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Help, Im only firing on 2 and 4.

Discussion in 'XJ Technical Chat' started by Reed whitehead, May 15, 2020.

  1. Reed whitehead

    Reed whitehead New Member Premium Member

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    I put new coils on my X J Sega750 now its only firing on 2 and 4. How is this possible and what do I do to resolve this?
     
  2. Polock

    Polock Well-Known Member

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    move the wires around so the good coil is now on the bad plugs and see what happens
     
  3. raskal

    raskal Active Member

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    honestly, I'm just getting lost of the newfangled words here.

    First I heard about an Atari, and now a Sega. Is the Xbox next???
     
  4. XJ550H

    XJ550H Well-Known Member Premium Member

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    re install spark plug caps
     
  5. Reed whitehead

    Reed whitehead New Member Premium Member

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    First off thank you for replying both the coils are brand new but I bought them on the Internet 1 coil does 1 + 4 the other coil does 2 + 3 I'm getting spark on 2 and 4 could half of each coil not be working is that possible I have a timing light when I put it on number one I get nothing if the timing lights dark I put it on number 2 The Flash normal I put it on 3 and it's dark I put on for and it flashes normal how is that possible the coils weren't long enough to fit where the the stock ones go so I fabricated a little bracket to attach them at both ends
     
  6. Reed whitehead

    Reed whitehead New Member Premium Member

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    Sorry about that it was my very first post and I did not read it before I posted it will not happen again
     
  7. Reed whitehead

    Reed whitehead New Member Premium Member

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    I put the old spark plug caps back on but I want to get new ones are 5 ohm spark plug gap the right one
     
  8. Rusty81

    Rusty81 Member

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    Flip the leads to the other plugs and see what you get. Check the input to the coils (resistance) and check the TCI output
     
  9. XJ550H

    XJ550H Well-Known Member Premium Member

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    this link is about relays switches and charging system and ignition system

    it has all the ignition specs
    Spark plug caps:
    1981-83 models: 5K +/- 20% = 4,000 to 6,000 ohms per cap acceptable range
     
  10. Reed whitehead

    Reed whitehead New Member Premium Member

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    So I need 5000 ohm not 5 ohm?
     
  11. XJ550H

    XJ550H Well-Known Member Premium Member

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    yes 5K ohm caps which is 5000 ohms
     
  12. Polock

    Polock Well-Known Member

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    the timing light test is strange. the two plug wires from the same coil are actually the opposite ends of the same wire. try checking resistance from each plug wire to ground. there should be no connection
     
  13. SQLGuy

    SQLGuy Well-Known Member

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    Basically, no, you can't really have one of these coils fire just one plug.

    The way they fire is in series. It's a bit of a difficult concept, but the high-voltage (secondary) side of the coil is a makes a circuit through the plugs and doesn't care about "ground". Ground is just a place where a bunch of other things are connected together.

    So, when the TCI transistor turns off, the magnetic field in the coil collapses, and this forms a several thousand volts potential between the two wires/caps. As this potential builds, it gets high enough to arc across the spark plug gaps, and the current from the secondary coil starts to flow and continues until it can't maintain a spark anymore.

    Current flows from the secondary winding, through one high-voltage wire, through the cap, down the center of the spark plug, across the gap, into the body of the plug, into the block, into the body of the "other" spark plug, across the spark gap, up the center of that plug, through its cap, through that wire, and back to the coil.

    So, if you pull a plug and just let it hang free, it can't effectively spark, because it has no way for current to flow into or out of its body. The loose plug needs to be resting against the block so the whole circuit can be tested.

    Not sure about your aftermarket coils, but stock ones have a bit of a weak spark that can be hard to trigger some timing lights.

    To be clear: one coil runs 1/4, and the other one runs 2/3. There's no reason to be trying other arrangements. If you want to test a wire, the best way is to use a spare plug, switch the cap to it, and rest is against the block. If you do that for, say, 4, and get spark, but don't when doing it for 1, then something is wrong internally in that coil.

    Edit: alternatively, if you want to test both plugs at once, you could do that with the caps on two plugs, and those two plug bodies touching each other. Then you don't need a block to complete the circuit.
     
  14. XJ550H

    XJ550H Well-Known Member Premium Member

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    it could be that your spark plug caps have failed. switch the caps to see if the problem follows the cap
     
  15. Polock

    Polock Well-Known Member

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    is it possible the primary side could get reversed and run two cylinders on the wasted spark cycle?
    i don't want to think about it in my present state of mind :)
     
  16. SQLGuy

    SQLGuy Well-Known Member

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    Primary side can't get reversed like that. If you swap primaries, then you'll get spark on all four plugs, but 1/4 and 2/3 will be 180 degrees off, and it's not going to run, and will probably cough and bang a bit as unburned fuel misfires.

