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XJ900F poor running - Spark issues

Discussion in 'XJ Technical Chat' started by Trodd69, Feb 21, 2020.

  1. Trodd69

    Trodd69 New Member

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    I haven't had a chance to take a video to upload but will do in a few weeks when back in the workshop but here goes:

    My 1988 XJ900F hasn't run properly since I got it (which is how I came into ownership).

    Idle is the main problem, with tickover possible but then just off tickover like pulling away from lights the engine has a tendancy to bog and die. Once you can get past this and onto the main jets the power is there without issue.

    I found an interesting point after borrowing a timing lamp that in each of the two cylinder pairs only one cylinder looks to be firing every stroke with the other randomly firing around half the time - this also confirmed with a Fluke on the ht leads, the good cylinder showing 1000rpm and the other showing around 400rpm.

    Any thoughts or experience welcome...

    Works so far:
    Full ultrasonic-clean carb rebuild including all new seals (yes even butterfly shaft)
    New coils
    New HT leads
    New non resistive plug caps with 5Kohm plugs
    Gunson colourtune idle mixture adjustment
    Carb balancing with Morgan Carbtune
    All valve lash brought into spec
    New original style air filter installed
     
    Last edited: Feb 21, 2020
  2. XJ550H

    XJ550H Well-Known Member Premium Member

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    I would look at the plug wires swap them see if the issue follows the leads. coil should fire both leads every time for 1 and 4 and 2 and 3 being a wasted spark ignition
    .


    NGK BPR8ES is the plug speced for the 87 xj900 it is a resistor plug could be you need resistor caps too


    Spark plug caps:

    N, FN, and F models: 10K +/- 20% = 8,000 to 12,000 ohms per cap acceptable range


    Spark plugs:

    N, FN, and F models: 5K ohms per plug
     
    Last edited: Feb 21, 2020
  3. Trodd69

    Trodd69 New Member

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    Thanks for the suggestion,

    The issue remains regardless of coil location (I have 3 sets of coils all with the same symptoms) and lead position. It's strange to me that with the wasted spark system and the TCI firing 1/4 and 2/3 that I can be seeing an issue on only one of each of these pairs, so 4 will fire fine but 1 firing erratically etc.

    After the above steps I've taken so far I'm leaning towards the TCI unit. My next step before that will be to check the voltages up to the TCI in-case there is an issue there.
     
  4. cds1984

    cds1984 Active Member

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    Just to throw my experience of a crazy failing TCI in to the mix.

    The TCI I had on my 900 which was an aftermarket from NZ (but from ebay) basically stopped advancing from the idle circuit after a few Kms of riding and warming up.
    Taking off from a set of lights was lunacy after that, until the mains kicked in and it tried to throw me off of course.

    Drove me mad getting to the final conclusion and replacing the rest of the ignition bits first. (plus a bit more)

    So... looks like you have done all the rest of the ignition circuit, maybe worth taking a look the state of the original TCI and refurbish or grab a second hand one to compare your outcome.

    I grabbed the original TCI that came with the bike and also made the bike just die willy nilly and replaced all the capacitors... runs a charm now! woot.
     
    Trodd69 and Dave in Ireland like this.
  5. Trodd69

    Trodd69 New Member

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    A bit late, but thanks for the post, I'm now also at the point of looking in the TCI. I've changed the capacitors out and got new transistors also whilst I'm in there. Glad to hear someone else has had very similar symptoms, hopefully I'm also on the right track.

    Will have some spare capacitors and transistors once I'm done if anyone else needs to do the same...
     
  6. bigfitz52

    bigfitz52 Well-Known Member Premium Member

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    Before you replace the TCI, and I know you're just gonna roll your eyes, but humor me: Replace the spark plugs. These bikes are hard on plugs to begin with, and all the fiddling that accompanies carb tuning can murder a set of plugs. This comes from experience. And you tend to swear even worse when it turns out to be just a saturated plug after having tried you name it...

    Also: the TCI unit is expecting the stock resistor plug caps and plugs. Running something other than that could beat up on TCI units. I know if you "run" one with un-grounded plug leads and no plugs to fire you can kill or wound it. My only experience here is in ferreting out flaky units, I've never attempted a repair.
     
  7. biggo

    biggo Member

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    Check what voltage you are getting to TCI unit and ignition coils with motor running .I had similar issue ended up being voltage drop used relay to give power to TCI nad coils direct from battery .
     
    Trodd69 likes this.

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