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timing and vacum on 8ba

Discussion in 'The Hokey Ass Message Board' started by 325window, Feb 1, 2009.

  1. 325window
    Joined: Apr 12, 2008
    Posts: 3

    from new york

    need some help with my 8ba i have a 318 dis in it and was wondering if the vacum advance should be at idle or under part throttle. i have two carb bases one with vacum port above the plate and one below i think the one with it below the plate is advance at idle the other is the opposite? thanks matt

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  2. tommy
    Joined: Mar 3, 2001
    Posts: 14,757

    Member Emeritus

    You might want to check this out. Normally vacuum advance distributors use manifold vacuum (high vacuum at idle) but the flatty is a different animal with different values.

    Beautiful car!
    Joined: Jun 15, 2006
    Posts: 3,385


    My vote is for idle. The vac advance should complement the mechanical advance. When rpm is low vac is high and you have vac advance when rpm goes up vac is low and you have mechanical .
    The 318 is a VERY adjustable unit the vac diaphrams are adjustable using a allen wrench thru the vac tube and the mechanical advance is adjustable uing the ecentric pins the springs swing on. Looking into the distributor you can see the large (high end) and small (low end) springs. Depending on the model year the springs may be adjusted by bending tabs or tuning the eccentric pins. Can send a pic of the tool i made to do this if needed...
    Lets just pick a cruise number for the sake of explanation,,,
    Lets say the engine really likes 20 degrees of advance after starting. At idle the vac advance is 14 degrees intial timing is 4 and mechanical advance is 0. 14 vac+ 4 balancer+0= 20 degrees.
    Now lets go to 2000 rpm intial is still 4 , vac is 0 and mechanical is 14.
    4+0+14 = 20 degrees.
    Vac is a overlap of mechanical.
    A properly tuned vac advance could add 3-4 mpg to a gasoline engine. The nature of the flathead with dual carbs make the vac unit very hard to tune.
  4. jetmek
    Joined: Jan 12, 2006
    Posts: 1,846


    hey matt...welcome aboard. those 94's utilize an average vacuum reading from 2 points in the carb. 1 at the venturi and other below the throttle plate the stock dist is calibrated to work properly from that source. ford books say to never draw dist vac direct from the manifold. with your setup all bets are off. id suggest manifold vac with that dist. if you have to have it. be careful tho as too much total timing can cause overheatng. i dont use more than 24* on my stuff ..(mech advance only)

  5. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,682

    Bruce Lancaster
    Member Emeritus

    There isn't anything much that would be easier or cheaper to experiment with! Assuming that your "above" tap is the normal Ford Loadamatic port, go in there and securely plug the little port that goes straight in to the venturi; leave the port going down from the port (and be sure you have a late base with hole to match it) and you have ported vac. Try it both ways and tell will likely want to block some of the travel on the vac advance either way.
    When you experimenbt with manifold vac, you will likely have to turn down the idle speed back to normal and readjust...again, easy and cheap.
  6. flatjack
    Joined: Feb 13, 2007
    Posts: 941


    14 + 4= 20?
  7. 325window
    Joined: Apr 12, 2008
    Posts: 3

    from new york

    ok from the start the motor is a 51 8ba merc crank 276 big valves exhaust has ben opend up intakes also. the carbs have 3.5 pv and 50 jets when first started and on road it baging pretty hard i think it was detination or ping dont know how a flat motor sounds in eather condition so i disconected the vacum from the advance the noise went away so i ran it that way for the season ran pretty good no over heating good power and so on . 7mpg looking for a little more so i want to try vac adv .dose the mec adv take over for the vac when the plates are partly open or do they work in conjuction to give you total adv i have a base plate with the venturi port pluged and the vac port below the plates so i have manafold vac i think.the distubtors mac adv is 18 degrees dont know what the vac adv is. thats whear im at right now thanks matt

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