take a couple weeks for the bucket to need more gas, we were really safe back then

[TxSportCoupe]

I’ve been doing this longer than most have been alive. I live in the “toughest weather” city in America, Lubbock Texas. I’ll tell you want I do and include a pic and y’all can do as you wish. Run a long body fuel pump on rubber. Keep it away from the motor, use a metal fuel filter. No glass or clear plastic and a phenolic wood spacer under the carb. The phenolic birchwood wood spacer under the carb is key! A return line to keep that unused fuel circulation instead of deadheading it to the top and letting it boil is a big help, if one has the ability to run a return line. I can run my .030 over 400 race gas motor all day long in the 117° heat and never vapor lock. I call it “the floating fuel supply setup”. Came up with with the design purely out of desperation. Even the regulator is floating and I separated from it’s bracket by a piece of thick carb gasket material The idea was to not have any place for heat to “transfer in”, anywhere. And, it worked.


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[grivera]

I like what you've done there. Has the small fuel gauge been reliable and leak free? If so, what brand is it?

[JSchmitz]

Your fuel line is awful close to your upper radiator hose. For a test, you could wrap it in aluminum foil and see if it makes a difference. You could use a sweeping 90 hose end and route it farther away from the heat. Looks like -8 AN from pump to carb? Do you need a line that big? With a return line it's not as bog of a deal oversizing the lines. Because the fuel recirculates and stay a lot cooler. With a deadhead system it makes a big difference in the residence time, or linear feet per minute, of the fuel traveling through the line. Just gives it more time to heat up. You could try a -6 line and see what happens.

[mchell]

From Robbmc using their mechanical fuel pumps. Note the use of a “vapor” return line and a “non-return ”style regulators
when using a mechanical pump. Tried this myself to no avail. No one can replicate your setup. Use what works for YOU. The vapor return made zero difference with vapor lock on my setup:

Pressure Regulator:
In most cases, a pressure regulator is not required. However, if the carb(s) in use requires a pressure lower than the pressure supplied by the pump, a non-return regulator such as RobbMc PN 1046 or 1050 can be used between the pump and the carb(s). Also, using a vapor return line with the pump will reduce idle pressure about 1 psi which may eliminate the need for a pressure regulator.

Vapor Return:
There is a metered 1/8 NPT port on the bottom of the pump for a "vapor" return line. A vapor return line allows a small amount of fuel to circulate from the pump back to the gas tank, keeping the pump and fuel cooler. This reduces the chance of fuel vaporization ("vapor lock") and can also improve hot restarts. Many Pontiacs came from the factory with a ¼” vapor return line connected between the stock fuel pump and the gas tank. That same line can be connected to the RobbMc pump by using an 1/8 NPT to 1/4" barb fitting. If your car does not have a factory return line, any 1/4" or larger return line can be used by installing the appropriate fitting. NOTE: When using the vapor return line, it is normal for the fuel pressure to bounce at low rpm. If you are not using a vapor return line, plug the port with union an 1/8 NPT plug.

[JLMounce]

Originally Posted by Formulabruce

Pierre1967 More than likely your engine temps are NOT even, and are higher than what you Indicate in the middle of the engine.
Do you use an IR gun? Where do you use it?

The steel braid will soak up more heat than the rubber hose will.

[Sirrotica]

No one has touched upon Pontiacs fix in the later T/As. They used foam wrap insulation on the steel fuel line right up to the carb. I have used this insulation before on a motorhome with a carb to cure vapor lock.

On the motorhome I also built a small air scoop below the bumper, connected it to 2 inch exhaust pipe which ran up to the carb, then flared out into a sheet metal shield in front of the carb. The cold air would travel up the pipe and cool the front of the carb. I ran the fuel line also inside of the pipe. The shield also kept the fan from blowing air that had already come through the radiator from heating the carb and bowl. it didn't look pretty, but it functioned very well.

This enabled my parents to drive it during daylight hours, before they had to wait for the sun to go down, and the ambient temp to drop before they were able to drive it. They also picked up 2 miles per gallon going from 8, to 10 MPG with ram air to the fuel lines...........

