[Brentco]

Well, stalling rarely ever happens at high speed. Stalls almost always happen at low rpm — idle or braking to idle. There’s probably enough fuel flow to keep the carb fed at high rpm, but when he brakes hard the rpm drops and the jets are uncovered, causing a stall.

[Sirrotica]

Unless the stuffer block has been removed it shouldn't uncover the jets. Q jets are known to be unaffected by fuel slosh on road courses and oval tracks.

I've run them on both with virtually zero trouble on autocross courses (entails very hard braking, and tight corners). Also on oval track cars they perform flawlessly.

I learned that in the old Doug Roe Q Jet performance book back in the 70s.

[Brentco]

I’m thinking of a situation where the fuel bowl level is running very low because of the pump’s difficulty moving vaporizing fuel, and hard braking pushes what little fuel is left in the bowls away from the jets, starving the engine.

[Schurkey]

Originally Posted by Brentco

I’m thinking of a situation where the fuel bowl level is running very low because of the pump’s difficulty moving vaporizing fuel, and hard braking pushes what little fuel is left in the bowls away from the jets, starving the engine.

Point taken.

A fuel-pressure test when hot should be performed.

[Brentco]

Fuel pressure test is good, and other good points as well — especially about never using the “cheap-junk unshielded worm-gear clamps“!!

["QUICK-SILVER"]

Originally Posted by Brentco

I’m thinking of a situation where the fuel bowl level is running very low because of the pump’s difficulty moving vaporizing fuel, and hard braking pushes what little fuel is left in the bowls away from the jets, starving the engine.

Q-Jet jets are in the front of the bowl. Metering wells are forward of the jets.

Hard braking will raise fuel level in the metering wells and cause nozzle drip. Which can flood an engine out and make it hard to restart.

Add engine braking to the mix and high vacuum will suck way too much idle fuel in and cause an engine to flood and stall. And that's another hard to restart.

Rear facing float can cause the carb to load up during hard braking.

OP could raise idle speed, lean out idle mixture screws, add a dash pot, etc...

Clay

[lust4speed]

I keep seeing reference to a 1/4" vapor return line, and while the line is that size the stock fuel pump has a restriction at the top of the cone that measures about .067". If someone were to just assemble a 1/4" return using a "T" that would be a lot of fuel being returned to the tank. Would be great for purging the fuel line but might leave the carb a little short on fuel at the top of 2nd gear.

Getting back to the OP's original question, we've installed a "T" right before the carb inlet and threaded in a .072" Holley jet to reduce flow back to the tank pretty much like GM was thinking. That did help last summer on a friends '67 GTO. We just did a return on the '65 Catalina wagon with the 496 that currently has a Holley mechanical pump. Since it was an AC car, the return line was right there so we got lazy and put the "T" right after the pump, installed a jet, and connected it to the return line. We'll see if the shortcut method works when temps get over the 100 mark. Our thinking is that there is a very limited amount of fuel flow when at idle or moving in traffic and the idea is that some areas of the fuel line get hotter than other areas, and getting fuel past the heat source should help it not flash into vapor.

Attached are both a stock deep fuel pump and a standard fuel pump cut open showing the cone. The opening of the cone would be at the top of the pump reservoir allowing it to bleed off vapor first, and then fuel after the vapor.

[Brentco]

Good info and pics of the cut open fuel pump!

Some cars also came with charcoal canisters and additional lines to the fuel tank, which also involve fuel vapors, just to confuse things further.

[z man]

This has been mentioned earlier, but a typical in-line electric pump with a max pressure of 7 psi, mounted into the feed line above the tank on my 68 GTO has not only assisted in filling the q- jet prior to starting, but will push the fuel past any areas prone to vapor lock. This can be turned on together in series with the mechanical pump and will not push past the inlet on idle causing a flooded condition . Yes you can run both pumps all the time, but I don’t. I do have a return from the mechanical pump and I am using a check valve by the electric pump to direct pressure toward the carb and not the tank. Simple inexpensive. Might want to throw a clear filter on the inlet line for a quick visual check until it works out , then a metal replacement. Should be good to go IF that’s the issue??

[TRADERMIKE 2012]

Mikes Reply:

To solve Vapor lock is to answer the million-dollar question. IMOP

Everything said in this post helps.

Band aids I was told, and I started off with the under-hood Temps. so hot it knocked you back with the radiant heat in your face.

A) check your fuel lines have no holes in it from a screw or something else.

B) I have 1966 Pontiac Tri-Power Rochester Carbs. and they like 3-6 psi. only, low pressure. Install a Fuel Pressure gauge before the Carb.

C) Block off the Cross over between the Heads and the Intake Manifold, in my case the Fuel heats up under the center Carb.

