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Old 08-06-2017, 01:59 PM   #1
The Swed
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Vented gas cap?

Here's pictures of my cap is it vented? I'm having vapor lock when the truck gets hot so it's time to rule out all the simple things first. I do not have a vented tank.

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Old 08-06-2017, 03:55 PM   #2
AussieinNC
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Re: Vented gas cap?

That cap should be vented...Does removing the cap cold and running the truck until its hot cure the issue?

More likely fuel boil out of the carb ...describe the scenario for us please...

Does the lack of fuel occur after a hot run then hard to start after a shut down?

Engine specs ?
Headers?
Heat insulator between carb and manifold?
Electric fuel pump or standard mechanical?

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Old 08-06-2017, 04:49 PM   #3
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Re: Vented gas cap?

The carburetor already has the phenolic spacer between the intake and the carburetor. Yes the truck is hard to start after it sets in when it gets really hot and the idle drops down to 300 rpm normally at 600 rpm. I did remove the cap at idle and the condition remained the same I also checked the routing of of the fuel it's perfect. The fuel filter was completely empty but the truck was still running on vapor! Maybe a cracked fuel line?

Carburetor is an Edelbrock 650 with an Edelbrock intake full length headers 350 motor with an RV cam, mechanical fuel pump, points.
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Old 08-06-2017, 04:57 PM   #4
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Some more pictures.
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Old 08-06-2017, 04:58 PM   #5
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Re: Vented gas cap?

Drop in idle speed and hard to start hot ...empty filter...all symptoms of an air leak into the suction line in the tank...

Is the tank in cab ?
Does it do it more often when fuel tank level drops below a certain level?

Hook up an electric pump and route around the mechanical pump...mount the pump as close to the tank as possible...pumps push much better than pull...

Doesnt have to be an expensive one to test with...

You could always take the entire tank out of the test by setting up a temporary tank and electric pump in the bed...

I have seen these pickups split about half way down...works fine when tank is full...issues start after tank level drops below crack in pickup pipe...

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Old 08-06-2017, 05:10 PM   #6
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Re: Vented gas cap?

Just saw your pics...CLEAN TRUCK !

That metal fuel pipe, pump to fuel filter is awfully close to both heater hoses and could be incurring heat soak from the water hoses...
Wrap the fuel pipe with some heat wrap and maybe try and move it away from the heater hoses...

Also...how old is that mechanical pump? I have seen the valve sets inside the pump work ok when cold, but drain back when hot...

Have fun...
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Old 08-06-2017, 06:41 PM   #7
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Re: Vented gas cap?

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Old 08-06-2017, 07:01 PM   #8
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Re: Vented gas cap?

Run it with gas cap off until she is hot, which will rule out the cap.
Do not run it this way with a full tank as any right turns will spill gas on your truck.
Do not put a rag in the filler neck as it may create a Molotov Cocktail. Plus rags fall into the tank to cause other issues.

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Old 08-06-2017, 07:09 PM   #9
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New pump with a 1000 miles plus miles and I use a plastic boat gas tank and had the same issues. The only thing that hasn't been changed is the steel fuel line from under the tank to the fuel pump. Btw I replaced the sending unit in the tank behind the seat after cleaning and coating the tank.
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Old 08-06-2017, 07:25 PM   #10
AussieinNC
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Re: Vented gas cap?

Sounds like you have checked a lot of things off the list...

Check how close the fuel supply line runs to the headers...shield it where it runs in the chassis rail past the headers...

Wrap that steel line from pump to carb with heat wrap...

Test with an electric pump pushing from as close to tank as possible...

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Old 08-07-2017, 12:15 AM   #11
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Re: Vented gas cap?

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Old 08-12-2017, 12:45 PM   #12
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Re: Vented gas cap?

Just a shot in the dark here...

While I don't have this problem in my truck, I've always had it in my air-cooled VW. When the engine is shut off, the carburetor begins to heat soak. After a few minutes the gas will start to boil and "dribble" and collect on the throttle causing a rich restart.

I have to hold the accelerator to the floor while cranking to "pass the gas"!

I've never seen this happen in my truck, but if your fuel level is too high in the float I suppose this could happen. To check for this, once the engine is warmed up quite toasty, shut it off, and watch down the throttle to make sure no gas dribbled down the onto the throttles. Good luck, and please share what you find!
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Old 08-12-2017, 01:05 PM   #13
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Re: Vented gas cap?

Hey man, my truck has been doing the same thing. My dad suggested checking the rubber line under the cab where the hard line goes through the floor. Micro cracks can suck air, but not leak fuel. I will check mine this afternoon when I get home from work. To my knowledge, mine has never been changed, which would make it a 50 year old piece of rubber.
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Old 08-12-2017, 06:49 PM   #14
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Re: Vented gas cap?

I would try a thicker phelolic spacer.
That one looks to be 1/4" or 3/8"

I'm running a 1/2" and solved my problem with heat soak
Try a 3/4" even. Plenty of clearance under the hood.
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