Ignition timing with/without vac advance....sticking?

Electrical & Ignition

  1. swisherred

    swisherred Well-Known Member

    Messages:
    958
    Likes Received:
    463
    Joined:
    Jan 19, 2016
    Location:
    winston salem, nc
    I'm running a stock 67-383 non hp with a Mopar Performance electronic dizzy and a GM ignition module conversion with 50kv coil and .042 spark gap.

    I'm tuning everything again after rebuilding my 1806 carb and I've run into an issue with timing. With the vacuum advance hooked up and the initial timing set to 12.5ºBTC my max timing is upwards of 50ºBTC...way too much (checking in park in the driveway). If I disconnect the vac advance my max settles in about 36º. Is my VA bad? Sticking? Can it be adjusted down? It's my understanding that it should not add to the total timing because it will drop out once manifold vacuum drops at high RPM. Someone suggested that it may be sticking and still adding to the timing. Checking it with a vac pump it adds a total of 15º.....36º+15º= 51º...which is what I'm getting with it connected.


    With the VA disconnected and the line plugged the car runs awesome...even at part throttle. Good response and power. Throw it back in and its not nearly as good.
     
  2. Trace 300 Hurst

    Trace 300 Hurst Professional Tinkerer FCBO Gold Member

    Messages:
    978
    Likes Received:
    543
    Joined:
    May 27, 2018
    Location:
    Ponte Vedra Beach, FL
    I think your dizz is working just fine.

    It looks to me that you're hooking the VA hose to a direct manifold source, instead of the intended "ported vacuum" nipple. At idle, with the throttle blades closed, that ported nipple will provide zero vacuum and your initial timing will be 12.5. As you're cruising down the road under light throttle/light load, your timing will go to 50+, which is just exactly what you want. When you go WOT, the vac goes to (near) zero and your max will be 36 due to the centrifugal weights in the dizz.

    So if you're hooked to a nipple that shows vac at idle, that's the wrong one.

    I have 14 initial, the dizz is limited to 18 by this thing FBO Mopar Distributor Limiter Plate which gives me 32...staying on the safe side of detonation. With my dizz's vac hooked to a ported nipple, I get something like 52 when I open the throttle just a bit and hold it at say....2400 rpm.
     
    Last edited: Nov 27, 2019
    • Like Like x 3
    • Agree Agree x 2
  3. swisherred

    swisherred Well-Known Member

    Messages:
    958
    Likes Received:
    463
    Joined:
    Jan 19, 2016
    Location:
    winston salem, nc
    I'm definitely hooked to the ported...no vac at idle.....your saying its normal to hit 50deg? Will it still do that driving it or will the load keep it at 36?
     
  4. swisherred

    swisherred Well-Known Member

    Messages:
    958
    Likes Received:
    463
    Joined:
    Jan 19, 2016
    Location:
    winston salem, nc
    When I rev it without the vac it stops at 36...reving with the vac it goes to 51.....all just in the driveway not driving.
     
  5. Trace 300 Hurst

    Trace 300 Hurst Professional Tinkerer FCBO Gold Member

    Messages:
    978
    Likes Received:
    543
    Joined:
    May 27, 2018
    Location:
    Ponte Vedra Beach, FL
    Ahhhh...that explanation makes more sense. I assume that when you quit the reving it goes back to 12.5? If so, you're perfectly fine. That's exactly what it's supposed to do.

    Now stop worrying and go enjoy Thanksgiving. :thumbsup:
     
    • Like Like x 1
  6. swisherred

    swisherred Well-Known Member

    Messages:
    958
    Likes Received:
    463
    Joined:
    Jan 19, 2016
    Location:
    winston salem, nc
    Thank you....just didnt want to blow it up with advance
     
  7. cbarge

    cbarge World Famous Barge in a Budget FCBO Gold Member

    Messages:
    2,589
    Likes Received:
    3174
    Joined:
    Nov 22, 2010
    Location:
    Kingston Ontario Eh?
    Recurving the distributor will slow down the advance which is what you want.
    With today's corn fed gas timing is everything. The 50 year old specs no longer apply.
    I do not run vacuum advance. Sure my MPG may drop but rather spend more at the pump than repairing damage from detonation.
    12.5 to 14 initial with 34 to 38 degrees all in at 3000 rpm is good for a 383 stock or light mods like mild cam and duals. Vacuum should be between 17. ( slight mods)and 20 inches ( totally stock)
    Every engine is different so test and tune. Drive between adjustments to see what makes her happy.
    The motor should start first click every time without long crank time with little or no throttle.
    Hope this helps.
     
    • Agree Agree x 1
  8. swisherred

    swisherred Well-Known Member

    Messages:
    958
    Likes Received:
    463
    Joined:
    Jan 19, 2016
    Location:
    winston salem, nc
    It does take a couple more revs to start than I want...two or three pumps and it starts good though... Still have to adjust the new electric choke and some fine tuning. Adjusting the secondary air valve helped eliminate my mid pedal bog...rebuilding it eliminated the part throttle bog...pulls good now....will fiddle with it a bit more.
     