D) You use some or all the ideas from the other 6 posts being offered won't hurt. I am giving you the most important things to do first.

E) Use the Factory Shroud and HD truck type Temp. actuated Clutch Fan, seal the Radiator off. Install a Thermostat with the 3 holes drilled in it for bypass. I use 180 degrees in Florida.

F) Use Rec-90 Fuel with no Alcohol.

G) Make working Air Induction or Ram Air, for cool air sealed to the Pan to Carbs.

H) Make a Sidekick.

I) Use a high-pressure Radiator Cap to increase your boiling point of water and 50/50 Antifreeze in distilled water.

J) Extra Fan on one side, used as a pusher Fan.

K) Small HD aftermarket Starter with Torque.

L) I added a double-sided heat shield type Phenolic under my Intake manifold. This keeps the Heat from getting under all my Carbs., then I wrapped the bowls on each Carb.

M) I Wrap my Fuel lines and route the Fuel from the Tank, past the Elect. pump, that is lower than the Tank to start, to a bypass hose back to the Tank. I have the sock in the Fuel Tank to screen out Debrees then a filter before the pump Plus the next filter is up near the P.S in the Grill, attached to the Fuel Pressure Gauge.

N) I made an Aluminum Plate with Phenolics over and under to keep the Bowls from heating up, they are installed under the 2 front Bolts of each Carb.

O) Rebuild the Carbs. Offen and keep a good Tune-up on your Vehicle.

p) My 1966 Pontiac has all new AC parts and the AC heats up the Under-hood Temps. by 30 degrees "F". So, if I stop and go into the store, I leave my Hood open to release the Heat soak that develops the moment you turn off the key, that is where the water Wetter works to eliminate Heat soak.

Q) Run HD Batt. and a Sudbury switch in the Trunk. Run The largest Cables forward with no joints to complete the circuit.

R) I have even wrapped the upper Radiator Hose.

S) I use a 4-core copper Radiator/ Remanufactured Core, Oil, Power Steering and Trans. Coolers.

T) I Wrap my Spark Plug Ends to protect them from the Headers as well.

U) I Wrap my Headers in Header Wrap Cloth, and they are Ceramic Coated inside and out.

V) The Inside and under the Carpet is treated for Water Proofing and Heat Insulation Installed Behind the Vinal Interior Panels and over the metal Floor pans plus under the T- Tops in the Roof area. This is to Keep the Heat out and the cool in while using the AC.

W) I created Insulated T-Tops that keep the Sun out and there Insulated as well and covered with the same Ceiling Vinyl that I used when I did the Head-liner. This works well and anyone with T-Tops knows how hot it can get in the summer. Since I had mine Benched, I sealed the Metal to Glass with Black Silicone Sealant. My Rubber is all new throughout the Vehicle and the T-Tops don't leak.

X) The Water Pump gets the Divider Plate inside adjusted to a tight tolerance. See my other Posts for more if you need to. See John Wallace's Site for the way to deal with the inside of the Water Pump in early 1967 Pontiac Motors.

Y) I Run dual Fuel feed lines forward and join them before the Carb. For Extra Fuel volume.

[Kenth]

Vapor lock is caused by heat. Excessive heat from the engine, exhaust system, and/or outside temperature can cause the fuel to vaporize in the lines. Some winter fuel blends and high altitude can lower the boiling point of the fuel. This can also contribute to vapor lock.

The remedy is using an electric fuel pump pushing the fuel from the fuel tank to the carb.
And, since the carb is vented to atmosphere at all times there is NO way for vapor lock to occur in the carb.
Also, learning the principles of the combustion engines you will find that the intake manifold and carb needs to be heated to convert liquid fuel to a combustible gas/mixture.
Cool air to the carb venturies may be beneficial if you look for the last horsepower.

[Sirrotica]

When the fuel boils in the lines and is sent to the carb in vapor form, the float, and needle and seat cannot control a vapor. Yes the vapor lock can happen in the carb.

Cooling all the components is beneficial. As I said before, nissan had a thermostatically controlled fan under hood to cool the carb because keeping the carb cool, and the gasoline in liquid form where it can be controlled by the float, and the needle and seat is essential. Otherwise the vapor flows into the carb uncontrolled making an over rich mixture.

Heat is needed in the intake manifold to keep the fuel in suspension in the mixture, this is the exact reason that insulating spacers keep the two extremes isolated, cool carb and heated manifold.

A carb will not act as a vapor injector, because there is no control over the fuel inlet in vapor form. The fuel needs to be kept cool until the air and fuel are mixed, then heat will keep the fuel from dropping out of suspension.

GM realized this and made a spacer out of layers of gasket material and sheet metal alternately sandwiched together to dissipate heat from the bottom of the carb, and keeping the two components isolated from each other as much as possible.

