Timing related thought

Discussion in 'General Maverick/Comet' started by satchelmcqueen, Jun 7, 2019.

  1. satchelmcqueen

    satchelmcqueen Member

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    Since ive been learning more and more about how to set the timing and carb and vacuum advance i have a qestion that i cant seem to make sense in my head.

    Ok so lets say for example i set base timing at 12 adv and that gives me 35 total at 3000rpm. At idle that gives me 800rpm in park.

    Now when i hook up and adjust vacuum canister to lets say 12advanced it now shows 24 adv at idle. Fine..... Now i back off the idle screw and redo air mixture if needed and keep doing this until idle is back at 800 like i want. Im fine with that to as this is what i was told to shoot for.

    What i cant understand is....doesnt that mess with the initial and or total timing that was set BEFORE hooking up the vacuum pot?

    In other words whats the difference in using the idle/mixture screws to bring idle rpm back down to 800 vs just simply moving the distributor instead, like was done at the very first step?

    Is the distributor timing totally separate from the carb/vac pot even if rpm changes?

    I hope this makes sense.
     
  2. groberts101

    groberts101 Member

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    Sorry I don't have time to really expand on where you're thought process is going wrong here, saw your other thread and didn't have time for advice tgere either, but I will simply say that you need to separate the various curves so that they overlap correctly. It's exactly the same for the carb too.. treat and tune each circuit independently but with emphasis on all of them overlapping and adding up to proper ratios during various throttle positions.

    Think of vac advance as a no-load/light load advance assist that only exists to aid lean mixture scenarios such as idle and light steady-state cruise. The less dense a mixture is, and especially during slower rpm/piston speeds/mixture movement, the more spark advance is required to light off that less dense/sparsely populated fuel mixture.. ie less crowded fuel molecule spacing in the chamber takes longer to burn.. so more spark lead required.

    And in my experience, you missed out on some extra performance and tire spinning throttle response by ignoring stumanchu's distributor tuning recommendations in your other timing thread. More work yes.. but worth the extra time. I may have even helped him dial that curve in?.. but if my memory is off he did a very good job of studying combustion dynamics to come up with that calibration. Either way I'm positive his tires hate him for it. lol

    If I can squeeze the computer time in I'll post a few links to help you understand the combustion dynamics revolving around ignition timing. Good luck with it all.
     
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  3. Crazy Larry

    Crazy Larry Member

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    Your vacuum advance should not be engaging at idle unless it is connected to a full manifold vacuum source. Not sure why yours is.
     
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  4. satchelmcqueen

    satchelmcqueen Member

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    Its the idle screw and tha ks to your advice last week i got it all dialed in. But Ive been messing aro7nd with manifold and ported. I went with what you said a few threads ago though. Just messing with manifold now and experimenting seeing what effects different things. Trying to understand it more.
     
    Last edited: Jun 7, 2019
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  5. satchelmcqueen

    satchelmcqueen Member

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    Ty for the reply. Im not ignoring stumanchu's advice though. Im just thinking outloud and asking questions. I just noticed how the idle effects the timing that was previously set and am trying to understand how it all relates and effects the other parts of the tune.

    And theres some really bad articles ive read that is way off compared to the advice here.

    Throttle cable came in today so when i get back from vacation next week ill have this on the road finally. Cant wait!
     
  6. Krazy Comet

    Krazy Comet Tom Supporting Member

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    If advance is connected to ported vacuum on carb, idle speed will not be affected. This was Ford's factory setup & method I still use on the 428. Some like to use full manifold vac which can improve off idle throttle response, especially on a light car with plenty of rear gear/converter stall. Both of my carbed cars have 3:00 cruising gears, manifold vac drops as quickly as I crack the throttle.

    End result, at part throttle cruse, vacuum advance will be approx same using either connection.
     
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