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Bad carb cause vacuum leak symptoms?

Discussion in 'The Garage' started by norville, Nov 1, 2006.

  1. norville

    norville 1/2 ton status

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    Folks,

    Can a bad carb give you the symptoms of a vac leak? Choke a warm engine and Idle increases and engine smooths out.

    Gummed up injectors from varnished gas cause this??? Internal vac leak the carb is a 1405 edlebrock.

    The gas is good now, but it runs like crap.

    I tried the carb cleaner tricks but no luck finding the leak.

    thanks

    bob
     
  2. diesel4me

    diesel4me 1 ton status Premium Member

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    some can...

    I've never had an Edelbrock Performer have an internal vacuum leak--but have seen some Q-jets and other carbs that did..also,the intake manifold can crack under the carb,and let exhaust gasses that normally would heat up the fuel mixture pass into the intake,creating an internal vacuum leak and dilution of the fuel mixture..leaky intake gaskets can let air sneak in thru the lifter galley...

    Sometimes using the wrong base gasket on the carb,or the "bowl" gasket can block off or leave open passages that shoudn't be!..therefore,it is crucial to MATCH UP the gaskets with the ones you get in the rebuild kit when rebuilding a carb,especially Rochester carbs..once someone puts the "wrong" one on,the only way to tell is to try BOTH,and its a tedious process tearing the carb apart TWICE!...:doah:

    Try checking ALL the vacuum hoses real good,if the carb base gasket does not seem to be your "leak" source--Power brake booster,all vacuum operated accesories like A/C contols,PCV valve,and also make sure the EGR valve is closed at idle (if you have one)..maybe a hose is cracked ,etc..
     
  3. norville

    norville 1/2 ton status

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    I checked/replaced all the hoses and change the base gasket.

    The carb has a slight crackling noise coming from it and the a/f screws seem to have little effect. As a matter of fact I was able to turn them all the way in and it kept running. both in . When I shut it down it kept running for a few and then chuffed off.?>?>?>

    is it the carb? I hate throwing money at problem.

    bob
     
  4. diesel4me

    diesel4me 1 ton status Premium Member

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    .........

    I had a 1405 on my 79 C10...it made a bit of crackling noise with no air cleaner on it--some noise like that is normal,its the sound the air fuel mix makes as its sucked past the throttle butterflies..if its real loud,it could be an intake valve sticking open slightly,or a bad exhaust lobe on the cam ,that causes exhaust to back up in the cylinder and tries to escape thru the carb next time the intake valve opens on that cylinder..

    It is possible to have a crack in the carb's casting that would leak vacuum,but I'd think it would leak gas too,either externally or internally..since the idle mixture screws dont seem to change anything,it is possible the carb might be faulty--maladjusted float level can cause the mixture screws to do little or nothing too however..clogged up air bleed holes,or stuck metering rods/valves can cause symptoms like this too..

    Best "cheep" diagnosis would be to swap on a borrowed carb known to run ok from some other engine,if your not feeling confident about tearing the carb apart to look for the cause...Edelbrocks are pretty simple though,and they have a good website with lots of tips and pictures..only way to learn is by doing..:crazy:
     
  5. norville

    norville 1/2 ton status

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    I have no friends.

    Thanks,

    This was a running truck.

    What does the fuel flow at idle look like?? At the venturies??

    My carb at idle the little brass jiggers that stick into the bowl. Kinda just dribble fuel out. If I give it some gas it comes out different brass tubes. Could my idle circuit be clogged?

    Not loud enough to be in the engine. Also the plugs were sooty.

    I'm not to afraid to tear things aparts it putting them back together that a pain.

    thanks for the help.

    bob
     
  6. diesel4me

    diesel4me 1 ton status Premium Member

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    go for it...

    It's a good bet some crud or water got in the carb, and plugged up some passages..

    I'd look over the exploded diagrams on Edelbrocks web site,and take the carb off,remove the top,and check float level,make sure the floats are not leaky and full of fuel (causes them to sink and flood the motor)..then blow compressed air thru all the jets and idle miture screw holes..you may need a gasket for the top bowl cover,I think they can be had separately,if you don't want to buy a complete "jiffy kit" to rebuild it..speed shops carry them..

    Your carb is running rich if the plugs are sooted up--could be choke troubles ,(not opening quickly enough,or at all!)-or a leaky float,wrong float level..the spark plugs may be misfiring from being fouled,and carboned up,you may have to replace them to get it to run good,even after repairing the carb..good luck!..
     
  7. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    Don't forget that if its on a Q-jet intake , the leak could be at the base if you don't have the adapter and TWO gaskets ( both sides ) .
     
  8. diesel4me

    diesel4me 1 ton status Premium Member

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    yeah!..

    Also if the adapter has the wrong side facing the carb!--I've seen lots of them installed "upside down"!...:crazy:
     
  9. norville

    norville 1/2 ton status

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    Thought od that also.. but it's a square.square. And I soaked everything down ith carb cleaner and even tried propane can't find a leak.

    bob
     

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