Msd pro billet in a small block 360 backfire at idle

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  1. 70Roadkill

    70Roadkill Well-Known Member

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    This is weird. I got a pro billet msd, new from someone never used. Popped it in, and backfired at idle. Swap in the original one that I locked out and runs fine. I wanted to go from the locked out one to a mechanical advance. Even if I lock out the msd it backfires. I’m running a pertronix hp digital box, basically same as an msd 6al and a blaster 2 coil. Checked the gap and it was 20 thousands on the air gap. Anyone ever run into this?
     
  2. fklskv

    fklskv Well-Known Member

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    The gap should be .007 Im thinking.
     
  3. Dubob

    Dubob Well-Known Member

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    Maybe the polarity of the wires from box to distributor? Not at all familiar with the pertronix boxes.
     
  4. 70Roadkill

    70Roadkill Well-Known Member

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    same wiring as a 6al
     
  5. 70Roadkill

    70Roadkill Well-Known Member

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    Also off idle runs great at cruise and wot. Only at idle
     
  6. pishta

    pishta I know I'm right....

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    gap is fine (.018-.030) are the pickup wired polarity correct? polarity issue will trigger the falling edge instead of the leading edge, and alter timing. IF you checked the wires to the manual, not the issue then unless programming is changed? 1000 RPM rotary switch position
    Magnetic Trigger (Falling Edge) ON 187mJ 4
    Magnetic Trigger (Falling Edge) ON 145mJ 5
    Magnetic Trigger (Falling Edge) OFF 187mJ 6
    Magnetic Trigger (Falling Edge) OFF 145mJ 7

    Maybe try the other setting to see if it changes anything.
    Pertronics calls out points/electronic and magnetic as 2 different settings...
     
    Last edited: Jul 5, 2020
  7. 70Roadkill

    70Roadkill Well-Known Member

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    I’ll have to check that out closer in the manual. Let me do a little reading in a bit. Working on fixing water leaks right now.
     
  8. 7e5hangten360

    7e5hangten360 semi-pro

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    what i have found with the msd blaster coils is most of the time you have to switch the +positive and -negative wires at the coil, unless running all msd components for ignition. i know it doesnt make sense. i believe that is because a msd box trigs the coil on the positive side of the coil where as most other boxes trig on the negative side, which is what we are used to. maybe you should try it.
    most of the time, you should use a msd coil with an msd box. but use another brand coil with other brand boxes.
    you might want to try your old coil to confirm
     
    Last edited: Jul 9, 2020
  9. 7e5hangten360

    7e5hangten360 semi-pro

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    after re-reading your post, it seems that it is the msd distributor /polarity at the distributor that is the problem, as someone mentionned.
     
  10. 70Roadkill

    70Roadkill Well-Known Member

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    I plan on giving a try this weekend to see what happens
     
  11. BVoelzke

    BVoelzke Dart Racer

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    I have tried MSD Billet distributors on my small blocks two times now and both times had various problems and eventually went back to a stock type distributor. However I have ran a 7AL2 box and Blaster coil for years with no problems.
     
  12. 70Roadkill

    70Roadkill Well-Known Member

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    I flipped the 2 wires in the dist connector and went for a drive yesterday and let it idle for a while. No issues. Now need to figure out springs and hot start hard crank. Had timing set at About 14 deg at idle. Light silver and blue in there now and a 14 deg bushing.
     
  13. yellow rose

    yellow rose Overnight Sensation

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    Verify that the bushing is actually 14 degrees. I have handfuls of them and it’s hard to find two the same.

    That 14 degree bushing gives you 28 degrees of mechanical advance, and that’s a TON. I’d you really have 14 initial, at some RPM you’ll have 42 TOTAL. That’s a TON.

    Depending on several things, the smallest bushing is use is the 10 degree part, and I forget what color it is and verify it’s actually 10 degrees.

    With that bushing you’ll have 20 mechanical and you can get 15-16 initial.

    I make my own bushings quite a bit. A 9 degree bushing gives you 18 mechanical and 17-18 initial.

    An 8 degree bushing would give you 16 mechanical and 19-20 initial.

    If the converter is loose, you can bring the curve in pretty quickly. If the RPM drop from neutral to in gear is too much, you need get more initial from your starting tune up.

    Any ignition box will retard with RPM. When it starts to retard, and how much is always a guess because two identical boxes will start retarding at different RPM and may not retard the same amount.

    I’ve seen as high as 6 degrees retard at 7500 (I’ve seen more retard but most guys don’t even turn that much RPM) so if you set your total when the distributor stops advancing at say...2500 RPM and you don’t check total timing at whatever max RPM it will most likely have less total than what you think.

    EDIT: crackedback pull my head out of my can so in the above if you have the 14 degree bushing, that means you’ll need 21-22 initial to get 35-36 total. If you need that much initial than that’s what it needs. You have to give it what it wants.

    Working on the bench I use distributor degrees and do the math in my head, but crackedback is correct. I prefer to do it that way, but it can be confusing...it just caught me.
     
    Last edited: Jul 12, 2020
  14. pishta

    pishta I know I'm right....

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    Score! There is a saying: "when all else fails, RTFM"
    :thumbsup:

    EDIT>> did all Pertronix models use that little plastic reluctor overlay? I though the old ones triggered off the original points lobes. Always thought that was a crazy way to trigger an electronic distributor as the lobes are 'soft' when it comes to tripping a hall effect or magnetic sensor. Pointy finger reluctors ie. Mopar electronic offers much sharper timing signal (trailing or leading edge) wise than a round rubbing block lobe
     
    Last edited: Jul 12, 2020
  15. crackedback

    crackedback FABO Gold Member FABO Gold Member

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    MSD bushings are in crank degrees.

    Black bushing is 18* on the crank, IIRC. The bushing I was making were in crank degrees also. One of the quirks of MSD and actually a bit less confusing for most people. "Math is hard!"
     
    • Thanks! Thanks! x 1
    • yellow rose

      yellow rose Overnight Sensation

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      Dammit, my bad. That is correct. Got my arse out in front of my head.

      Did you find the MSD bushings inconsistent? I can run out and measure some of them and they’ll all be different sizes. Talking about the same color/size bushing.

      I’m also not a fan of the springs MSD sends out. I need to source some so I can get what I want.
       
    • crackedback

      crackedback FABO Gold Member FABO Gold Member

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      The MSD bushings are OK. Never seen them be seriously out of spec. You will get some stacking tolerance issues. I chuckled about a scolding regarding the CNC tolerance, being +- .001 and the amount of change in advance. It is so small you couldn't see it on a manual timing light. We are talking on a 10* bushing something on the delta of .05ish degrees... REALLY... that's not going to be significant or an issue on anything that runs on pump gas.

      I used to make these with a hillbilly lathe. A vise, flat file held in vise, drill with mandrel holding a bushing. Then grab the Very Near calipers and spin that puppy out. LOL

      Black was the largest diameter and was 18* red smallest diameter at 28* with 4-5 in between those two, wanna say 25-23-21-19.
       
    • mopar65

      mopar65 Well-Known Member

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      So is the MSD Billet Dist considered a Magnetic pick up? I have the same set ip as the OP and trying to see if i have mine set right. Should it be set to falling edge on or Off.
       
    • furrystump

      furrystump Well-Known Member

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      The company “4 seconds flat“ makes a 10 degree bushing for them.
       
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