Many people have testified that after the heat passages are plugged in the intake manifold that the car runs badly until some heat transfers from the heads to the intake, that can take 5-10 minutes before the hesitation and stumble goes away. Of course worsened in colder ambient temperatures.

Then the greenies insisted that adding 10% ethanol to pure gasoline would save the planet. Both pure gas, and E10 begin to boil at the same temp 115 degrees, but at 173 degrees E10 is pure vapor. Pure gas needs to be in the neighborhood 400 degrees to fully vaporize, therein lies the difference between the two fuels, and why pure gasoline can still be controlled by a float, and needle and seat.

If the carb and fuel lines can be held below 173 degrees E10 can still be metered by a carb, if not, the best bet is pure gas. One other plus is pure gas can sit for probably 8-10 times as long before all the light hydrocarbons evaporate. Having the light hydrocarbons missing lowers octane, and the ability to start a cold engine.

[JSchmitz]

Heat is not the only thing that will make fuel boil. Lowering the pressure below atmospheric can cause it also. Make sure you don't have a restriction on the suction side of the pump. Such as a kinked line or hose, a clogged sump filter, etc.

[Schurkey]

Heat is the only thing that will make fuel boil.

Reducing the pressure makes the fuel boil at lower temperature/less heat.

So, yes, restricted plumbing on the suction side of the pump makes things worse. Which is one reason to not have a fuel filter on the suction side of the pump--and if you do, use a "coarse" filter, with a fine (smaller micron rating) secondary filter on the pressure side of the pump.

[JSchmitz]

Originally Posted by Schurkey

Heat is the only thing that will make fuel boil.

Reducing the pressure makes the fuel boil at lower temperature/less heat.

So, yes, restricted plumbing on the suction side of the pump makes things worse. Which is one reason to not have a fuel filter on the suction side of the pump--and if you do, use a "coarse" filter, with a fine (smaller micron rating) secondary filter on the pressure side of the pump.

I guess I should have said that excessive heat is not the only contributing factor that can cause fuel to boil. That was the point that I was trying to make. I understand the principal very well. Liquid can boil at room temperature in a vacuum. So you are technically wrong. Heat is NOT the only thing that will make fuel boil.

[Schurkey]

Fuel in a vacuum will not boil unless the temperature (heat) is high enough.

Vacuum does not make fuel boil. Heat makes fuel boil. It's just that you need less heat if you have more vacuum (lower pressure).

[TRADERMIKE 2012]

Administrator, Mike is trying to share Text and Photos to the PY Forum and when I try to do so, I am being sent to Facebook and I can't figure out why?

This time I went down to the bottom of the page to write to you and I am not posting a photo.

I tried to send a Photo and I was directed to my face book page. somehow I got crossed linked. When I use manage attachments, I find myself on face book. It happens the moment I am using manage attachments!

[Tom Vaught]

I HAVE USED ONE OF THESE: (FOR THE LAST 20 YEARS) IN THE SUCTION SIDE OF THE FUEL PUMP. WORKS GREAT AND LOW PRESSURE DROP WITH HIGH FLOW. The screen diameter is 4inches. Different micron screens are available.

https://www.obergfilters.com/wp-cont...s-Oct.2018.pdf

Tom V.

[Schurkey]

I can't quote a prior post, or I get sent to FeceBook.

Noticed this with Post 56.

Fakebook must be destroyed.

[TRADERMIKE 2012]

Originally Posted by Tom Vaught

I HAVE USED ONE OF THESE: (FOR THE LAST 20 YEARS) IN THE SUCTION SIDE OF THE FUEL PUMP. WORKS GREAT AND LOW PRESSURE DROP WITH HIGH FLOW. The screen diameter is 4inches. Different micron screens are available.

https://www.obergfilters.com/wp-cont...s-Oct.2018.pdf

Tom V.

Mikes Reply:

Nice Filter system and one way check valve. I bought an inline filter for my Power Steering fluid and it had a fine Screen in it.

I wanted to mention and don't quote me on my numbers, the Rec-90 - unleaded Fuel Boils at around 186 degrees F and the Pump Gas out there with Alcohol Boils sooner about 173 degrees F and just that little difference with my Air Induction allows me to keep out of Vapor lock, so far. The Temp. activated Clutch Fan HD version is Far superior to the OEM one that I originally used. The Holden? 3-hole Thermostat by-pass performs as they said it would. While using the AC the under-hood Temp. climb some 30 degrees F, so the use of a Mini Starter was inevitable. My side -kick counteracts the AC via a remote switch on my Dash. It seems that the Clutch Fan does most of the work, plus I can turn on the Pusher Fan when I need some help on a hot day.

This is the type of phenolic I use under my Intake Manifold, custom to my standards though.