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Carb cutting in and out

Discussion in 'The Garage' started by Citizen Rider, Aug 30, 2006.

  1. Citizen Rider

    Citizen Rider 1/2 ton status

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    I just got done trouble shooting my engine as i had a burned through wire that was misfiring on my header. Engine runs much better now. Timing is unknown where it is set exactly, but is set close and runs decent.

    Carb is a quadrajet and about 2 months old and ran for less than a month.

    The truck has trouble starting and idling when cold, it dies if i dont hold the gas slightly while going from Park to Rev or drive. The carb does need adjusting more but works fine sometimes and idles at start fine, or other times it just wont stay running.

    Driving the blazer it runs fine and has no problems. But once it gets warm and the gas pedal has been depressed for quite a while, the engine cuts in and out. It goes from rumble, no noise, rumble, no noise, rumble. Literally nothing like it died and kicks back in. If i dont let off the pedal it bogs down and looses speed as it tries to stop running. But if i let off the gas and let it cruise along, No cutting in and out while im off the pedal and if i wait about 5 sec. i can give it gas and not have any problems eigther for the rest of the night or It will come back about 2 min. later.

    Im totally lost on what it could be, i know nothing about carbs, and the last engine guy i told this to said nothing to me as to what it could be or what to fix it.

    Any clues? Im hoping i can take it to the shop that is working on my 95 silverado and maybe they will know.
     
  2. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    Sounds like the choke is not adjusted properly. Possibly at least the fast idle.



    This almost sounds like it could be an ignition problem.

    I don't know how it is with engine ignitions, but with many electrical systems if you let something short out too long it can cause other components to fail. It's possible the shorting ignition wire messed up your coil or ignition module.
     
  3. sledheadak

    sledheadak 1/2 ton status GMOTM Winner

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    did you replace the fuel filters?
     
  4. Citizen Rider

    Citizen Rider 1/2 ton status

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    New carb included new filter at the fuel line mount, and i put a filter inline. All clean.

    The cap and rotor are 2 months old.

    And the choke is a vaccum controled unit, and would close while starting or a small jar would close it and kill the engine. A guy i know used some tiny wire to hold it open(never gets below freezing here) but im wondering if its even holding anymore.
     
  5. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    The choke isn't just for cold days. Your engine is cold relative to its normal operating temperature. Even if the the block was 90° from ambient air temp, it would still need to heat up another 90°+ for normal operating temp.

    For example, this morning the choke was active on my '00 Tundra. Last night I drove the K5 around a while and the choke activated on it for a little bit when I first lit it off.

    I'm not saying it won't run, it just won't run well. Just like you described.

    I mentioned the coil and ignition module, not the cap and rotor.
     
  6. Citizen Rider

    Citizen Rider 1/2 ton status

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    I agree with the choke concept, but the choke closes ramdomly by itself and the vaccum being pulled through holds it down. Also once the blazer is started, the choke doesnt open up once it warms up. I dunno, Carbs and chokes are a hole new concept for me.

    What are the ways i can test the coil and ignition module?
     
  7. 1985_K5_Silverado

    1985_K5_Silverado 1/2 ton status

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  8. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    I have never heard of a vacuum choke on a Quadrajet before . That hose isn't going to vacuum , it goes to the hot air chingas coming off the intake ( where the divorced choke coil is on older models ) . There are tubes that go into that port in the head ( the port you block off when doing intake gaskets and have no hot air choke ) , and the tubes run to the carb and attach with a little vacuum hose .

    So basically , no hot air , no choke :D

    I have seen quadrajets come divorced originally , hot air second , electric third .
     
  9. diesel4me

    diesel4me 1 ton status Premium Member

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    might not be the carb...

    I agree with those who are suggesting an ignition problem,or a fuel delivery problem..a defective fuel pump can cause this,so can pinholes in the metal fuel lines coming from the tank,or the rubber hose that joins them to the tank near the right rear wheel is collapsing internally..then,I'd do a pressure and volume test on the fuel pump (or simply replace it,if its old and "iffy")..

    It could very well be a pick up coil crapping out intermittently too,or the coil or module..or a wire to the ignition switch grounding out or losing contact--possibly the switch itself also..time to start trouble-shooting!..the worst thing is it never screws up while your trying to diagnose it,right??..:doah:
     

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