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Set timing by vacuum gauge?

Discussion in 'The Hokey Ass Message Board' started by Orn, Jun 13, 2009.

  1. Orn
    Joined: Jul 17, 2005
    Posts: 1,106

    Orn
    Member

    Did anyone use this method? I’ll try this on my 248 straight 8 with good results. According to the manual it should be 4-degree btc and with this number the vacuum reading was a steady 14 hg. I advance the timing and reach the top limit at 22 hg and then back down to a steady 20 hg. The engine runs smoother then ever before and absolutely no sign of detonation when test driving uphill and no hard start. The compression on these engines is like 6:1 and with today’s fuel (95 octane) compared to the 70-75 octanes on the late forties it seams logical to use a little higher advance or am I asking for trouble here? What you think?
     
  2. langy
    Joined: Apr 27, 2006
    Posts: 5,730

    langy
    Member Emeritus

    Yep used that method for years.
     
  3. riceman
    Joined: Oct 8, 2006
    Posts: 743

    riceman
    Member

    It sounds like your motor likes it at 20Hg/in. The condition of the motor, if it was rebuilt, has better made parts, better ring sealing and machine tolerances better than originally mfg. Too many variables like higher rpm at idle, cam type and timing, compression ratio, oil type (SYN) and iginition parts. Drive like you stole it.
     
  4. rodsnhawgs
    Joined: Oct 4, 2006
    Posts: 214

    rodsnhawgs
    Member
    from WV

    been using it for years especially since the decline in gasoline quality. Way back when (early as mid 70s) used it as confirmation in conjunction w/ timing light...still do.

    BTW, also handy in adjusting carbs.
     
    Last edited: Jun 13, 2009

  5. BLAKE
    Joined: Aug 10, 2002
    Posts: 2,783

    BLAKE
    Member

    Can you shed some more light on the details of 'the procedure'? Do you find highest vacuum reading, and then back off by two or to a steady reading? At idle? Something else?
     
  6. Orn
    Joined: Jul 17, 2005
    Posts: 1,106

    Orn
    Member

    Yes, you’re right. I advance the timing for the highest vacuum at normal idle (400 rpm) and then back off 2 hg/inch. The idle will pick up, so you'll have to back it down so the centrifugal weights in the dist don’t interfere. Test drive your car and listen for any ping under light acceleration as well as under heavy acceleration and back off if needed.
     
  7. REJ
    Joined: Mar 4, 2004
    Posts: 1,612

    REJ
    Member
    from FLA

    I use this method on every car that I have. When finished, you can put a timing light on it and see where it is at.
    Nine times out of ten, it will be dead on where it should be.
    Robert
     
  8. montclaire
    Joined: Jul 24, 2007
    Posts: 501

    montclaire
    Member

    I've always done it by ear. Use your right foot for a confirmation.
     
  9. When you set the timing with a vacuum gauge, do you disconnect the vacuum advance like you do with a timing light?
     
  10. plym_46
    Joined: Sep 8, 2005
    Posts: 4,018

    plym_46
    Member
    from central NY

    attach vacuum gauge to the intake manifold after it has reached proper operating temp. loosen the dizzy, with the car at idle, everything attached, trun the dizzy to atain the highest steady vacuum reading. depending on your cam shoud be between 15 to 21 inches of vacuum. Once the highest steady state vacuum is reached, tighten the dizzy, the re adjust idle speed as necessary.

    If you can not attain a steady vacuum reading you got other problems that need attending to. Check the secongchancegarage website for the section on what the vacuum gauge readings mean.

    If you can not conveniently attach the vacuum gauge to the intake, try attaching it to your power brake booster tube if so equipt. The vacuum wiper port is also a handy spot.
     
    Last edited: Aug 21, 2009
  11. carcrazyjohn
    Joined: Apr 16, 2008
    Posts: 4,843

    carcrazyjohn
    Member
    from trevose pa

    Yep That's the way I do them .I only drop 1 though unless i have starter issues.
     
  12. Thanks Plym 46, Good info.
     

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