1985 CT70 goes wide open throttle on its own...

Discussion in 'Tech Area' started by 85CTrider, May 22, 2020.

  1. 85CTrider

    85CTrider Member

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    I have a super nice 85 ct70.. yes 85... it usually runs perfect... now it likes to just start going WOT on its own after warmed up... it starts on first kick, then i dialed in the idle so it idles fine, air screw is out like 2.5 turns. bike runs fine.... i go for a short ride and come back,,, put it in neutral. then all of a sudden it revs to wot.??? the throttle slide is not stuck,,, the throttle handle goes back and forth with spring in it. all the carb and manifold parts and tight together with good seals/gaskets. carb is tight on air box looks good other then rubber boot a bit hard but dont think that should cause this... is it an air leak>?? and if so, why does it start fine until i ride a block then starts to rev high?? can it be anything other then air leak? stuck float? can throttle glide be stuck but handle still spring? its scary when wot and i have to kill it by turning off..
    any ideas???
     
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  3. kirrbby

    kirrbby Well-Known Member

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    Air leak would be a good first guess.
    You could also check the needle and slide to make sure the clip is still on the needle, the Wclip is still in place, and the spring is springing.
    In order to rev, the engine needs air, and fuel.
    But if you only add fuel, it SHOULDN'T have enough air to rev, IF the slide is closed.
     
  4. 85CTrider

    85CTrider Member

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    yo kirby.. i have have this issue sine last summer when i covered up the bike for another year.. now same issue... my question is - if the throttle grip has spring in it when i turn the grip - does that mean its not stuck and ok?? or can the grip still have perfect spring in it and there be a problem? i may have lost a throttle slide part when taking it apart a bunch of times... like the w clip or c lip etc... but every time i take the slide out i put it back in perfect so it slides all way down..
    it cant be a fuel problem as bike will run as long as i have rev controlled... as for air leaks - the bike runs fine until it warms up.. i go for a ride down the block come back - put in neutral and then it revs like mad and scares me to shut it off.... am i missing something here????
     
  5. racerx

    racerx Administrator
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    The answer to that question is "yes".
    Now, we need to figure out what you're missing. Based on what you've said, it sounds like the return spring or/and retainer clip (or plate) are missing...allowing the slide to rise inside the carb. A few detail pix might be very helpful.
     
  6. 85CTrider

    85CTrider Member

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    ok.. i will pull slide out and examine it more closely.. but i know the spring is there as there is spring in the grip.. can it still be missing a clip and have spring in it??
     
  7. racerx

    racerx Administrator
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    Yes, easily.
     
  8. 85CTrider

    85CTrider Member

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    Here are all photos.. anything out of ordinary other than black plug? is there a gasket in the choke area? IMG_0217.jpg IMG_0221.jpg IMG_0223.jpg IMG_0224.jpg IMG_0226.jpg IMG_0217.jpg IMG_0221.jpg IMG_0223.jpg IMG_0224.jpg IMG_0226.jpg
     
  9. red69

    red69 Well-Known Member

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    Is there a gasket under the carb cap?
     
  10. 85CTrider

    85CTrider Member

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    there is but it looks pretty flat... - you mean where the throttle cable goes in the carb?
     
  11. red69

    red69 Well-Known Member

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    Yes, and it is a flat gasket, not an o-ring.
     
  12. racerx

    racerx Administrator
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    After thinking about this, I wondered if you weren't dealing with "whiskey throttling"...unintended throttle application due to the throttle cable being pulled tight, when the front end is turned to the LH limit. That's an easy thing to check.

    Might be an optical illusion but, it looks to me like your return spring is ~10mm short. It should be under significant compression, even with the cable fully extended, beyond idle. If it doesn't fit snugly against the "W" clip, engine vibration could allow partial throttle opening. It doesn't have to be WOT, to seem like it is. Under no load, 1/4 throttle is enough to send rpm skyrocketing.
     
  13. OLD CT

    OLD CT Well-Known Member

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    It would be a good time to see if you can raise the needle clip one notch to lower the needle. That will cure the black plug blues.
    When was the last time you lubed the cables, The throttle cable looks dry and dirty. One broken cable strand can give you symptoms like you described.
     
  14. 85CTrider

    85CTrider Member

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    Great ideas guys. But here are more details. Whisky throttle not likely as it happens with steering straight and not turned left. As for cable. It’s slick and springy as possible. There is perfect spring in the grip. I have had many bikes and the throttle works fine. As for the spring it’s stock. I have had this bike for 10+ years. No changes to parts or carb. My only thinking is air leak. But all parts are put together so good. Where would most likely spot be? I only put throttle cap on by hand twist. Should it be any tighter? Air box rubber is crusty a bit. Manifold is good. Where do I look??
     
  15. Gary

    Gary Well-Known Member

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    Start it up, take a unlit propane torch and aim it anywhere you suspect a leak engine note will change when propane enters engine. Leak will be after slide,leak before slide like the connection between carb and filter will not affect running
     
  16. 85CTrider

    85CTrider Member

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    Ok. Put a new plug in it. Started right up. I dialed in idle and air screw after it was running very high. I then sprayed carb cleaner over all possible leaks areas and no changes in speed whatsoever. Rode down street and back and let it sit. It idled perfect. No high reving. Weird. Seems fine. Did the carb cleaner spray do something. ?? The last thing I messed with that could cause this was the air box. I bought a new filter last year.
     
  17. racerx

    racerx Administrator
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    It takes next to nothing, in terms of spring pressure to keep the throttle closed. With the idle stop adjusted to allow the slide to close completely, the slide should close with an audible click, if cable tension is released quickly enough. You could remove the handlebar end of the cable, then pull & release it by hand.

    This phantom throttling only makes sense, imo, if the return spring is too short. Try removing the cable from the throttle slide, then install the slide & spring. If the idle speed keeps varying as you say, the spring is not returning the slide to idle position...and that is an easy fix.
     
  18. kirrbby

    kirrbby Well-Known Member

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    How about this possibility. While running/riding, the intake is usually cold. When you stop and idle for a while, it heats up. Maybe it's leaking when it gets hot...metal expands, maybe warps a bit.?
     

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