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Vacuum leak?

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Old 05-02-2012, 06:30 PM
seawulff seawulff is offline
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Vacuum leak?

I haven't been active for awhile and started driving my '56 F100 recently since the weather is better. Whenever I accelerate, but more so when the engine is cold, there is a delay/near stall when I step on the accelerator pedal. If I accelerate slowly it is much less noticeable. Sometimes when idling I hear a faint high pitched whistling sound. Does this indicate anything in particular to anyone? The carburetor has been rebuilt <250 miles ago. The engine is a '57 312 if that makes any difference. I put in an electronic ignition conversion awhile ago and this is a new problem. When I listen under the hood while idling I don't really hear anything different but there's a lot of noise in that compartment! Thanks.
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Old 05-02-2012, 06:41 PM
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carb rebuilt 250 miles ago... did you keep the old leather accelerator pump or put in the new cheepo plastic? I put in the cheepo plastic and have it planned to tear it back apart this weekend after 500 miles of troubleshooting and hating on it (thankfully I saved the old parts).

a whistle like that could very well be a vaccum leak and that would also be a candidate, but what you describe "hesitation when you step on the pedal" is most likely accelerator pump.

If you have the leather, soften it up with mink oil maybe even "toast" it (not really toast, just warm it up to about 100 degrees so the pores open up) to get the oil to soak in.

mechanics stethascope (sp) blocks out the engine noise and you can wave the end around near all the typical parts of the carb. I found a tiny leak in mine that way and resolved it, but still the hesitation.
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Old 05-02-2012, 06:58 PM
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[quote=seawulff;11784253 there's a lot of noise in that compartment! Thanks.[/quote]
Remember, you're hearing a Y block with those mechanical lifters.......Music

Make sure your advance is working. Might move the pump linkage to get more fuel.
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Old 05-03-2012, 10:24 AM
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Quote:
Originally Posted by brain75 View Post
mechanics stethascope (sp) blocks out the engine noise and you can wave the end around near all the typical parts of the carb. I found a tiny leak in mine that way and resolved it, but still the hesitation.
If you don't have a stethoscope use a 2' length of pipe.
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Old 05-03-2012, 11:27 AM
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How long has the 312 been sitting? Even a few months through the winter I have had gaskets shrink up and leak thus causing a vacuum leak.

Try getting her warmed up and gently re-torque the intake and carb base.

And I agree on a bad accelerator pump as well.
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Old 05-03-2012, 11:44 AM
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Back in the day Rochester 4bbl's were notorious for their accelerator pump's drying out, we would add a 1/2 quart of tranny fluid to the gas tank. We found this worked 90% of the time, and told the cust to do this every 3rd or 4th tank. It also is good for getting rid of carbon.
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