    If you had one bad channel, and miswired the good channel, then you would get spark on only one mismatched pair. So, let's say for instance, that the OP really has a bad TCI - 2/3 being bad, but he thought the problem was a bad coil, so he replaced both coils, and then miswired the 1/4 one to 2/4. This would result in exactly what he's seeing: spark only on 2/4... but it's really spark on 1/4. But it sounds like he's pretty sure that he has them connected correctly. So, IDK.

    I think the more likely answer is that he has a fuel problem, but he's checking spark inconsistently: check 4 with plug out and resting on block - good; check 1 with plug out and not resting on block - no spark...
     
  17. Reed whitehead

    Reed whitehead New Member Premium Member

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    First off I would like to thank everybody who tried to help me you guys are awesome here's what I did I finally got the opportunity 2 ohm out those coils the input side on both coils with 5.3 the output side a full coils was 15 point three ohms so I threw them in the garbage the coils I took off the original one s. Low side was 3.8 high side was 12 seven the other one was 12/8 something like that I order some new ones they ohmed out at 2.75 and 12.8. I bought one new spark plug cap it came at 4.9 the other three out at 5.22 and 5.8 and 9.09. So I know I need three new caps. I also left the spark plug wires maybe four inches longer than they were stock could that helped cause backfiring and it not wanting to start
     
  18. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    I can't understand what you are saying here.........the INPUT side of the ignition coils, which is internal to the coil, is what is called the PRIMARY circuit., and is measured where the two small wires from the wire harness attach. Measuring the resistance across these two points (harness wires, or, if like most aftermarket coils, at the screws which hold the harness wires to the coils) should read somewhere between 2.5 - 3.0 ohms (two to three, not thousands of ohms), unlike what you'll find on........:

    The OUTPUT side of the ignition coils----which include the actual spark plug wires-----and which is called the SECONDARY circuit, and which should have about 11,000+ ohms of resistance.

    Both circuit "sides" (inaccurate word, but electrical guys learn to live with it) actually have a range of usable ohm values that are considered "in spec", for your bike it is:

    Ignition Coils:

    Primary side (input from main wiring harness):
    2.5 ohms +/- 10% = 2.25 ohms - 2.75 ohms acceptable range

    NOTE: most aftermarket coils designed for these bikes have a slightly higher resistance value, maybe 3.0 to 3.5 ohms, and this is not ideal, but okay.


    Secondary side (spark plug wires, without their end caps):
    11K ohms +/- 20% = 8,800 ohms - 13,200 ohms acceptable range NOTE: this resistance measurement is done with neither the spark plugs nor the spark plug caps attached to the end of the plug wires!


    The hard-plastic spark plug CAPS (don't call them boots!) have a resistor embedded within them, and so measuring the resistance from the input of the cap to the output side of the cap should have the following values:

    Spark plug caps:
    5K +/- 20% = 4,000 to 6,000 ohms per cap acceptable range


    Stock spark plugs have no resistance within them, although many model spark plugs do have resistance built into them (for a variety of reasons), the stock NGK BP7ES spark plugs for your bike are non-resistive:

    Spark plugs:
    0 ohms per plug



    The only plug cap that you have which is out-of-spec is the one you measured at 9.09 ohms...............although, even in that condition, it would not cause the problems that you describe. Completely fouled, or spark plugs gapped at incredibly incorrect amounts, might give you odd readings. Check and replace the spark plugs if unsure, they are the cheapest component in the system to replace........

    The ignition coils that you threw out (with, if I'm reading your post correctly, had 15.3K (15,300) ohms, which is out of spec, but again, won't cause the problems you are experiencing (it'll produce a very weak spark, but would still work).


    Final note: the original rubber-sheathed spark plug WIRE (the thick stuff coming out of the ignition coils and running to the spark plug CAP) is actually zero-resistance wire, and basically lasts forever. Now, you may ask, well if it is zero-resistance (0 ohms), then why, when I measure the resistance thru them and thru the SECONDARY side of the coils, do I get those 11,000+ ohm readings? It's because the spark plug wires are attached directly to the secondary wire "winding" (in other words, a "coil" of wire) inside the ignition coil, and it's that winding of wire ---which, if stretched out flat and straight, might actually be a 50-foot length of wire----that accounts for those mega-ohm readings. But the short length of spark plug wire attached to the internal windings has zero resistance.....
     
    Last edited: May 24, 2020
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  19. joejr2

    joejr2 Active Member

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    Thanks for all that info. I was about to go through all the components in my ignition system for problems and now I won't have to look up the specs
     
  20. Polock

    Polock Well-Known Member

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    those specs aren't the whole story , you have to test from a plug wire to ground. anything but open circuit, no connection and that coil fails
    this can make one side fire but not the other
     
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