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Thread: 350x with 400ex carb dying at wide open

  1. #1
    Join Date
    Dec 2019
    Location
    Jackson MI, USA
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    54

    350x with 400ex carb dying at wide open

    Hey guys, unfortunately my 85í 350X with a 400ex carb has developed an issue where it will die after itís been at WOT for more than 7-8 seconds. By looking at the plug I can tell itís running really lean, and it has one small dead spot in the throttle if you gun it right off of idle. The dead spot has always been there, as I had to jet the idle jet up to a #45 right after I got the carb (itís a Chinese knock off). The stock main jet has been perfectly fine, so Ive never changed it.

    The bike runs great, even at wide open, so I doubt jetting is my problem. Like I said, it only dies after 8 seconds or so of being at wide open. Anybody else have this issue? Iíve already tried increasing fuel flow from the tank, and cleaning the screen in the inlet of the carb as Iím suspecting that Iím running out of fuel in the bowl. If anyoneís had this issue or has any advice, leave a reply! Thanks!

  2. #2
    Join Date
    Jan 2009
    Location
    ohio
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    1,732
    You sure your tank is venting ?
    If its on the internet its got to be true they can't put any lie's on the internet

  3. #3
    Join Date
    Jul 2005
    Location
    Manheim, PA
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    5,651
    YOU NEED MORE FUEL. You can not put a carb on a different model machine and think it will run correctly. I put a 2014 OEM brand new 400ex carb on a healthly mild built 300ex...i ended up needing 45 pilot and 178 main....that's crazy huge compared to a stock 400ex. (38 pilot / 138 or 142 main stock) You should be around the same area depending what exhaust and air filter. Chinese carb stuff is not designed like OEM stuff. The sizes of jets are not even close. I only buy jets from www.jetsrus.com

    If you are running corn93 = ethanol gas, it will burn hotter than normal non ethanol gas. The hotter burn does not help to jet, it can leave the white electrote barely tan on a properly jetted.

    Buy a range of jets, start fat and work your way back. You are just sucking fuel faster then carb can keep up. Also if you have a metal tank with rust in it and old petcock i'd replace with plastic tank and new petcock. I just fixes a scramber 500 which needed 10 mins to warm up on choke in summer time...its fuel filter and petcock were clogged so bad it was just starving for fuel.

    Alot of carb issues are not carb issues at all. They are intake, or maintenance issues.

  4. #4
    Join Date
    Apr 2011
    Location
    Mexico
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    Remove your fuel line and let it drain into a bucket for a minute to see if flow decreases with time. If it does, remove the fuel cap and see if that improves it. If it does you likely have a blocked vent. If it doesnít help then you likely have a plugged petcock or in-line filter.

    If fuel flows freely and consistently for a minute I would remove the carb and look for junk in the float bowl. You may have a large piece of something that remains on the bottom at low rpm but is getting sucked up against the opening of the main jet at WOT and then falling back to the bottom of the bowl once the engine stops.

    If itís not that, check your float height and if itís good you can try a larger main jet, but I doubt thatís your issue if you can get 8 seconds of strong running time out of it before power drops off.

    You can try opening up you fuel screw a 1/4 turn at a time to get rid of you flat spot, but if you get to three turns out and itís still there you may need a larger pilot, or to raise your needle.
    Last edited by El Camexican; 05-24-2023 at 08:30 AM.
    It sucks to get old

  5. #5
    Join Date
    Nov 2019
    Location
    Florida
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    1,022
    I have seen aftermarket petcocks that are half the size (output) as the OE petcock, I run a Honda 400EX carb on all my 350x's.
    The smaller petcock may also have a small fuel line.

    MrC.
    mrc_builds on YouTube Channel

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  6. #6
    Join Date
    Dec 2019
    Location
    Jackson MI, USA
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    54
    So let me explain some of the troubleshooting Iíve done. I have indeed checked to see if my fuel cap is venting, and it is, and flow from the tank is a steady strong stream (even after 1 minute +). I do agree with it needing bigger jetting, as it does appear to be running lean. Iím just unsure at this moment what else could be wrong.

    The float doesnít have any holes or faults that I notice, however the float needle looks as though itís seen better days. When I pulled it out it had a white ring right where it contacts the seat, something that I havenít really seen before. What Iím wondering is if my needle is sticking to the seat. My grandfather has said that heís had to polish needle seats in the past to keep them from causing the needle to stick, but I believe that was with a brass needle as well as a brass seat. My needle is rubber, theoretically removing the idea that the seat needs to be polished. Nevertheless Iím going to attempt to investigate that today if I have the time to.

