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  1. #1
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    Default HELP: carb idles then revs to 5k then dies

    The stock carbs on my 1976 KZ400 needed work.
    I had already planned on swapping for a single carb and already had a Keihin PE35 in hand.
    So, today, I spent a few hours building a manifold out of copper tubing.
    Slapped the carb on, hooked up a piece of tubing to a plastic bottle full of gas, and let her rip.
    The bike started and idled for about 10 seconds then revved up to 5k on it's own then died.
    This sequence happened 3 times in a row.
    Adjusting the idle screw seems to do nothing.
    What could be causing this? I'm assuming I will need to change the jets but, the idle then rev then die thing has me scratching my head.

    Secondly, I'm having trouble finding jets for this carb. Never bought jets before. What's a good source? All I know about the carb is that it is a Keihin PE35. Is that enough info to go off of or should I look for a part number on it?






  2. #2
    Jetblack
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    It's a racing carb, so you most likely don't have to go on a jet hunt...

    Make sure you're not putting the slide in backwards, or that it is hanging up on the idle screw, sometimes the spring like to get into the action and twist the slide out of place, when screwing the top on... if you back the idle screw out the slide should drop flat into the carb body where you cant see under it or past it. 1.5 to 2 turns in or to where it barely lifts to a small crack is a good starting point for adjustment.

    The other adjustments make be ok at this point... but the slide or cable is binding causing the high revs... the idle jet acts like a mini throttle so if it's too far in it will raise the slide to however many rpms it's height dictates.

  3. #3
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    First off, thanks a bunch Jetblack! You always seem to come through with help and knowledge.

    The slide has a slot that runs the whole length of it and there is a tab in the cavity wall that keeps the slide from twisting during it's operation.

    If I completely remove the idle screw, there is still a gap. It's eliptical shaped and is about 1/8" at the widest / in the middle.

    I don't have a throttle cable hooked up yet. To rev the engine, I was just lifting the slide with my finger.

    I will get a video today and post it. It runs smooth and idles kinda high at about 2500 then, after about 10 full seconds of idling, it revs on it's own. no matter what I do to the idle screw or move the slide manually, it revs to 5000 then dies.

    Like I said, I'll try to get a video today and post it up.

  4. #4
    Jetblack
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    Here's an exploded view of it so you can clean it well. Make sure it has all the little washers and o-rings springs etc. present and in the correct order. If you mean you have the cap off and sliding it by hand the cap being off will mess with the air mixture as well as not having a filter on it... you can palm choke the filter side to add a bit of restriction. You just want to make sure you don't have any intake leaks etc..

    So give it a good cleaning and bench adjustment if you haven't and set it up with the cap on etc., like it's gonna magically just run right... in other words no one off testing. You're not going to know how a bike or carb is actually going to run unless it is setup in final form... yeah pulling all that shit back off is a pain... but if cleaned proper once on; it comes down to adjustment and checking for vacuum leaks.

    The site the below image came from: http://www.allensperformance.co.uk/exp-pe.html


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    Thank you again sir.
    That picture is worth... well, a thousand words.
    I'll use it to do a thorough cleaning and check to make sure I have everything.
    When I was manually moving the slide, it was through the mouth of the carb. I did have the cap on.
    Looking at the picture above, it looks like I can move the clip (#9) and allow the needle to sit at different heights...
    I wonder if the needle being too low is not allowing the slide to bottom out?

    Anyway, first things first... the carb is back off the bike so, next free time I get, I'll get to disassembling and cleaning.

  6. #6
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    Got the carb torn apart tonight... Won't get a chance to actually do any cleaning until tomorrow morning or monday...
    Is there anything that I can soak this stuff in? I figured I'd go at it with carb cleaner and a toothbrush when I get the time but, in the meantime, seems like I could be soaking it... Simple green?


  7. #7
    Jetblack
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    Pinesol is my personal favorite really hot water 50/50 mix for the bodies; and acetone/nail polish remover for the jets in a seal-able jar it evaporates off pretty quick and melts plastic so a glass container. Let the main body parts soak over night rinse well hand rubbing with water then dry with a shop cloth then assemble and bench adjust.

    Look in the air jet passages make sure there are no O-rings/washers jammed up inside them.
    Last edited by Jetblack; 05-24-2013 at 10:33 PM.

  8. #8
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    This video is really good.

    http://www.youtube.com/watch?v=-2NIDcqv59I

  9. #9
    Jetblack
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    When learning shit seeing things being done is better comprehended than when read a lot of times...

  10. #10
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    definitely...
    I'm not one the lucky people that can learn without actually doing it or seeing it...
    there are a lot of things about these carbs that i've heard / read that i had a vague idea about and this vid made them a whole lot clearer.

