On ignition timing
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Thread: On ignition timing

  1. #1
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    On ignition timing

    Hi guys! Long time no see.

    I have a small issue setting my ignition timing, previously i have run it at ported vacuum, and it has been running good.

    Now i have done some more work on it this winter, and just yesterday i read an article about timing and it said that ported vacuum is for emissions controlled engines and i will lose on fuel economy, power etc by running with the distributor vacuum through that port.

    So i tried hooking the distributor up to manifold vacuum. And it runs much better, pulls much stronger up to 4500 rpm but then it starts to misfire BADLY. And i mean reaaally badly, the whole car jerks.

    The issue is, i set the timing with the vacuum disconnected as you should, i set it to about 8 initial. I dont remember what the mechanical advance was but the total timing with vacuum hooked up was about 38 i think, this was at ported vacuum.
    But when i switch over to manifold vacuum the timing jumps to about 40 degrees at idle, and when trying to turn the distributor to adjust i have very little play before it almost dies. I have also tried adjusting the vacuum advance on the little screw in the vacuum canister without results.

    Is my distributor just junk or am i doing something wrong?
    1972 Mercury Cougar XR-7 2dr HT
    1975 Saab 96 v4
    1993 Saab 9000 2.3T
    2006 Saab 9-5 Aero Sportcombi

  2. #2
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    Re: On ignition timing

    Ported vacuum is the only way to go in my opinion.
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  3. #3
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    Re: On ignition timing

    What kind and make distributor are you running? I'm having similar issues, with a White performance HEI (Chinese) distributor. My previous setup was a EFI dist from a 95 GT, never had problems setting the timing, could even do it by ear. Same thing with a Pontiac HEI i used to have, I would dump some race gas in it, 11to1 comp, and bump the timing up about 10 just by ear and would pick up about .2 in the quarter. I can't make sense of the timing I have now even with a light. Currently using ported vacuum.

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  5. #4
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    Re: On ignition timing

    First of all, if your engine ran much better with the ported vacuum, then all means go back to ported and see if she misfires at 4500 rpm. My guess is that you will experience the same problem, because ported and manifold vacuum are only different at idle and light load. When you switched from ported to manifold vacuum, you have to adjust your idle and possibly your mixture screws as well.


    Your ignition advance should be all in around 2800 to 3000 RPM. At WOT, your vacuum drops out of the equation and your total advance is a product of initial and mechanical advance only.


    The other problem I see here is your vacuum is advancing 32 degrees but the total advance should only for a 351 should not exceed 34 degrees. Is your vacuum advance a stock can or is it aftermarket? Some vacuum cans allow you to dial in the rate of advance whereas other vacuum advances allow you to adjust the total advance so you really need to know what you have.


    Coach Jack

  6. #5
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    Re: On ignition timing

    I'm with Jack on this one, ported vs. full manifold vacuum hooked up to the distributor will only make a difference at idle or while cruising. I also use ported vacuum btw.
    Think you might have a different issue.
    For comparison (i have a 351W), I run:
    11 initial, 35 total at 2500 rpm
    With vac attached (ported): 11 inital, 55 total at 2500 rpm
    And it pulls to 5000 rpm without problems

  7. #6
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    Re: On ignition timing

    Hello again! Just a quick reply since im at work. The thing is, when i run ported vacuum, it runs good and all, and pulls up to 5500-6000 rpm no problem.
    But when i run manifold vacuum, it launches much harder and pulls stronger up until it starts to misfire just around 4500 rpm.

    I also have one of those cheapo china HEI distributors, its probably junk. I just got one because it worked so good on my big block olds i used to own.
    I think i will just go for a pertronix ignitor II in the stock distributor.
    1972 Mercury Cougar XR-7 2dr HT
    1975 Saab 96 v4
    1993 Saab 9000 2.3T
    2006 Saab 9-5 Aero Sportcombi

  8. #7
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    Re: On ignition timing

    Quote Originally Posted by Odmark View Post
    Hello again! Just a quick reply since im at work. The thing is, when i run ported vacuum, it runs good and all, and pulls up to 5500-6000 rpm no problem.
    But when i run manifold vacuum, it launches much harder and pulls stronger up until it starts to misfire just around 4500 rpm.

    I also have one of those cheapo china HEI distributors, its probably junk. I just got one because it worked so good on my big block olds i used to own.
    I think i will just go for a pertronix ignitor II in the stock distributor.
    Try more initial timing with ported vacuum and see what that does. I run 14 degrees initial and only 22 mechanical in the distributor, no vacuum advance (somewhat built 408 Cleveland). A little hard starting when hot even with a high torque starter but otherwise it runs great.
    Last edited by 1969XR7Vert; 1 Week Ago at 07:56 AM.
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  9. #8
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    Re: On ignition timing

    I will fiddle around with it a bit more on ported vacuum, then i will get back to you guys! Thanks for the replies!
    1972 Mercury Cougar XR-7 2dr HT
    1975 Saab 96 v4
    1993 Saab 9000 2.3T
    2006 Saab 9-5 Aero Sportcombi

  10. #9
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    Re: On ignition timing

    Something has to be wrong with this junker distributor. I tried running ported vacuum again and fiddled around with the initial timing. And it always runs.. alright. I mean it does not jerk or anything, but its no where near as responsive as when i hooked up to manifold vac.

    When i swap over to manifold vac it idles much smoother, and still it pulls so much better, especially from a standstill. And i have noticed that it does not always misfire on manifold vac either, sometimes it just goes.

    Anyway something is very odd, i think i will try to swap back to the stock distributor to see what happens. And then like i said go for a pertronix ignitor II kit.
    Last edited by Odmark; 4 Days Ago at 12:53 AM.
    1972 Mercury Cougar XR-7 2dr HT
    1975 Saab 96 v4
    1993 Saab 9000 2.3T
    2006 Saab 9-5 Aero Sportcombi

  11. #10
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    Re: On ignition timing

    The mechanical advance curve of your distributor is not matched to your engine. You need to go back to ported vacuum. Run more initial mechanical advance, probably 12 to 15 degrees initial advance, and then make sure all of the mechanical advance is in by about 2800 RPM. Total advance needs to be about 38 degrees.
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  12. #11
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    Re: On ignition timing

    light advance springs required

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