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Discussion Starter #1
I'm having problems with pressure building up in my carb.after 5 minutes of running the pressure darn near stops any gas from going into the filter,it lets just enough in to keep it running.the pump is putting out lots of gas.1 liter in 20 seconds in a volume test.but if I were to remove the gas line from the filter it would shoot the gas from the carb out. if I didn't keep my finger over it to prevent it.after I release the pressure from it you can see air been forced from the carb into the filter.I went back 30 minutes after releasing the pressure and small air bubbles could still be seen coming from the carb end of the filter.does anyone know what the heck is going on?is this fuel percolation?


Terry
 

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Is there something wrong with the way the car drives?

I am guessing you have a clear plastic or glass fuel filter and wonder why it looks empty. If that is the case it's normal.
 

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Discussion Starter #3
Your right Royce,I have a clear glass filter on it.the car dies on the side of the rode if I keep driving it.I should mention that I'm on my 4th fuel pump as I keep blowing the diaphram in them from the pressure.after 5 minutes you can see the gas drain from the filter,I don't now if it's air or pressure but something is stopping the gas from going into the filter.I even put on a rebuilt motorcraft carb with a screw in filter and same thing.the car starts running rough and labors to keep going.if I remove the fuel line the gas comes shooting out from the carb.could the heat from the engine be boiling the gas in the carb causing this pressure?

Terry
 

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You should have a 3/8" thick plastic heat insulator / spacer between the carb and intake. It keeps fuel from boiling in the carb. Is it there?

Here's where to get one if not:
http://www.mansfieldmustang.com
 

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Just a thought...but if the float's needle valve in the carb wasn't sealing when the carb was full of gas, would the gas coming in just boil out down the carb and flood things, or could it be that the gas isn't being shut off by the float valve, and gas just keeps coming in and building up pressure?

Though that wouldn't build up more pressure than the fuel pump was putting out, I wouldn't think, so I wouldn't expect a pile of blown diaphragms (can I say that in here? :) ) from the fuel pump...

Just a thought (and barely one of THOSE!)
 

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There should not be any aluminum parts on a stock GT390 intake other than the water neck. The carb spacer is 3/8" thick stock. Any aluminum spacer between the carb and intake is not right.
 

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Sounds like you're getting vapor lock. When the filter is empty -- there ain't no fuel getting up to the carb. And that's what you're seeing.

Sounds like the blend of fuel you're running has too low of a vaporization temp. Try insulating the feedline from the pump to the carb.

Is the line hot to the touch -- like way too hot to touch?
 

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Ala Royce's suggestion: Try putting a phenolic spacer between the carb and intake. They don't transfer as much heat as an aluminum spacer and can be had at just about any parts store on the corner or from summit, speedway motors, and Jeg's in a couple of days.
I think the carb is boiling the fuel.
You'll probably need to rebuild the carb after you get the problem fixed too.
 

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Here's the spacer you should have....

It's not my idea, the Ford engineers determined that a plastic spacer was needed on GT 390-4V engines between the carb and the intake to prevent heat related problems.


One other thing, if you are running a Holley mechanical fuel pump that could be the main problem. They are junk.

 

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And to think I threw that piece away when I changed carbs on 68 Kat...NOT!!!

Royce your experience and opinions are almost always right on the mark (OK, you've never been wrong that I can see). With the notorious hot running reputation of the 390 it makes perfect sense.

Why are Holley mech fuel pumps junk?

Ken
 

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Discussion Starter #13
Thanks for the input guys.I had a Holley fuel pump on it but it went south after 8 months,now I'm running a Carter M6905 ala Royce's recommendation.the filter looks clean everything from the gas cap forward has been replaced with new parts.I have routed the gas line away from the block,water pump and p.s pump,I have not felt the line to see if it's hot after it happens,to busy scratching my head wondering what the?I'll try a different spacer to see if that helps it.I've added a picture of the spacer that's on it now.it's suppose to rain for the next 3 days so that seems like a good time to go over the carb.if anyone has any more ideas I'm ready to try just about anything.

Terry
 

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OK, that is the correct spacer for a '69 390. Lets try to think of another scenario. Is the carburetor of a known quality and have you tried borrowing a known good carburetor to try on the engine? Those Motorcraft and Autolite carbs can be tricky to set the float level on and have hot start problems but they don't usually act up in the manner you are talking about.

Visit the FE Forum (http://www.fordfe.com) and search Holley fuel pump for many tales of woe.
 

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Discussion Starter #15
The Holley carb that's on it now was a brand new out of the box unit.the motorcraft one that I put on was a rebuilt unit.I'll see if I can borrow my bro's 600 Holley his car is running fine.I'm also going to try a different spacer and see if that helps any,couldn't hurt.I've emailed Holley to see if they have any suggestions but haven't heard back from them as of yet.Thanks again guys.

Terry
 

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Terry, I pulled a 625 Carter off my 390 and have a 750 on there, you want to try that one. It ran real good. Let me know. I could COD it up on the bus.

Rick
 
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