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Discussion Starter · #1 ·
i got a 1979 f150 with a holley carb on it a spacer under carb and a good return line system and it runs good on road because i use the secondaries and fuel is moved through but when on dirt road where i don't open the secondaries the gas will set in the back bowl and after awhile will start to boil and flood and stall motor what do i do?
 

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Welcome to the FFO!

I understand the engine stalling. However I would need more clarity about when.
Coming to a stop
Accelerating
Maintaining speed - if so what speed

You state the fuel is boiling in the secondaries.
Wouldn't the fuel be boiling in the primary too?
If the fuel is ONLY boiling in the secondaries and you are driving slow enough to not use the secondaries, how does the engine stall?

You state you have a return line. Fuel return line?
Is the carb stock?
What engine?

mid to late 1980s Class C motorhome chassis with 460 4V and Holly carb had known vapor lock issues. Most of the time it was related to fuel pressure too low. And it doesnot take much of a drop in pressure for the issue to show up. If you are getting vapor lock (fuel boiling) I would suggesting it is happening in the front and the rear of the carb.
I would guess your fuel pump may not be developing the needed fuel pressure.
If the fuel filter has not been replaced in the last year do that.
Checking fuel pressure as well.

Many people have gone to an electric pump back by the tank. The pressurizes the entire system.
If you have a return line then you can dial up the pressure a couple of PSI. The higher the pressure, the higher the boiling point for the fuel.
And with a return line fuel can move into the carb and out of it without picking up as much heat.

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It is also better to use steel fuel lines instead of rubber
Rubber holds heat more
 

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Discussion Starter · #5 ·
it has a 400 motor and i put a new radiator in because it had the wrong one in it and it was small at idle when it stalls and i got out and took breather off and just secondary side was boiling and dripping out booster
 

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Discussion Starter · #6 ·
it is a 570 brawler carb, primary side isn't boiling because it is using gas out of their and secondary is just sitting and starts to build heat
 

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Under low load the secondaries are closes and not being used.
Under that circumstance it is a 2 BBL carb. The engine is running off of the primary side only. And if the primary side is not experiencing vapor lock the engine will run well.

Since this is a non-stock system with a return fuel line, the above vid can be helpful to some degree.
Again a fresh fuel filter is critical!
Correct float setting (both sides) is as critical as well. And this may need to be adjusted a bit to get the best setting for your application.
The stock fuel pressure would be in the 5 to 7 PSI range. If not you could have a vapor lock issue because the fuel is not under enough pressure.
With a return, I would want a higher pressure than that. Guessing 2 to 3 PSI more.
You can pressurize the whole system from the back. (recommended) Or install closer to the engine. You can leave or remove the mechanical pump.

Depending on the CFM for that carb you may be over carbed
That carb can be had in 570 to 770 CFM
If a stock 400 at 5500 RPM max - 600 CFM would be the place to be.
CARB SIZE MATTERS: What size carburetor do I need? – Performance Improvements

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You are looking for a solution to cool the back of the carb. And I bet the whole carb is hot with the back end getting slightly hotter. Or the float level back there is different than the front.

Possible solutions for that would be -
Larger spacer depending on under hood clearance
Wide shield around (attached to the bottom of carb) the carb
Create greater air movement. Ducting, fan or ???? If you have electric radiator fans, a manual switch to kick them on when the problem shows up.
A switch to turn off the electric choke as that is still generating heat
Shielding of the front part of the exhaust system placed on top of the pipes so heat generated from there goes behind the vehicle not up into engine compartment

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i got a 1979 f150 with a holley carb on it a spacer under carb and a good return line system and it runs good on road because i use the secondaries and fuel is moved through but when on dirt road where i don't open the secondaries the gas will set in the back bowl and after awhile will start to boil and flood and stall motor what do i do?
Hello 70's Ford Trucks,

I too have this problem on the Holley style carburetor. The fact that fuel is boiling in the carburetor is caused obviously by heat. This leads to some important questions as you have to determine is the engine running excessively hot or are you at a higher elevation and or using fuel with a high alcohol content. The more alcohol and or elevation (less atmospheric pressure) will cause fluids to boil at lower temperatures. The higher the alcohol content the lower temperature petroleum will boil. All these factors intermingle.

