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  1. #1
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    Default KZ400 won't return to idle

    I couldn't find an issue like this in the forum for my bike; but excuse the re-post if this has already been answered. I chopped a 78' KZ400 and finally have it running. It has straight pipes and screened velocity stacks. I have not yet changed the jetting on the carbs but plan to run 70 primary main (current is clear but too corroded outside to read#), 95 secondary main (current) and 35 pilot (Current and plugged). Here's my issue: Once it's warmed up it starts and idles fine; but after it's revved once it never returns to that low idle.

    I'm only running it with a single throttle cable; but its not sticking there. Throttle cable has acceptable play/no resting tension. I took it for a run around the hood and it takes off just fine, shifts fine, doesn't pop or foul. Upon stopping it will not return to idle, which makes for one fun rocket start. So I basically built a death trap, I guess.

    Any ideas as to what I'm missing?

  2. #2
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    Slide may be sticking might check there I had one I bought guy said it wouldn't run right and needle was sticking. Also try spraying some carb cleaner around carb and boots may have vacuum leak good luck

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    Thanks for the tip. Appreciated.

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    Quote Originally Posted by Cascadian View Post
    Thanks for the tip. Appreciated.
    don't go crazy with that carb cleaner. it'll eat up the rubber.. just a quick blip as it's running on each intake boot at the engine.

    since you have velocity stacks stick your finger in there and move that slide up. see if it snaps back.. if not then clean the hell out of those carbs. check the throttle as well.. make sure it closes.. make sure that butterfly isn't loose/broken...

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    Quote Originally Posted by loddytoddy View Post
    don't go crazy with that carb cleaner. it'll eat up the rubber.. just a quick blip as it's running on each intake boot at the engine.

    since you have velocity stacks stick your finger in there and move that slide up. see if it snaps back.. if not then clean the hell out of those carbs. check the throttle as well.. make sure it closes.. make sure that butterfly isn't loose/broken...
    Definitely won't overdo it on the cleaner. Slides snap back just fine and everything seems to be clear. Should the needles be tight in the slide or have some play?

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    you mentioned only using a single cable. Did it start as a dual? Check/replace the return spring and if there is no sort of return spring it might help to rig one up. Maybe if it has one put a stronger one on?

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    Should have a little wiggle a least mine does

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    Quote Originally Posted by deathmetaldan View Post
    you mentioned only using a single cable. Did it start as a dual? Check/replace the return spring and if there is no sort of return spring it might help to rig one up. Maybe if it has one put a stronger one on?
    Return spring is there and snaps back like a charm. Idle screw backed all the way out doesn't work either.

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    starting to sound like a vacuum leak since the idle screw wont do anything. Im guessing you checked that the choke works as it should or maybe that you dont have the choke turned on by mistake?

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    Quote Originally Posted by deathmetaldan View Post
    starting to sound like a vacuum leak since the idle screw wont do anything. Im guessing you checked that the choke works as it should or maybe that you dont have the choke turned on by mistake?
    Choke only has an effect on initial idle and start up. The stuck rev happens with choke closed and wide open. Weird.

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    yeah, i would say vacuum leak if it just comes down slow but not all the way. Check the carb boots over VERY well, its very easy to miss a split, check the carb tops to make sure they are on all the way and the rubber gasket isnt pinched.

    I had a virago that had a cracked carb boot and spraying it with wd40 made the problem go away. You could try that (spray it all over the rubber) before you go ripping everything apart. (assuming you dont like taking things apart)

  12. #12
    JetBlackII
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    Um how did you set the idle? The bike should be fully warmed up, a/f screw(s) set to around 2-2.5 turns out from lightly seated to start with; then set the idle. If you don't do it fully warmed up; it can shut down on deceleration, or just sitting at a light/stop etc.

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    Quote Originally Posted by JetBlackII View Post
    Um how did you set the idle? The bike should be fully warmed up, a/f screw(s) set to around 2-2.5 turns out from lightly seated to start with; then set the idle. If you don't do it fully warmed up; it can shut down on deceleration, or just sitting at a light/stop etc.
    As you said. Idle was set warm. Fixed vacuum issue at boots, which helped a bit. noticed a pretty bad oil leak from the top end and inconsistent exhaust pressure from pipes. At this point I'm thinking I need to go over the engine and make sure it's not a bigger issue. I having a hard time figuring out how it would even idle if it's something in the top end, let alone idle variably and too high. Then again, I'm new to this so what do I know.

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    Quote Originally Posted by Cascadian View Post
    As you said. Idle was set warm. Fixed vacuum issue at boots, which helped a bit. noticed a pretty bad oil leak from the top end and inconsistent exhaust pressure from pipes. At this point I'm thinking I need to go over the engine and make sure it's not a bigger issue. I having a hard time figuring out how it would even idle if it's something in the top end, let alone idle variably and too high. Then again, I'm new to this so what do I know.
    we all started somewhere. No harm in asking, somebody probably had the same problem before. If you could post pictures of what you found that would probably help everybody to help you

  15. #15
    JetBlackII
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    if you have oil in the top end and differing pressure at the exhaust; compression test that shit... could be bad/broken ring or the valve guides need replacing. Those are the A typical reasons for getting oil up top where the shit doesn't belong.

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