1972 CT90 idles fine, runs bad at higher rpm

Discussion in 'CT90, CT110, ST70, ST90 Discussion' started by Lugnuts, May 9, 2018.

  1. Lugnuts

    Lugnuts New Member

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    I have a new carb. that adjusts fine, points are good, valves are adjusted. When I get to mid throttle, even with no load, it breaks down popping and running rough, no power. Could I have a bad condensor?
     
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  3. Rizingson

    Rizingson New Member

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    I've had the same problem that stumped me for a while, turned out to be the spark plug. Switch plugs and problem went away. I'd spent time looking at the carburetor for the problem.
     
  4. racerx

    racerx Administrator
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    These motors are a little outside my wheelhouse. With bikes, I've worked with magneto ignition, mostly. Rpm-dependent + load independent sounds like lack of voltage, at the plug. I'd definitely try a new plug, you've got someone Else's experience and that's the easiest ignition test going. While you're at it, ground the plug against the engine, while cranking it over (via the kickstarter); you should see a nice blue-white spark, jumping across the electrodes. Usually...but not always...a fouled plug, with healthy ignition, will have the spark jumping elsewhere. Similarly, a fat-n-snappy spark usually means that everything in the ignition circuit is healthy, including the condenser. The operative word, here, is usually. As rpm increases, there's less time to saturate the coil and condenser. That leaves battery voltage, the point contacts, and condenser health as the remaining variables.

    If you've got a voltmeter, check the battery voltage, too. With a battery ignition, battery voltage matters. The only practical way, for most of us, to test a condenser is by throwing a new replacement at the problem.
     
  5. Lugnuts

    Lugnuts New Member

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    OK, I appreciate the comments. It is a new plug and battery but we all know how long batteries sit on the shelf. Especially 6 volt. I will try your suggestions and get back to you guys.
     
  6. bruces

    bruces Active Member

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    I am no expert on those engines ,but I am wondering if the ignition advance is working properly
     
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  7. Lugnuts

    Lugnuts New Member

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    Thanks, I'll check that as well.
     
  8. motodevo

    motodevo Active Member

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    A point gap thats too wide will cause similar symptoms
     
  9. b52bombardier1

    b52bombardier1 Well-Known Member

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    Is it starving for DC volts after riding for a few minutes? Check it with a multimeter. But yea, definitely take a look at the spark advancer. Absolutely, you must use a Sharpie marker to make note of how the two major advancer sub-assemblies come apart and go back together. Otherwise, you have a 50-50 shot at reassembling it wrong. And you might think that this should not be a problem . . . it looks the same either way . . . right????? WRONG!!! If reassembled wrong, you will have a great spark but it will occur 180 degrees wrong in the four stroke combustion cycle.

    And you will say a lotta' really bad words and upset the people around you while you are trying to figure all of this out.

    Rick
     
  10. MikeCT90

    MikeCT90 New Member

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    I agree with the other that you want to make sure you're spark advancer is clean and can operate freely. If you do takes yours off to clean it and are not sure you are getting it back together correctly then feel free to take a look at a post I made on my CT90 blog on how to reassemble spark advancers. He is a link to the post: https://ct90restoration.blogspot.com/2017/10/ct90-spark-advancer-assembly-build-up.html

    There is also a possibility that maybe you're running a little rich and it might help to raise the clip one notch on the needle in the slide in your carb to lean it out a bit mid throttle. Here is a link to a post I did on carb adjustments that might be helpful: https://ct90restoration.blogspot.com/2017/10/adjusting-and-jetting-ct90-carb.html

    Mike
     
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