If you are at a high elevation and or using a fuel with alcohol in it and your engine isn't running excessively hot, there's nothing you can do to stop the boiling with the current carburetor. That's just physics. The reason why the primary bowl is not boiling is because the air flow (engine running) through the narrowing venturi's cause the temperatures to drop and that cold portion of the carb absorbs heat from the primary side bowl keeping the fuel cooler and not boiling. Shut the engine off and wait a few minutes and more than likely that side will boil as well.

If you are confident the engine isn't overheating and you want or have to run regular alcohol laden fuel, you'll need a different a style carburetor or fuel injection. If you want to stick to a carburetor then as much as it pains me to say a Quadrajet carburetor is the way to go. The single small central fuel bowl keeps cool by the primary venturi's and since the secondaries draw off that same small central fuel bowl, there's no boiling of an overheated secondary fuel bowl.

Obviously fuel injection will cure that too as it's pressurized and fuel under pressure will boil at a much higher temperature. The term vapor locked was used mostly as describing when the fuel pump could not pump liquid fuel up to the carburetor as the fuel line to the fuel pump was so hot every time the pump would try and suck fuel from the tank this caused the fuel to see lower atmospheric pressure and boil right there in the line before the fuel pump. The pump then would suck vapor with much less mass than a liquid and not fill the carb and the engine would obviously starve of fuel.

Vapor lock doesn't happen on the pressure side of the pump because that 5-9 PSI raises the boiling point of fuel and it will stay liquid at a higher temperature. Some people call fuel boiling in the carb vapor lock, but that is not 100% the case. When fuel boils in the carb, the float drops and allows more liquid in always keeping the fuel level more or less constant. It's the violent bubbling action that carries liquid fuel into vents and boosters causing it to get sucked down the engine in an unmetered fashion. Where vapor lock starves the engine of fuel, carb fuel boiling causes excessive fuel to be ingested by the engine. You can see how the terminology is in conflict.

Having a fuel return line at the engine driven fuel pump, fuel filter or fuel pressure regulator allows a constant decent flow of fuel from the tank and back to it. This keeps slower moving fuel (returnless) from stagnating in the fuel line and absorbing excessive heat causing it to boil before the pump. With that it seems you have that covered.

Todays fuels with the higher alcohol content are really not carburetor friendly and everyone should expect that as it's formulated for fuel injection systems. Carburetted applications make up such a tiny percentage of the total fuel used, it's not even on the radar of major petroleum companies. It just means we are the ones that have to adapt.

In my case, I live at 6500 feet altitude and our fuel is alcohol based. Now I can use AV-Gas (100 octane leaded fuel) but at 5 bucks a gallon, it's not feasible. I have a full size Chevrolet and my old Quadrejet was just plumb wore out (25 years of use) and so I decided to switch the intake and carb. I bought a brand new Quick Fuel Brawler 750 vac sec thing. In the heat of the desert with the factory air con blasting and city style driving, it does what yours does, just all kinds of driveability problems from the fuel boiling in the carburetor. My engine runs at 165 degrees (160 stat) and it's still a problem in 85+˚F weather with the air con on. At idle with the air cleaner off and bonnet up I can watch both bowls boiling through the sight glasses. The old Q-jet never did that because of the small centrally located fuel bowl was kept cool when the engine was running. My choices are toss the brand new Quick Fuel carb and buy a new Q-Jet or buy multiport fuel injection for its factory equipped 454.

Sorry for the long diatribe but sometimes explanations need a bit of length :)

Cheers
 
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