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Thread: XB12X Backfiring through intake and exhaust

  1. #1
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    XB12X Backfiring through intake and exhaust

    Hello All, This is my first time posting but definitely not my first time visiting this site, it is a gold mine!

    Some background: I recently got a 2006 Buell XB12X off of a friend, it had been sitting in his shed for a couple of years. He had drained the tank prior to storing it. He replaced the intake seals, spark plugs and a rocker cover gasket as maintenance about 6 months before I got it but could not get it running right, hence why I got it cheap.

    My problem is as it states in the thread title. The bike will start and run, but after about 20 seconds of running it starts to backfire through both the intake and exhaust. If I take the bike for a ride it is very low on power and constantly dies. I have searched this forum and tried a number of things suggested as shown below.

    Things I have replaced:
    Spark Plugs
    Spark Leads (front one was faulty)
    Battery
    Injector Seals

    Things I have checked (and sometimes double or triple checked):
    Timing (it's timed correctly and for the front cylinder)
    Injectors (have cleaned them and spray pattern looks fine, they are also plugged in correctly and squirting fuel into the correct cylinder)
    Compression (around 130psi for both cylinders)
    Fuel Pressure & flow (sits bang on 50 psi while the bike is running)
    TPS (performed reset with ECMspy multiple times and tried to set the idle but it's hard when it keeps backfiring and dying)
    Exhaust Valve (according to my mate this was faulty and disabled, I have enabled it, and it works, but it's position makes no difference to the backfiring)
    Earthing points (have cleaned them)
    Battery (is charging while running at about 13.4v)
    Coil (tested with a meter, resistance is in spec)
    Intake seals (I rotated the engine and checked the seals, put some non setting compound on them to make double sure they sealed)
    Throttle Body (cleaned off built up carbon in it)
    Wiring (all wiring looks ok and everything is plugged in properly, there are no signs of wear or cuts)
    Fault Codes (it is not showing any fault codes)
    Fuel (is new 95 and it has about half a tank)

    Any thoughts on other things to check or what could be the cause of the problem? Could it be the exhaust O2 sensor or exhaust seals? I'm stumped on this and don't have much left I can think of to check.

    Thanks in advance for any help.

    Kane.

  2. #2
    Senior Member Cooter's Avatar
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    Hello and welcome Kane. Thank you for a complete and well written post! Holy Lord it's nice to have a little info to go on Usually we get the FaceBook dropouts that post "HELP!! my bike won't start!, tell me what it is right NOW, you idiots!"

    These are pretty simple machines, you've done a great job of checking a lot of things, so let's try a couple quick checks and maybe we get lucky:

    You have ECMspy. Whats the current AFV and EGO correction? I would be OK with re-setting them both to 100.

    Repeated cold starts (even with a perfectly running Buell), will foul plugs quickly. You've probably just replaced them... but if you've tried to start it more than a couple times, do it again... every time. I know it's pain. The fact that it starts ok and then runs bad means they're probably OK, but it is free!

    The O2 sensor isn't active that fast on a cold engine, so it's not doing anything. However running bad will clog it fast, you might want to get one to swap out AFTER she's running better. Check the parts interchange thread for applicable numbers, or I have one I can send you.

    For the TPS re-set, I want to make sure you are performing the WHOLE process, (using the idle screw set to 0, first), re-set, set TPS degree... and not just punching the TPS re-set button? It will act exactly like you describe if not done correctly, and its possible the PO did that and screwed ya over (PUUNNN!) before you even bought it.

    Backfiring through both the intake and exhaust is most certainly a timing issue. Good that its statically timed correctly but the fact that it runs ok for 20 seconds and then starts the issue tells me a CPS (or the circuit) could be failing. CPS like to fail with heat. There is a single wire from the CPS to the ECM (GN/W), gets modified for timing each cylinder then it splits to 2 (Y/E and BE/O) and goes to the coil. Check continuity while wiggling (the wires, lol), and use a timing light shining on the sensor to see if it's jumping around while running. Alternatively just replace the sensor. I hate throwing parts at something but you're pretty far down the wormhole right now.

    This (Page B-19):
    Screen Shot 2021-12-19 at 10.08.50 AM.jpg

    from here: https://buellmods.com
    Last edited by Cooter; 12-19-2021 at 05:29 PM.

  3. #3
    Senior Member Barrett's Avatar
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    Sir: combustion ignition out the exhaust is an after-fire. Combustion ignition up thru the intake tract is a back-fire.
    After-fire typically ignition related with fueling secondary. Back-fire typically fueling related with ignition secondary.

    1-Your compression test, if accurate and done correctly, is revealing a motor in poor health. Was said test done with a quality compression gauge....full operating temp...throttle fully opened?
    2-A new or known healthy XB engine typically has 170-180 PSI. Yours is well below even the minimal acceptable level.
    3-Do you have after-fire.....back-fire....or both?
    4-Remove the front belt pulley black plastic cover...separate all electrical plugs...check and clean and lubricate every single one paying particular attention to burned/cracked/shorted wires and plug ends.
    4-With NO fault codes either active or historical and all the tests done as you stated...if this XB was in my shop....and the engine exhibits both after and back-firing......I would:
    Immediately perform a proper compression test
    Immediately perfrom a proper leak-down test which will reveal where the internal problem lies
    Immediately replace the ECM with a known good one containing stock mapping

    Other than well-below-acceptable cranking compression limits...all your indicators point towards a failing ECM.