There is a 12 volt fan system they use to clear the bilge on inboard engine boats from fumes before starting. I had also thought about using one of these to pull cool air into a piece of tubing, rather than an air scoop, to cool the fuel pump then run it up to the carb to encase the fuel line and further cool the carb, and attendant parts. I know it isn't show car looks, but it is efficient, and it would work very well from my past experience with the motor home. If in stop and go traffic the fan would keep air circulating without need for the car to be moving.

The idea to cool the carb with a fan comes from a design Datsun/Nissan used before they had fuel injection, while still using a standard carb. They had a thermostatic controlled fan that when the under hood temp was over a certain range, would blow air on the carb and intake area. The fan was similar to the same style that some older trucks used on the windshield to defrost the windshield using cabin air. Crude looking, yet very effective. This was in the early 80s when E 10 had already come onto the scene, but not all cars were yet fuel injected. The under hood fan was a band aid, but it does work well for carbureted engines with E 10 fuel.

I used to have this problem when I was racing dirt track cars also, if the cars were stopped to clear the track and be realigned for a restart, the cars would be idling and the fuel would get hot while sitting still. Upon restart the car would bog and sputter until you worked the hot fuel through the system. For me this was a problem that I encountered in the mid 70s, and have tried many things to counteract the low boiling point of E10 fuels in a carb.

I had also toyed with the idea of a 1/4 inch return line off of the carb inlet nut right off of the front of the carb on a Q jet that keeps recirculated fuel cool just in front of the needle and seat.

As has been said each car, and situation may need different approaches, but good to be able to look through what has worked for others, and decide what may work best in your own situation.

There are a lot of good thoughts in this thread, I just wanted to add my

Things that will work, but they won't win any beauty contests for under the hood of show cars. I tend to gravitate towards form and function, over aesthetics though.

[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.

[Don 79 TA]

i had a serious issue of this when i moved to the higher altitude and heat in NM.
My system was deadheaded and had been for decades, which worked fine where i was on Long Island.
Running that setup here, had nothing but issues, fuels bowls would be empty when the car died, then after a short cool down... wallaha they filled up, but a short drive same issue (the temps at the time were hitting 110+ outside
went with a nice return setup and no issues what so ever, something i had been meaning to do for so long too, but laziness took over

other issue i had was my vacuum pump would never shut off, turns out there is an adjustment that is needed for higher altitudes
live and learn....

[dataway]

Does the return on an OEM mechanical pump on a 68 AC car qualify as they the type of return required to help with heat related fuel supply issues ... or is a return closer the to carb required?

[tjs72lemans]

The return on the pump is sufficient.

[25stevem]

Thanks to you in post 31 for finally drilling down to bottom of what’s going with the OPs car!

[dataway]

While we are at it ... can someone outline how a vapor return line differs from the fuel return at the pump?

[Schurkey]

Originally Posted by dataway

While we are at it ... can someone outline how a vapor return line differs from the fuel return at the pump?

Really simple. In nearly all cases, they're the SAME THING.

If the return plumbing is 1/4 tubing, and it connects to an OEM fitting on the engine-driven fuel pump, that fitting will be internally-restricted to .030--.060; it will pass vapor readily but not a lot of liquid fuel. It's a vapor return system primarily intended to prevent/reduce vapor lock of the fuel pump. Often seen on cars with factory A/C.

If the return plumbing is nearly the same, or even larger than the fuel supply plumbing, it's from a return-style regulator that will need to bypass nearly all the fuel pumped, right back to the tank at idle. If the return plumbing is restricted, the regulator may not be able to reduce the pressure sufficiently.

[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.

[tjs72lemans]

Maybe I'm disillusioned, but, vapor lock causes fuel not to enter the carb area due to no fuel flow via fuel line. That would cause a stall. With no fuel, that would cause a hard start until fuel had a chance to enter carb.

[Schurkey]

Originally Posted by Brentco

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.

Originally Posted by tjs72lemans

Maybe I'm disillusioned, but, vapor lock causes fuel not to enter the carb area due to no fuel flow via fuel line. That would cause a stall. With no fuel, that would cause a hard start until fuel had a chance to enter carb.

Vapor lock could cause stalling and difficult re-start.

But it wouldn't be limited to stalling while braking hard at low speed.

Originally Posted by Pierre1967

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.