Opinions on this engine miss and vacuum guage reading

Discussion in 'Street/strip 400/430/455' started by 69 stepchild, Apr 28, 2022.

  1. 69 stepchild

    69 stepchild Well-Known Member

    Hey fellow Buick guys, I could use your opinions on this slight miss I'm dealing with on my 455. Last year I posted about my oil pressure issue (lack of) which ended up with a complete lower end tear down and bearing replacement / crank polishing. The engine now has great oil pressure (60 at start up) but I'm chasing this slight miss and the engine has never really performed as well as I'd expect. Engine specs: TA112 cam, stock heads (which were overhauled professionally) , standard bore, timing at 10 degrees and new 800 qjet bought from reputable Buick guy/vendor. It idles very good but a small noticeable miss is there as throttle is increased. Here are the things I have checked and what I've found:

    Vacuum Reading: 18# with engine warm that decreases to 17# intermittantly. Sticky or burnt Valve?

    Checked carb and noticed that gasket between air horn and base of carb is seeping fuel/wet

    Plugged PCV valve and corresponding port at carb (idle quality suffered)

    Checked function of vacuum advance (works and holds vacuum)

    Plugs all looked good great.

    Any thoughts on what direction I look next?
     
  2. 73Stage2

    73Stage2 Well-Known Member

    Points or hei? Forget about base timing and set all in 32 degrees at 2500. Look for issues in distributor, wires, and read your plugs.
     
    69 stepchild likes this.
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    X2. One of my pet peeves is when someone posts their initial timing. It really means next to nothing. It's one of the reasons I wrote the power timing thread.

    https://www.v8buick.com/index.php?threads/power-timing-your-buick-v8.63475/

    Mechanical and vacuum advance add timing anytime the engine is off idle.

    The carburetor gasket is wet with fuel? That tells me the carburetor is flooding. That will contaminate your engine oil and lead to other problems. Pull your dipstick and see if the oil smells of gasoline.

    Those are two things you need to resolve.
     
  4. 69 stepchild

    69 stepchild Well-Known Member

    It’s a GM Hei. I did an overhaul on it when I did the engine install. I’ll pull the plugs again tonight.
     
  5. 69 stepchild

    69 stepchild Well-Known Member

    Yes upper carb gasket is wet to touch with fuel!
     
  6. CJay

    CJay Supercar owner Staff Member

    That just means it's leaking. Not necessarily flooding
     
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Fuel level on a Quadrajet is below the air horn gasket. OP says 800 QJ from reputable vendor? If it is a Holley, yes, it could just be the gasket. If the base gasket is wet and seeping, it's flooding.
     
    Last edited: Apr 28, 2022
  8. CJay

    CJay Supercar owner Staff Member

    Fuel tends to slosh around inside a carb. Gaskets keep the fuel inside. In the normal course of operation, the gasket is wet on the inside and dry on the outside. Because a gasket is wet and leaking COULD just mean the gasket had failed. And it's the air horn gasket from what he's saying

    Would you agree to that point?
     
  9. Schurkey

    Schurkey Silver Level contributor

    Air horn gaskets on Q-jets often seep/leak, due to the tendency of Fookin' Apes to crank the hold-down bolts, which over time leads to warpage of the carb.

    I've heard of folks using double gaskets there. I've never done that. And there are expensive, heavy steel plates available which can supposedly un-warp the air horn and main body. I've never done that, either.
     
  10. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    If that were true, everybody would have wet gaskets, especially the way we drive. I don't think the air horn gasket should ever be wet and seeping fuel. The QJ has a single small fuel bowl, and fuel level is normally tightly controlled. It shouldn't leak IMO. Maybe Ken Gies can weigh in on this. Now a Holley is entirely different. Fuel slosh in those fuel bowls can be a problem with fuel moving away from the jets, hence the need for jet extenders. It still shouldn't leak though. High float levels would flood the engine from fuel slosh in a Holley.
     
  11. CJay

    CJay Supercar owner Staff Member

    The air horn gasket should not be wet. That is a true statement. Again....the point I'm trying to make is that saying the carb is flooding because the air horn gasket is wet and seeping fuel is not a necessarily a true statement. Could be...could also be it's just leaking which is a very common issue. And yes, double gasketing is sometimes necessary due to warpage that shurkey alluded to
     
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Which is more likely though? Especially considering it is a relatively newly rebuilt carburetor.
     
  13. CJay

    CJay Supercar owner Staff Member

    I don't assume anything. Either one is just as likely
     
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Hopefully, the OP can do a little more investigating. It isn't running well, and there are probably multiple causes.

    To address the vacuum reading, a sticking/burned valve would exhibit a regular 4-5" dip in the gauge, and return to normal.
     
    69 stepchild likes this.
  15. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Glad to see you are "feeling your oats" again.:)
     
  16. 69 stepchild

    69 stepchild Well-Known Member

    Have a short vid of the vacuum gauge reading. Will try to post later. It is a regular flutter from 18 to 17 like a cylinder is dropping out.
     
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    If a cylinder were intermittently dropping out, due to a valve, it would not be a flutter, it would drop at least 4" and quickly return to normal.
     
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  18. Mark Demko

    Mark Demko Well-Known Member

    What your seeing on the vacuum gauge, (slight needle drop) is normal.
    Your gauge may just be over sensitive.
    Now if the needle was swinging back and forth, then yes there is a valve issue.
    Sounds like your good to go vacuum wise.
    Fix those other two issues and burn some rubber:D
     
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  19. CanadaCat

    CanadaCat Well-Known Member

    The dip of 1-2” is probably when the 5-7 cylinders fire. They fire sequentially, are next to each other and sometimes rob air from each other. It happens on a sensitive gauge or if there’s a bit extra overlap in a performance cam, or a combo of both.
     
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  20. 69 stepchild

    69 stepchild Well-Known Member

    96F886D5-A190-4ADF-A7A9-0AB73A7A3C0A.jpeg Plugs 8,6,4,2 in order. Plug 8 was a victim of my brute strength and lack of coordination
     
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