    XB/XR CPS failure almost always accompanied by wildly fluctuating tachometer indicator. NO mention of that here.
    Last edited by Barrett; 12-24-2021 at 11:39 AM.

  4. #4
    Senior Member 34nineteen's Avatar
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    Quote Originally Posted by Cooter View Post
    Hello and welcome Kane.
    Welcome Kane.

    You may call me Hawke.



    Since you and your friend have rotated the motor on this, and you said you replaced the injector seals. Have you checked to make sure you installed the injectors correctly into the correct ports and the plugs on the correct injectors? I forget the orientation, but it should be spelled out in the service manual.

    The injectors have a red and a white band on them and since the spray is directional and if they are reversed they will either be spraying on the manifold walls or in the wrong port. Also, make sure the wires are on the correct injector. They were labeled from the factory, but verify the wiring color, in case the labels from the factory were wrong. Once again, verify this through the service manual.

    Go in peace. That is all.
    Last edited by 34nineteen; 12-20-2021 at 03:29 AM.

  5. #5
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    Thanks for the info guys, responses below:

    Cooter:
    The AFV and EGO were already both set to 100.
    I followed the full procedure using ECMspy and did not just press "Reset TPS".
    I don't have a timing light, and will have to get some spare time to test wiring continuity. Another friend of mine agrees it's probably the CPS sensor or wiring - I will get back to you on this.

    Barrett:
    The Service Manual I have states that anything over 120 PSI is fine for compression. Is there a difference between the XB12X motor and other XB's in this regard? I used a compression tester I bought years ago from a industrial told shop, it's never given me any trouble before and has provided accurate values in the past. I cracked the throttle open and used the starter to crank the bike over to get the values I stated.
    Unfortunately I can't perform a leak-down test and I can't get the bike up to running temp - If I can find a way to do this I will let you know.
    Yes, I have both after-fire and back-fire, but they occur independently. It's mostly through the intake though.

    Hawke:
    Yes, the injectors are installed into the correct ports (I couldn't find it in the manual but found out from the web, red is in the front port), the connectors are snug on them and the connectors are installed with the GN / GY wire on the rear injector.

    So, I think I will chase the wiring continuity for the CPS first, then visit my mate who has a timing light and see what's happening there. If that doesn't work then i will look for a new ECM.

    Thanks for your help guys, I will come back with any new developments.

    Cheers
    Kane

  6. #6
    Senior Member Cooter's Avatar
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    I should have a stock ECM I can send you if it gets to that point. My moneys on the CPS though, ECM's are pretty hardy unless physically damaged

    So remove and check your ECM. Ulys with a low seat would contact and crack the housing causing many issues as you can imagine. Theres a relocating bracket available in that case, but if it's broken don't mess around and just get one.

  7. #7
    Senior Member 34nineteen's Avatar
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    Quote Originally Posted by Kano-Moto View Post
    Another friend of mine agrees it's probably the CPS sensor or wiring - I will get back to you on this.
    Also check to be sure the CPS wire harness is not rubbing on the hole in the cam cover through which it passes.

    And, did you check the TPS for proper operation, it should move gradually through the voltage with no dead spots or "jumps" in the signal.

    Lastly rule out a grounding issue. To test, get a jumper cable and attach to the neg terminal and a bare metal spot on the motor.



    Hawke - out.

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    Just a quick update on some investigations I did:

    * Removed the ECM - it looks fine, no crack and no damage
    * Tested Earthing to motor in various places and the connector for the CPS, all read less than 0.2 ohms. So no earthing problem.
    * With the ignition switched on I am getting 5V at the connector for the CPS on both the Power and Signal wire for the CPS. I assume it's a digital hall effect sensor?
    * TPS operation is smooth through the whole voltage range.
    * there are no nicks, cuts, abrasions or damage of any sort on the wiring for the CPS.


    Question: Do you know of a place that I could get a cheap CPS sensor? I am in Australia and the sensor is minimum $150USD and then $80USD to post which makes it about $320AUD for a $5 sensor in a fancy housing.

  9. #9
    Senior Member 34nineteen's Avatar
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    Quote Originally Posted by Kano-Moto View Post
    Just a quick update on some investigations I did:

    * Removed the ECM - it looks fine, no crack and no damage
    * Tested Earthing to motor in various places and the connector for the CPS, all read less than 0.2 ohms. So no earthing problem.
    * With the ignition switched on I am getting 5V at the connector for the CPS on both the Power and Signal wire for the CPS. I assume it's a digital hall effect sensor?
    * TPS operation is smooth through the whole voltage range.
    * there are no nicks, cuts, abrasions or damage of any sort on the wiring for the CPS.


    Question: Do you know of a place that I could get a cheap CPS sensor? I am in Australia and the sensor is minimum $150USD and then $80USD to post which makes it about $320AUD for a $5 sensor in a fancy housing.
    $150 USD is cheap. You may be able to somehow take one from a Sportster and fuse it to your XB plate. Honestly, I dont really know what the difference is between the two. The timing cup is definitely different, but I never could figure out what makes the XB one so special. Perhaps it has to do with triggering the ECM for single fire?

    https://st-paul-harley-davidson-buel...ors-/-lighting

  10. #10
    Senior Member Cooter's Avatar
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    There signal wire having voltage is just a matter of where the engine is at rest. Testing it with a timing light or oscilloscope is the only way to do it accurately.

    I looked in the parts cross reference thread and didn't see a CPS option. Strangly I'm thinking the cylinder lacking Blast might work?



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