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Technical Can't get this 396 dialed in...

Discussion in 'The Hokey Ass Message Board' started by alteredpilot, Sep 30, 2021.

  1. Got an L78 (396/375) that I can't seem to get dialed in. From what I know its a spec numbers matching numbers type rebuild with very low miles on it.
    When i got my hands on it, it had like 24* degrees initial timing and a big vacuum leak under the carb. The vacuum advance was hooked up to manifold vacuum.
    Got the vacuum leak fixed, put the right gasket under the carb. verified TDC #1. Set initial timing at 4* BTDC and it just does not like it. I can't keep the throttle plates closed under about 18* initial timing. and even then the motor is producing bupkiss for vacuum at idle.
    I've been banging my head against this thing for a couple of days and its time to step away for a minute and collect my thoughts, and see if you guys can think of anything I'm missing.

    Any Ideas?

    Thanks in advance.
     
    Deuces likes this.
  2. iagsxr
    Joined: Aug 26, 2008
    Posts: 158

    iagsxr
    Member

    Mechanical advance is stuck advanced.

    Vacuum advance diaphragm is blown out creating vacuum leak.

    Brake booster is bad creating vacuum leak.

    Harmonic damper has spun.
     
    Elcohaulic, Deuces and 41 GMC K-18 like this.
  3. jaracer
    Joined: Oct 4, 2008
    Posts: 1,221

    jaracer
    Member

    I'd check to see that the TDC mark is actually at TDC.
     
    Blues4U, Deuces, 67drake and 3 others like this.
  4. What type of distributor is in it?
     
    41 GMC K-18 likes this.

  5. choptop40
    Joined: Dec 23, 2009
    Posts: 4,010

    choptop40
    ALLIANCE MEMBER

    timing chain installed improperly ???
     
    1971BB427, egads and Truck64 like this.
  6. Truck64
    Joined: Oct 18, 2015
    Posts: 5,080

    Truck64
    Member
    from Ioway

    Yeah, see if the timing mark & pointer is telling the truth or not.

    If it is OK, then check valve timing, n.b. this is different thing than ignition timing.
     
  7. Rramjet1
    Joined: Mar 13, 2018
    Posts: 151

    Rramjet1

    Solid lifter motor. Check valve adjustment.
     
  8. SS327
    Joined: Sep 11, 2017
    Posts: 413

    SS327

    I would check the timing chain and gears then the harmonic balancer first. Sounds like cam advanced too much.
     
    egads likes this.
  9. Mr48chev
    Joined: Dec 28, 2007
    Posts: 30,027

    Mr48chev
    ALLIANCE MEMBER

    All good and very possible ideas and suggestions there.

    Simple things first:
    Check the advance weights and the advance diaphragm. You can just pull the hose off the carb end and suck on it to check the advance diaphragm. If you get resistance it should be ok if you suck air though the hose it's bad.
    From there I'd check to make sure that the timing mark is right and if it is right pull the front cover off. You may be battling someone's "performance mod" where they advanced the cam way more than the advised amount.
     
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  10. lippy
    Joined: Sep 27, 2006
    Posts: 5,668

    lippy
    Member
    from Ks

    Yes the 375-396 is solid lifter have a look at valve adjustment, although we don't know for sure what cam is in it now. Lippy
     
    Deuces likes this.
  11. lippy
    Joined: Sep 27, 2006
    Posts: 5,668

    lippy
    Member
    from Ks

    Is this an automatic or a stick? If it's an auto, good luck. Lippy
     
    Deuces likes this.
  12. MeanGene427
    Joined: Dec 15, 2010
    Posts: 1,939

    MeanGene427
    Member
    from Napa

    Bingo- always. Check and baseline everything, make sure everything is functioning correctly
     
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  13. squirrel
    Joined: Sep 23, 2004
    Posts: 49,795

    squirrel
    Member

    did you check total timing? should be around 36-40. Without knowing what distributor is in it, and what changes have been made to the advance curve, your 4 degrees initial is probably part of the problem.
     
  14. Put a vaccum gauge on it and set the timing and will also tell you what else may be wrong
     
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  15. Deuces
    Joined: Nov 3, 2009
    Posts: 21,275

    Deuces
    Member
    from Michigan

    Yep! No more than 2-4° max...
    Maybe I'm wrong on that....????:confused::(
     
  16. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,450

    Elcohaulic
    Member

    That initial seems real low. My initial timing on my 454 is 22. The mechanical gives it 14 starting at 1200 and all in by 2400..

    I would cap the vacuum advance and get it running good without the vacuum advance.. I like having the idle stay put and not fluctuating up and down while trying to keep the engine steady while parking..
     
  17. lippy
    Joined: Sep 27, 2006
    Posts: 5,668

    lippy
    Member
    from Ks

    And I wouldn't expect big vacuum numbers out of that camshaft .:D
     
  18. MeanGene427
    Joined: Dec 15, 2010
    Posts: 1,939

    MeanGene427
    Member
    from Napa

    Yep, 4 is gonna be way too low- going from 24 to 4 is quite a jump
     
    Blues4U likes this.
  19. Blues4U
    Joined: Oct 1, 2015
    Posts: 5,933

    Blues4U
    Member
    from So Cal

    If everything else checks out, set the initial timing for the highest vacuum reading, and/or the highest rpm, but backed off just slightly.
     
  20. Tickety Boo
    Joined: Feb 2, 2015
    Posts: 1,402

    Tickety Boo
    Member
    from Wisconsin

    Most stock Chevy distributers that I played with should have about 22 degrees of machanical movement, so at 4 degrees initial total timing all in would be only about 26 degrees.
    At 18 degrees initial + the 22 machanical movement the total "all in" should be about 40 degrees,
    How does it run and drive at 18 inital ?
     
    Deuces likes this.
  21. This all started because it was pinging.
    I'm trying to track down the engine builder to see what cam is in it, but at low advance we have real low vacuum. also not sure about cam timing. With the throttle plates all the way closed and the idle screws all the way in, its still idling at 850 RPM and running rich.
    I'm pretty sure the power valve is open at idle. and that's what's giving us grief.
     
    Tickety Boo likes this.
  22. Deuces
    Joined: Nov 3, 2009
    Posts: 21,275

    Deuces
    Member
    from Michigan

    How's the float level on the carb???...
     
  23. Doublepumper
    Joined: Jun 26, 2016
    Posts: 1,080

    Doublepumper
    Member

    Fuel pressure...?
     
    Deuces likes this.

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