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Weak Spark

Discussion in 'XJ Technical Chat' started by Dtperrell, Jan 28, 2019.

  1. Dtperrell

    Dtperrell Member

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    I have an 81 XJ550 Seca that I have fighting to make run for a while now. I cleaned and soaked the carbs and replaced the seals for the throttle shaft because I thought that was my issue. Now I'm encountering a weak spark.

    I followed the manual and tested the resistance in my ignition a while back and thought it was my end caps because they were reading a ton of resistance. After replacing with NGK end caps and resisitive plugs I'm still getting a weak spark. Coils tested good on secondary circuit but both tested 3 ohms on primary circuit which I think may be my problem.

    At this point I'm digging for any possible solution because its becoming a money pit. With a fresh charged two day old battery on the first push of the start button it will sputter for a second and that's all until I fully charge again. Even a squirt of starting fluid didn't do a thing.
     
  2. XJ550H

    XJ550H Well-Known Member Premium Member

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    the spark is weak to start with . you are not supposed to use resistive spark plugs
     
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  3. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    That's a bit high for the primary side:

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

    but it's not hugely out of range, and shouldn't cause the symptoms that you are experiencing. It sounds more like a bad battery (even new batteries can be bad; it's more common that you'd think) or dirty electrical connections somewhere in the system. If you have a voltmeter, you can see what the voltage is on the battery (fully charged state) and then WHILE THE STARTER MOTOR IS TURNING......it should not drop below about 10.5V DC (and even that's borderline; 11+V is more-better-er). The TCI needs a minimum of about 10VDC to operate......and while the starter will spin the engine over like mad with low voltage, the TCI falls on its face at less than 10 volts.....which can lead to all sorts of confusion when a battery low-voltage condition occurs!
     
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  4. Dtperrell

    Dtperrell Member

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    The OEM end caps are 10k ohm, aftermarket NGK end caps are 5k ohm. I have 5k ohm end caps and 5k ohm plugs which brings it back to OEM 10k ohms.
     
  5. Dtperrell

    Dtperrell Member

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    I am going to test this battery tonight while starting. The last battery I had in it would drop to around 9 volts when starting so I replaced it but I haven't tested this one yet.

    Would it burn up the plugs if I used non-resistive spark plugs with the 5k ohm end caps? I'm thinking so that's why I haven't tried it..
     
  6. Dtperrell

    Dtperrell Member

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    Just tested. I was getting 11 volts while cranking with this superstart ext9 AGM battery.
     
  7. Chitwood

    Chitwood Well-Known Member

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    I agree that all you need are the 5k caps and non resistor plugs. That's what spec is on my 650, could be wrong but I'm pretty sure the 550 is the same
     
  8. Dtperrell

    Dtperrell Member

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    My Clymer confirms that the OEM spark plug end caps are 10k ohms. I'm just not sure if it would cause any issues using 5k ohm caps with non-resistor plugs
     
  9. XJ550H

    XJ550H Well-Known Member Premium Member

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    look at the overall spec
    Secondary side (spark plug wires, without their end caps):
    11K ohms +/- 20% = 8,800 ohms - 13,200 ohms acceptable range
    Spark plug caps:
    10K +/- 20% = 8,000 to 12,000 ohms per cap acceptable range

    per the xj4ever catalog
    HCP21721BLK This kit comes complete with black 8mm plug wires already installed; wires have black non-resistive plug boots installed. Since these plug boots replace the use of the stock resistive plug caps, it is recommended that you also purchase a set of resistive spark plugs (HCP13514 standard resistive plugs or the HCP12974 hi-performance iridium resistive plugs). Spark plugs are not included with the kit. Fits all XJ550 models.
    $ 279.95

    so just the 5k resistance of the cap with non resistive plugs work @chacal may confirm this
     
  10. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Not really, the secondary resistance primarily controls the DURATION of the spark (and thus the energy expended per unit of time). Increasing the secondary resistance creates a LONGER spark time, but at a reduced energy output per unit of time (thus the total amount of electrical energy used remains the same, but since it is "spread out" over a longer time frame, there is less energy per unit of time). Engine design parameters (combustion chamber design, piston designs, intake port design, camshaft profiles, etc.) are the main determining factors in deciding upon the choices in secondary resistances (and, it won't make much of a difference in real-world, street-driven vehicles, although it may effect starting in extreme conditions hot or cold).
     
