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Thread: Help! I Need Diagnostic Assistance

  1. #1
    Moderator Phil Tarman's Avatar
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    Help! I Need Diagnostic Assistance

    I noticed this once or twice on my way towards Portland and during the 3-Flags Classic, but it got much worse during my Kansas journey. Thursday night when I turned off I-70 and onto I-135 southbound towards Wichita, I was bucking a headwind and couldn't hold the speed limit with my cruise control. It's always disconnected if the throttle had to open past a certain point, but up until this started, that was almost always when going up a fairly steep grade at speeds of 75-80mph. But Thursday, it wouldn't hold 70 and when I tried to take over manual control, I realized that if I opened the throttle past the point which is usually good for 70-75mph on level ground, the bike would start to miss and slow down. That got really bad in the strong winds I was in Monday afternoon and evening and was bad today until I got to Colorado and the wind quit. Even with no wind, a speed of more than 70-72 would cause the bike to feel like it was missing until I backed off the throttle. Then it would accelerate up to about 70-72 again, but that was the max.

    I've been thinking about what could cause this and my first guess is bad spark plugs. I guess it could be something else in the ignition circuit. I'm inclined to think it's not bad gas -- that was my hunch when it happened in Oregon and on the 3-Flags Classic because it always seemed like it happened after I'd filled up. Not this time!

    What else could it be? What would you check first?

    Any suggestions are greatly appreciated!


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    Phil Tarman
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  2. #2
    Growing old disgracefully JQL's Avatar
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    Re: Help! I Need Diagnostic Assistance

    Could it be a faulty TPS (Throttle Position Sensor)?

    See:

    If you think it's plugs the quick and cheap "fix" (way to check) is to change the spark plug caps or make sure they are correctly seated. Always use the NGK ones.

    As an aside, I had the NGK caps and one was slightly corroded/tarnished inside which caused an interesting fault when the ambient air temperature was high (OK otherwise!).
    Last edited by JQL; 10-11-2017 at 05:51 AM.
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    John
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    Re: Help! I Need Diagnostic Assistance

    John, I guess I should have mentioned that I don't have an MIL light. So I don't think it's the TPS.

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    Graham gbrand42's Avatar
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    Re: Help! I Need Diagnostic Assistance

    My guess is a weak spark under load. First check in my case would certainly be plugs & caps/leads.
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    Re: Help! I Need Diagnostic Assistance

    At this point, without a MIL to guide you, it is a crap shoot. At 70, you are turning about 5K rpm. How does it act at 5K in other gears? Does it lose power at 5K in 1st, 2nd, etc.? Could be spark, could be fueling.... sorry I can't be of more help.

    Mike
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    Site Supporter junglejim's Avatar
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    Re: Help! I Need Diagnostic Assistance

    I agree with Mike. And, from your description I still don't know if it is consistent or intermittent. I believe Mike is on the right track to first determine if it is related to rpm or load. Inconsistent/intermittent faults are a real bugger to diagnose. If the miss is regular and steady it is probably an ignition problem on one cylinder. If the miss is not regular it is more likely a fuel problem. But as mentioned above the spark plugs are the easiest and least expensive place to start. Note the condition of the old plugs when you remove them. Next I'd take it to a shop and have them measure your fuel pressure - that's pretty easy. After that your friends at Sun Honda should be able to help. Good luck.

  7. #7
    Site Supporter dnktng's Avatar
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    Re: Help! I Need Diagnostic Assistance

    The only simple thing to add that hasn't been mentioned is the air filter.

    There is a good troubleshooting guide on page 24-3 of the service manual for "Engine lacks power" to get some ideas. A few of the ideas are tire pressure, wheel bearing, clogged fuel line or muffler or air filter or oil passages, worn out engine or bad compression, spark plugs, low oil level, ignition timing & faulty ECM or CKP or TPS, and overheating.

    Just because you don't have a MIL doesn't mean that the TPS (or some other sensors) is sending a correct position to the ECM for the actual position. It may not be the most likely suspect, but don't eliminate a sensor because you don't have a MIL.
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    Re: Help! I Need Diagnostic Assistance

    I'd start with the simple stuff.. bad-ish fuel... get some seafoam in there and run a tank through from a good dependable gas source and go from there.
    Don't sweat the small stuff, it's all small stuff"

  9. #9
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    Re: Help! I Need Diagnostic Assistance

    I'll add some Seafoam today.

    The miss seems to be consistent in all gears and is more related to throttle position than rpm. I think. I'll check that too.

    Thanks for the suggestions!s

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    Re: Help! I Need Diagnostic Assistance

    My money is on the ignition, especially if you've never replaced the plug wires. I'd be guessing that everyone's NTs are going to be due for plug wires pretty soon. I don't understand why but it seems that marginal spark is only a problem under load - like climbing a hill or into a headwind.

    It wouldn't hurt to replace the fuel filter if that hasn't been done recently.

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