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MustangRanch

351 W Carb 'throb' at Idle

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Here's a video of the issue:

So I'm a Carb Noob.  I can adjust mixture screws, idle speed and rebuild them... But that's about as far as it goes.  I rebuilt this carb when I restored this daily driver 3 years ago.  If this were my snowblower I'd just rebuild the carb again.  Is that my only option?  

Is there something I can do besides just pull the carb and rebuild it again.  Adjusting mixture screws doesn't have any affect.  Increasing idle speed helps, but the idle speed ends up being way to high for regular operation.

Runs smooth as soon as I add some throttle, but back to idle and it throbs again.

Thanks for any help from you carb gurus.

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Check basic stuff like vacuum leaks, etc.  If all is okay rebuild the carb again.  With todays fuel rebuilding a carb every three years is not uncommon.  This is especially true for cars that sit for long periods of time.  As the fuel in the carb evaporates it tends to leave film build up.  Eventually small passages like the idle fuel restrictors plug.  Could have also picked up small debree that plugged an idle fuel restrictor.

Have you tried covering the carb with your hand when it is idling like that?  If so, and the idle quality improves it is definitely too lean from either a vacuum leak or plugged idle fuel restrictor or idle passage.

You can try removing the idle mixture screws and spraying carb cleaner through the passage.  Also, if you know where the air bleeds are spray carb cleaner through those as well.  If it is a Holley carb, the air bleeds at the top looking straight down, 2 in each bore, very small with brass inserts.

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You might want to pick up a new float as well.  Floats are typically replaced during a rebuild as a precaution.  If the carb has any vacuum pots attached to it for the choke operation, check those for correct operation as well.

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On 8/11/2017 at 3:44 PM, 1969_Mach1 said:

Check basic stuff like vacuum leaks, etc.  If all is okay rebuild the carb again.  With todays fuel rebuilding a carb every three years is not uncommon.  This is especially true for cars that sit for long periods of time.  As the fuel in the carb evaporates it tends to leave film build up.  Eventually small passages like the idle fuel restrictors plug.  Could have also picked up small debree that plugged an idle fuel restrictor.

Have you tried covering the carb with your hand when it is idling like that?  If so, and the idle quality improves it is definitely too lean from either a vacuum leak or plugged idle fuel restrictor or idle passage.

You can try removing the idle mixture screws and spraying carb cleaner through the passage.  Also, if you know where the air bleeds are spray carb cleaner through those as well.  If it is a Holley carb, the air bleeds at the top looking straight down, 2 in each bore, very small with brass inserts.

A quick flush with carb cleaner inside and out will cure most problems and point you in the right direction for probable repair needed.

plugged air bleeder will cause the problem in your video.

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what happens when you turn the idle mix screws?

1. your idle is too low, turn it up.

2. check the float level.

3. remove and plug the distributor vacuum advance line then check the timing then advance it 4 degrees and tell us what it does.

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