Idles fine but will not accelerate

Discussion in 'Tech Area' started by JohnnyT, May 27, 2018.

  1. JohnnyT

    JohnnyT Member

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    My K1 ct70 will start right up,idles fine, but sputters and will not rev up when I give it gas. Rebuilt carb. No change. Replaced carb with a quality new carb. Still no change. Replaced points too. Any suggestions?
     
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  3. red69

    red69 Well-Known Member

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    If you were talking about a model airplane engine, I'd say that it is too lean on the needle valve. Did you replace the condenser and maybe applied too much heat to the solder joint? If so, do it again with a new condenser and use less heat.
     
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  4. motodevo

    motodevo Active Member

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    Check your points gap again. Had similar issues on a fresh motor (new top end, points, etc), idled perfectly but popped and farted when revved up. I was sure it was carb related, cleaned carb multiple times, swapped carb for good known one, changed plugs, valve clearances, timing lined up correctly, etc. Ready to give up, checked the points gap, that i was certain was spot on one more time as i had literally checked everything else multiple times over and over. Was set way too wide, closed up the gap and problem solved. Many hours over weeks spent on that one.
     
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  5. JohnnyT

    JohnnyT Member

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    Checked the points again. Adjusted them a couple times. Didn't solve the problem. To me it sounds like it's missing and not firing right. New plug. New carb.
    Might be a coincidence, but all started after reinstalling the exhaust. Put note different mufflers without any change. Don't know what that would do to cause the problem I'm having. What else "ignition-wise" could it be? Coil? Im stumped and frustrated.
     
  6. OLD CT

    OLD CT Well-Known Member

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    Check the timing marks on the cam sprocket and the flywheel, @ TDC. you want to be sure they line up with the notches in the head/flywheel cover exact.
     
  7. racerx

    racerx Administrator
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    I would temper the definition of "exact" when applied to cam timing marks. With a lot of engines, even with all-new parts, the cam sprocket will be a few degrees away from precise alignment at TDC. As long as it's less than 1/2 tooth "off" you should be good-to-go. A full tooth off will be easy to spot.

    FYI, the only way to adjust a partial mismatch (can't be improved by moving the cam sprocket one tooth) is by R&R-ing the crankshaft timing sprocket. That takes a lot of work, experience and specialized shop equipment. Unless the crank sprocket was R&R'd already, it should be fine as-is.
     
  8. lukelaw1

    lukelaw1 Active Member

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    All good advice
    double check the valve lash on intake and exhaust. Pull carb off intake and put your thumb over the intake hole and kick over you should feel your thumb suck in and feel resistance, sucktion, as you remove you thumb.If not you have a vacuum leak at intake to head gasket or a valve hanging open
     
  9. racerx

    racerx Administrator
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    One typical aspect of a vacuum leak is that they typically manifest as hard starting and weak/wildly unstable idle. Once the revs build, the amount of "false air" becomes negligible. Might be worth looking into the intake port, for signs of blackening, which would indicate reversion, from a leaking valve. Here, again, the effects would be more noticeable at low rpm, start-up especially. And, I reckon you'd notice lack of compression, via kickstarter feel...IOW, it'd be difficult telling when the engine was on the compression stroke.

    Valve lash, OTOH, could do this and you'd probably not keep the engine running long enough to result in stalling, when a valve no longer seats, due to thermal expansion.
     
  10. JohnnyT

    JohnnyT Member

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    What would the symptoms of a bad coil be?
     
  11. racerx

    racerx Administrator
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    Most typically, there'd be no spark. Coils are normally all-or-nothing, they either work, or they don't. And Honda coils are incredibly reliable. If you have an ohmmeter, you'd most likely get an "open" reading, i.e. infinite resistance, same as if the test leads were in contact with nothing but air. It is possible...at least in theory...to get a much-higher-than spec resistance number; in the real world, however, I've yet to see a coil test as such. I've been working on small engines longer than the CT70 has existed as a model.
     

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