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  11. Dtperrell

    Dtperrell Member

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    @chacal that's a great explanation! Thank you. I'm going to try the non-resistor plugs tomorrow and see what happens
     
  12. Joshua Olkowski

    Joshua Olkowski Member

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    I'd like to know what happens. I also have a hard starting issue with an xj550 Seca that hasn't been ridden since 2005 before I bought it a year ago and I've had many no spark, hard starting situations since. This is the first time I've heard of non-resistive plug caps. I'm only about a year into bikes. After testing the coils and the caps it turns out I couldn't get any ohm readings on the caps at all and there was a bit of rust in there too so I'm planning on replacing mine. The coils tested fine. I thought all caps have resistors in them though so I'm curious to see what happens with yours. Thanks.
     
  13. XJ550H

    XJ550H Well-Known Member Premium Member

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    caps have resistors , boots are non resistive
     
  14. Dtperrell

    Dtperrell Member

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    Swapped out the resistor plugs for non-resistor plugs and it fired up. Now, obviously my carbs haven't been tuned yet but when it came on it started running rough and eventually started hanging almost wide open when I tried to give it gas. Thought it was rich but when I turned the mixture screw down it needed the choke to even idle so I started adjusting out. I went out about a quarter turn each time and still hanging idle with no noticeable difference. Then finally it just wouldn't start I assume because too much gas. Then my battery started to die so I'm calling it a night.

    Any input on what could be happening?


    UPDATE: I'm pretty sure its running lean. One of the plugs looked a little gray when I pulled it out. Tomorrow I'll try to work it out
     
    Last edited: Jan 29, 2019
  15. chacal

    chacal Moderator Moderator Supporting Vendor Premium Member

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    Some common causes of a high idle:

    - Engine has not been synched, as each cylinder "fights" the others for dominance, it can lead to a run-away idle situation.

    - A lean air-fuel mixture condition, which can be caused by a variety of problems......vacuum leaks, plugged or too-small fuel jets, etc. Old or "stale" gas may also cause a lean fuel-air mixture to occur temporarily (until the fuel is used up!).

    - A vacuum leak somewhere in the intake system.....intake boots, internal o-ring seals, etc.

    - Mixture screws not properly set or adjusted.

    - Vacuum piston sticking or stuck in a partially raised position.

    - Idle speed screw set too high, or set to create a "proper" idle speed when the engine was cold (and thus results in a "high idle" once the engine reaches operating temps). The idle speed when the engine is cold should be modulated via the use of the choke (enrichment) control system.

    - Butterfly valves opened too far; synch screws out of adjustment.

    - Throttle cable wear, adjustment, or throttle lever brackets installed incorrectly or interfering with other nearby objects (cylinder head fins, etc.).

    - Choke (enrichment) circuit is stuck "open"....this can occur even if the choke lever is rotated to the fully closed position, if for some reasons the choke plungers are not fully closing (cable wear, cable adjustment, bent finger brackets, or installation problems). In addition, even if the choke plungers are "closing" fully, if the choke plunger valve face or its seat are worn or scarred, this will allow fuel to leak part the plunger and richen the mixture even if the plungers are closed.
     
  16. JeffK

    JeffK Well-Known Member

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    FWIW: I have worked on bikes & ridden since the early 70's. I've personally met Lucas, the Lord of Darkness and his positive ground systems, I've road raced multiple bikes and God only knows how many different bikes I've worked on over the years.....that stated.....the spark on my Stock 12K mile Turbo was absolutely the weakest, most feeble spark I ever witnessed on a running engine, period. On a TURBO bike of all things, what were they thinking?

    Well, the reality was that Japanese coils were not that great for anyone in the 70-80's so it's not just on Yamaha. Replacing them with Accel coils made a huge difference in running but especially in starting!

    jeff
     
  17. Chitwood

    Chitwood Well-Known Member

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    Not to hijack, but as long as the primary resistance of replacement coils, accel or whatever brand, no harm should come to the TCI? Would one still need to run the resistor caps with those coils?
     
  18. Polock

    Polock Well-Known Member

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    correct
    resistor caps, plugs, wires isn't go matter one way or the other. nobody ever broke their bike by using the wrong plug caps
     
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  19. Chitwood

    Chitwood Well-Known Member

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    I had meant to say as long as the primary resistance is the same as stock spec...but polock got my point
     
  20. Dtperrell

    Dtperrell Member

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    I'm going to be running a custom 4-1 exhaust eventually anyway so I guess its time to go jet crazy!

    @chacal I assume you have a great selection of jets for these mikuni carbs?(I hope so)
     

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