  11. #11
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    well, got it all back together today and it's running 100% better.
    I can get it to idle at just under 2k.
    then, after a few starts, it got hard to kick.
    checked the plugs and they are black / sooty.
    so, i'm guessing, my next move is to lean it out.

    EDIT: come to think of it, maybe putting a filter on there would help
    Last edited by dooder; 05-27-2013 at 8:27 PM.

  12. #12
    Jetblack
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    Nice, the bevel at the bottom of the slide is a factory leaning method. Kerosene makes for a good plug/part/foam filter cleaner.

    If richness continues you can adjust that by the needle position


  13. #13
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    depends on what the carb was originally set up for, if it was a 2 stroke the jets are going to be completely wrong, you might get it to run but it will be way off, needle and needle jet are going to be the most inaccurate. The types of jets used for that application draw ALLOT of fuel at low and mid range throttle settings that are beyond the adjustment range of the jet needle with design differences in the needle jet that will require replacement.

    I would contact SUDCO and get their input for what jet you might need for this combination.
    Last edited by riceburner; 05-28-2013 at 6:51 AM.

  14. #14
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    Thanks Riceburner.
    I will be in touch with them...

    i'm going to need to eventually figure out my throttle situation because 1) the stock one is really coroded and I'm having trouble removing one of the screws. 2) I don't like the looks of it anyways.

    I'd like to go with something as minimal as possible.
    I really like these really simple ones:

    http://www.dimecitycycles.com/vintag...ly-68-2an.html

    http://www.dimecitycycles.com/vintag...-44-29480.html

    http://www.dimecitycycles.com/vintag...y-44-9771.html

    But I have no idea if they will work with a CR125 throttle cable.
    I'm pretty sure anything I do that's not stock will not be plug and play but, can one of these simple throttles be made to work with my carb? Specifically a CR125 throttle cable?

    If it turns out I can't use one of these types of throttles, I found this on ebay:

    http://www.ebay.com/itm/THROTTLE-NEW...bcb1c0&vxp=mtr

    Looks pretty much plug and play and, it seems like a good price to me. Or are there other suppliers that you guys might recommend?

  15. #15
    Jetblack
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    You're going from a dual carb to a single on a KZ400, normally that calls for quite a bump up in jetting, which a racing carb should already have going on but... did you happen to take note of the jet sizes when you had it apart cleaning it? Sizing should get cleared up pretty quick in that case, but another way to know...

    If you didn't note the jet size and can run it, screw the A/F screw in and if it dies before or when it seats, or dies when you back it out before it comes out... then most likely the jetting on it is perfectly fine as is and you need to make small adjustments like float level and needle height to clear up the small issues, if you can't get it to run through the range from idle to WOT.

    That cable mechanism should work, there quite a few that are just as cheap or cheaper that I think are the same throttle... if expense is an issue. I suggest a metal throttle housing, hard to tell if that one is or not, but I got a cable like that, off of there before it's waa little small in the stock throttle tube... but it probably fits the one that comes with it.
    Last edited by Jetblack; 05-28-2013 at 12:44 PM.

  16. #16
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    I did write down the numbers on the jets.
    the main jet was 142. could have been 742 but, I'm pretty sure it was 142.
    the slow jet is either a 68 or an 89. depending on which was was right side up.
    there was also a 4 pointed star symbol on the slow jet.
    so, it was either star68 or 89star.
    hope that makes sense...
    from what I've read, i think my jets are 142 and 68.
    although the slow jet number, whether 68 or 89 seems bigger than what most people are running with a similar carb.

  17. #17
    Jetblack
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    Quote Originally Posted by dooder View Post
    I did write down the numbers on the jets.
    the main jet was 142. could have been 742 but, I'm pretty sure it was 142.
    the slow jet is either a 68 or an 89. depending on which was was right side up.
    there was also a 4 pointed star symbol on the slow jet.
    so, it was either star68 or 89star.
    hope that makes sense...
    from what I've read, i think my jets are 142 and 68.
    although the slow jet number, whether 68 or 89 seems bigger than what most people are running with a similar carb.
    Well, screw the A/F in if it doesn't shut off before or when it seats it's too big... otherwise you should be good to go barring any vacuum leaks.

    You can refer to this thread if you haven't seen it one setting idle, a/f and pilot info: http://www.chopcult.com/forum/showthread.php?t=29958

  18. #18
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    Will do next time I get a minute to play with it. Probably tomorrow.
    Thanks for that link / thread Jetblack! i'll have to study it when I get a chance.

  19. #19
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    what did the carb come off of?

    On you low speed mixture screw, since it is located infront of the carb it is an AIR MIXTURE screw not fuel, so when you turn the screw in it will be richening the mixture, going out is leaning the mixture.

    2 strokes also run a very rich cut away on the slide, so it will most likely have to be replaced if the carb came from a 2 stroke.

  20. #20
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    it came off of an '84 CR125. 2 stroke i believe

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