Engine stumbles with vacuum advance

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

  1. AJ_72

    AJ_72 Veteran Member

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    Hey everyone. It's been a while since I've posted anything because, well, life. :)

    Anyway, now that she's gone and I've got myself stable again I figured it was time to pull the beast out of the garage after ~5yrs of sitting, and get her going again.

    Engine details.
    350 short block out of a 91 Z28
    Vortec heads
    Magnum 280* (230* @ 050", .480" lift, 110* LSA)
    Holley 650 dp
    Weiand dual plane intake.
    Flowtech ceramic coated 1-5/8" headers
    Thrush two chamber welded mufflers
    Was HEI with Accel top mount coil with Accel module.
    Now HEI with factory replacement coil with factory replacement module.

    Long story short, I'm at my wits end with this engine bog. I've tried running the engine with manifold and ported vacuum and either one causes the engine to bog at anything over 2,000 rpm. It doesn't matter if the car is sitting or driving. Same symptoms.

    I'm thinking the damper may not be aligned with the timing tab. The damper is new, as is the timing tab. I don't know what damper it is. A friend of mine donated it to the cause years ago. Using a dial-back timing light I've got the initial set at 12*. I haven't even bothered checking the timing curve because I haven't really even got the car driving "normal" yet. The reason I suspect it may not be aligned is because if I try manifold vacuum the car won't even start. The engine kicks back in distress.

    I've rebuilt the carb, set the idle mixture max vacuum +1/4 turn out, at idle with the vacuum advance disconnected. The car idles 700 rpm. With the vacuum advance disconnected the engine runs great. It pulls strong up to at least 4,500 rpm (I haven't "got on it" yet. I'm still not comfortable yet), but as soon as I drive it with the vacuum advance hooked up, it bogs bad at and over 2,000 rpm.

    I even changed out the vacuum canister thinking maybe the diaphragm on the old one developed a leak. Nope. Same problem.

    Can too much timing cause this same problem even though I'm not hearing any audible engine knock?

    Thanks in advance :)
     
  2. COPO

    COPO Veteran Member Lifetime Gold Member

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    Check your timing with the vacuum connected. If too high then you will need to limit the rod.
     
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  3. AJ_72

    AJ_72 Veteran Member

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    I checked it on ported and no change. I can't check it on manifold vacuum because the car won't start if I put it there first, and dies when I try to switch it from the transmission to the vacuum canister.
     
  4. AJ_72

    AJ_72 Veteran Member

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    Update:
    I tried retarding the timing to the extreme and went back to 0* on initial just as a starting point. I had to increase the RPM but even after that the engine chugged and idled bad. I set the idle mixture back up to ~13in/Hg but then the engine backfired through the carb when I tapped the throttle by hand.

    I went ahead and brought the timing back up to 8* BTDC and reset the idle mixture again. No change. The engine hates the vacuum advance. Without the vacuum advance hooked up I can sit in the car and rev the engine to 5000 RPM all day. As soon as I hook the vacuum advance back up and try to bring the RPM up past 2000 the engine will bog down and I can literally watch the RPM drop back down to 1500 while the engine is doing its best not to die.

    I'm going to pull the carb apart tomorrow and make sure no passages got blocked by debris and that I didn't overlook anything. BTW, this is the reason I thought the carb needed to be rebuilt in the first place. The engine was acting like this before the rebuild.
     
  5. mallard

    mallard Veteran Member

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    Very strange problem and I can't help but thinking you've already eliminated the carb as an issue. Any chance whatsoever that the cam isn't indexed properly?
     
  6. AJ_72

    AJ_72 Veteran Member

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    I don't know. It pulled strong through every gear after the engine swap before it got parked 5 years ago.

    I'm going to try draining the tank and probably swap out the fuel line next. It had less than 1/4 tank when it got put away and I've really only added about 5 gallons of fresh fuel since then. Maybe the gas lost its ability to burn.
     
  7. jeff swisher

    jeff swisher Veteran Member

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    You found your issue it is not the carb if what you wrote is true.
    Your timing curve is possibly exactly what the combo wants.
    I ran 906 vortecs with a roller cam with 228 @ .050 on a 110 lsa with .495" lift.
    Pretty close to the 280 magnum cam.
    Strong runner for sure and I made max HP at 35 total and down 9 HP at 32 total timing.
    But I did port my Vortec heads.

    Now I have NOT ran any Vacuum advance for decades.
    My initial timing is usually 18 minimum though and total all in by 2200rpm.

    Give it what it likes.. If it does not like vacuum advance then quit giving it to the engine.
    The idea is to make it run excellent.
    Some say they get better MPG with vacuum advance.
    That may be fact for them but it is not for everyone.

    Show me a 6000 lb E150 with 3.55 gears pulling 18 MPG with carbed 302 No OD.

    I built myself one and was shooting for 20+ MPG. Pulling a 15 foot bayliner it got 17.6 MPG. Low Compression shorted me and all the vacuum advance deals I tried killed my MPG and killed drivability.
    So No vacuum advance it was.

    Give it what it likes.
     
  8. AJ_72

    AJ_72 Veteran Member

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    I was thinking that, too. It's just odd how it used to like it but something changed. But as they say, time changes everything.
    Yes. That was a pun about timing as well as my divorce. :D
     
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  9. jeff swisher

    jeff swisher Veteran Member

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    If you are up for it you could pull all spark plugs.
    Then get where you can turn the engine by hand and have a buddy there.
    Place a zip tie or long drinking straw into cylinder #1 spark plug hole.

    Rotate engine by hand until that straw is at it's highest point. The piston will push it up.
    Make a mark on the balancer.
    What I do to get it right on the money is rotate until the straw reaches full height and then keep rotating until the straw begins to go back down.
    Then make a mark there and rotate the other direction and mark highest point and then another mark where the straw goes down again.

    You should have 3 marks and in the middle should be the TDC mark.

    Now you know for fact what TDC is.
     
  10. 80sz

    80sz Veteran Member

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    Idk , your issue is odd. I’m with Jeff, leave it off. Set initial to 18, rev up motor and watch light. What max out at?
     

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