    • Like Like x 1
  9. Trace 300 Hurst

    Trace 300 Hurst Professional Tinkerer FCBO Gold Member

    Messages:
    978
    Likes Received:
    543
    Joined:
    May 27, 2018
    Location:
    Ponte Vedra Beach, FL
    Nope, you're fine. And if you're not getting detonation (ping) at 36 total, that's even finer. When I go above 34* I get some rattle, but my engine has 11:1 slugs in it so I keep it at 32/33. With octane boost I can go to 36, but why bother? And that's why I love that timing limiter plate. You could choose to go with 18 initial if you wanted for a great idle, and install the plate in the 16 degree position. 16+18=34*, etc. It's a great tuning tool that allows you to get far away from the FSM 2.5* initial setting.
     
    • Agree Agree x 2
  10. Trace 300 Hurst

    Trace 300 Hurst Professional Tinkerer FCBO Gold Member

    Messages:
    978
    Likes Received:
    543
    Joined:
    May 27, 2018
    Location:
    Ponte Vedra Beach, FL
    If my engine has been run in the past ~10 hours, it starts so fast with a tiny tap of gas that you don't even hear the famous Chrysler starter whine. Instantaneous light-off, and that's really gratifying. That's what 14-16 initial gets ya, but you sure don't want it to go to 42 max with the stock-spec centrifugal advance. Boom!
     
    • Like Like x 1
  11. Davea Lux

    Davea Lux Old Man with a Hat FCBO Gold Member

    Messages:
    5,517
    Likes Received:
    1436
    Joined:
    Mar 27, 2017
    Location:
    Cornelius Or
    Most of the performance distributors have a small Allen screw inside of the vacuum diaphragm (Inside the hose bib.) that can be used to limit/adjust total vacuum advance as well.

    Dave
     
    • Agree Agree x 2
  12. Trace 300 Hurst

    Trace 300 Hurst Professional Tinkerer FCBO Gold Member

    Messages:
    978
    Likes Received:
    543
    Joined:
    May 27, 2018
    Location:
    Ponte Vedra Beach, FL
    Yes. 3/16th. That Allen adjusts spring tension, and does not limit arm/plunger travel. Given enough vacuum (30 inches?), the diaphragm will still pull the timing (against the spring tension) to the maximum advance that was designed into that particular vac unit.

    There are various vac units available with different amounts of vac advance but I've never used them. I fiddle with the "travel slot" in the arm if need be, but that's rarely needed for a mild motor.
     
    • Like Like x 1
    • Agree Agree x 1
  13. Davea Lux

    Davea Lux Old Man with a Hat FCBO Gold Member

    Messages:
    5,517
    Likes Received:
    1436
    Joined:
    Mar 27, 2017
    Location:
    Cornelius Or
    If it takes 30" vacuum, it is not going to move much.

    Dave
     
  14. Trace 300 Hurst

    Trace 300 Hurst Professional Tinkerer FCBO Gold Member

    Messages:
    978
    Likes Received:
    543
    Joined:
    May 27, 2018
    Location:
    Ponte Vedra Beach, FL
    Exactly!
     
  15. mr. fix it

    mr. fix it Old Man with a Hat

    Messages:
    6,728
    Likes Received:
    4466
    Joined:
    Aug 14, 2013
    Location:
    Welland, ON
    I was going to chime in on my Poly that is 10' at idle ported vacuum off the carb then 30' at 3000rpm
     
    • Like Like x 1
  16. Trace 300 Hurst

    Trace 300 Hurst Professional Tinkerer FCBO Gold Member

    Messages:
    978
    Likes Received:
    543
    Joined:
    May 27, 2018
    Location:
    Ponte Vedra Beach, FL
    That sounds reasonable. Do polys kinda “max out” at 30 vs the 34-36 of wedges?
     
  17. mr. fix it

    mr. fix it Old Man with a Hat

    Messages:
    6,728
    Likes Received:
    4466
    Joined:
    Aug 14, 2013
    Location:
    Welland, ON
    Yes I believe so
    They rely on the torque produced more than Tom’s on a stock build

    there may be more expert opinions on this topic
     
  18. 65sporty

    65sporty Old Man with a Hat

    Messages:
    7,039
    Likes Received:
    1677
    Joined:
    Aug 24, 2013
    Location:
    Marlette,Mi
    That limiting plate looks great, Thank You. I have broke down distributors and brazed up the slots to limit advance but this will be way easier.

    One other option would be to send it to @halifaxhops for a custom curve.
     
  19. Trace 300 Hurst

    Trace 300 Hurst Professional Tinkerer FCBO Gold Member

    Messages:
    978
    Likes Received:
    543
    Joined:
    May 27, 2018
    Location:
    Ponte Vedra Beach, FL
    The FBO plate is absolutely genius. Can't believe someone (like me!) didn't think of it 40 years ago when I was brazing and filing the slots, too. It makes tuning SO simple.....even a cave man can do it.
    UTEW0684.JPG
     
    • Like Like x 1
    • Agree Agree x 1
  20. cbarge

    cbarge World Famous Barge in a Budget FCBO Gold Member

    Messages:
    2,589
    Likes Received:
    3174
    Joined:
    Nov 22, 2010
    Location:
    Kingston Ontario Eh?
    Yes the FBO plate is a tuner's masterpiece. I have it in the Waygun's 318 dizzy.
     
    • Like Like x 1