Engine stumbles with vacuum advance

Discussion in 'Troubleshooting & Diagnosis' started by AJ_72, Aug 10, 2019.

  1. Bandit723

    Bandit723 Veteran Member

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    I'd like to know what you are all in with out the vac advance....
     
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  2. G72Zed

    G72Zed Veteran Member

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    Reading thru the posts, and assuming your timing marker/tab is accurate and the carb has been thoroughly cleaned (metering block E pak) out with new power valve(s), sitting 5 years you may have rust/dirt accumulated on the pickup coil,or broken frayed/corroded wire? have you tried a different cap/coil/module & rotor-button, perhaps they are faulty even if new.

    Also, could the pickup coil be out of phase due to the new, or even old vacuum canister rod location? could be enough that when you engage the vacuum (ported or manifold) once it's seeing vacuum advance, it's over advancing and it's arcing to the next pole, seen this a few times with different parts manufacturers and tolerance stacking.

    Vacuum advance that's been properly phased and set with a limiter to allow approx. 8-10* advance right at idle in performance applications has many benefits and very few negatives if any, again, exceptions to every rule, but not what you are experiencing.

    Clean idle, better throttle response, cooler exhaust headers and better "cruise" gas mileage (with the correct AFR) is my experience with vacuum advance.

    As a comparative, my SBC run 24 initial, with manifold vacuum advance for an extra 8*, so 32 at idle, and 48* at cruise speeds. Gained 2+ miles per gallon with vacuum advance (17 to 19.5 mpg.)

    Hope you find the issue AJ_72
     
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  3. cadillac_al

    cadillac_al Veteran Member

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    I was also wondering if the pick up wires in the dist are damaged.
     
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  4. K1ng0011

    K1ng0011 Veteran Member

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    I agree with the others verify top dead center and the mark on your harmonic balancer is correct. Also hook the vacuum advance up to manifold vacuum at idle. That way you are getting advance from the vacuum canister at idle. Then check it with a timing light and see how much vacuum advance the canister is providing. Need to figure out how much advance the vacuum canister is adding. Might need a timing tape if your harmonic balancer is not marked. As you increase your RPM your vacuum in your engine decreases. Your vacuum advance will do less and less until you reach wide open throttle where there is no vacuum in your engine and your vacuum advance will do nothing. The only times your vacuum advance will do anything is at idle if hooked to manifold vacuum and at part throttle down the street. Some vacuum advance canisters are adjustable as seen in the picture below. You might be able to adjust the canister to add less vacuum advance if it is adding too much. If you have a buddy that has an extra distributor maybe you could swap it in to see if it is something within the distributor itself that is causing the issue.

    Finding TDC: http://nastyz28.com/threads/timing.323727/
    Setting Timing: http://nastyz28.com/threads/idle-when-timing-70z28-lt1.325144/page-2#post-3712102


    [​IMG]
     
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  5. 1owner72

    1owner72 Veteran Member

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  6. 1owner72

    1owner72 Veteran Member

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    Get rid of all the old gas, put some cleaners through the carb.
     
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  7. 2ndGenCrazy

    2ndGenCrazy Veteran Member

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    EXCELLENT post and information
     
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  8. COPO

    COPO Veteran Member Lifetime Gold Member

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  9. AJ_72

    AJ_72 Veteran Member

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    Just an update;
    I have not done much else since my last post. One of the other issues is, the rings on this engine are shot. There's oil coming out of the breathers like there's an oil can leaking straight out of each one. I'm just going to buy a rebuilt short block next spring and go from there.

    I'm sorry to have wasted anyone's time. All of your suggestions were great and appreciated.
     
  10. jeff swisher

    jeff swisher Veteran Member

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    Hold on a minute.
    You may be correct but there are other things that will make oil come out of the breathers.

    Clogged drain back holes. Seen it.
    No baffles in the valve cover will allow a rocker arm/pushrod to squirt oil right out of the hole.

    Take an oil cap off a new car while it is running say an overhead cam one.
    You will get coated in oil if it is not baffled.

    Some aftermarket valve covers put the hole right over a rocker. Junk.

    But you may be correct in assuming the rings are shot.
    Usually if the oil ring is shot it will smoke out of the tail pipes.
     
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