    Iíve heard of people having issues having to do with air filters and air boxes, but it has the stock airbox and the same filter from when it used to run good. And the filter is indeed clean.

    Right now Iím going to keep investigating, and if I donít find anything Iíll tear my carb down all the way again and see if maybe thereís a clogged port somewhere that Iím missing.

  7. #7
    Join Date
    Dec 2019
    Location
    Jackson MI, USA
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    54
    Quote Originally Posted by MrConcdid View Post
    I have seen aftermarket petcocks that are half the size (output) as the OE petcock, I run a Honda 400EX carb on all my 350x's.
    The smaller petcock may also have a small fuel line.

    MrC.
    By the way, good content man! Love the channel.

  8. #8
    Join Date
    Apr 2011
    Location
    Mexico
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    8,972
    Quote Originally Posted by Fixer_Upper View Post
    So let me explain some of the troubleshooting I’ve done. I have indeed checked to see if my fuel cap is venting, and it is, and flow from the tank is a steady strong stream (even after 1 minute +). I do agree with it needing bigger jetting, as it does appear to be running lean. I’m just unsure at this moment what else could be wrong.

    The float doesn’t have any holes or faults that I notice, however the float needle looks as though it’s seen better days. When I pulled it out it had a white ring right where it contacts the seat, something that I haven’t really seen before. What I’m wondering is if my needle is sticking to the seat. My grandfather has said that he’s had to polish needle seats in the past to keep them from causing the needle to stick, but I believe that was with a brass needle as well as a brass seat. My needle is rubber, theoretically removing the idea that the seat needs to be polished. Nevertheless I’m going to attempt to investigate that today if I have the time to.

    I’ve heard of people having issues having to do with air filters and air boxes, but it has the stock airbox and the same filter from when it used to run good. And the filter is indeed clean.

    Right now I’m going to keep investigating, and if I don’t find anything I’ll tear my carb down all the way again and see if maybe there’s a clogged port somewhere that I’m missing.
    You may have answered your own question. Usually the needle and seat stick when the vehicle has been parked for a while, but there is a possibility that it isn’t fully opening, and therefore restricting the flow of fuel. I’ve also polished them in the past to clean them, toothpaste works great, but once they start leaking due to wear you need to change them. Do yourself a favor and replace it before you bother doing any more trouble shooting.
    It sucks to get old

  9. #9
    Join Date
    Jul 2005
    Location
    Manheim, PA
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    5,651
    Clean it all up, dont be scared to just jet it pretty fat while you are in there. Run a new fuel line around the carb in a spiral so you can get the tank off and on easy.

  10. #10
    Join Date
    Dec 2019
    Location
    Jackson MI, USA
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    54
    Quote Originally Posted by El Camexican View Post
    You may have answered your own question. Usually the needle and seat stick when the vehicle has been parked for a while, but there is a possibility that it isn’t fully opening, and therefore restricting the flow of fuel. I’ve also polished them in the past to clean them, toothpaste works great, but once they start leaking due to wear you need to change them. Do yourself a favor and replace it before you bother doing any more trouble shooting.
    Yeah, that’s what I’m going to do. I’ll likely replace the needle too because its the needle that came with the carburetor, which is made in china. Aka, the needle is 3.5 years old and was made of Chinese junk to begin with.

  11. #11
    Join Date
    Dec 2019
    Location
    Jackson MI, USA
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    54
    Quote Originally Posted by oldskool83 View Post
    Clean it all up, dont be scared to just jet it pretty fat while you are in there. Run a new fuel line around the carb in a spiral so you can get the tank off and on easy.
    It used to run perfectly before it started this WOT , but since then it’s appeared to be lean.

  12. #12
    Join Date
    Apr 2011
    Location
    Mexico
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    Quote Originally Posted by Fixer_Upper View Post
    Yeah, that’s what I’m going to do. I’ll likely replace the needle too because it’s the needle that came with the carburetor, which is made in china. Aka, the needle is 3.5 years old and was made of Chinese junk to begin with.
    Good plan, needles need to be perfect, any bend or scratch will change how they work.
    It sucks to get old

  13. #13
    Join Date
    Aug 2016
    Location
    East of Worcester ma
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    1,311
    I might as well add my 2-cents... It's a Chinese carb just buy another $40 dollar new one not knowing that any replacement parts u buy may not fit it anyway. You're lucky to have gotten 2-3 yrs out of it.

    shep

  14. #14
    Join Date
    Jul 2005
    Location
    Manheim, PA
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    5,651
    There are two sizes of 400ex carbs 35mm and 38mm was used on the later models with the A/F screw plugged with a welch plug for no adjustment due to emissions. If its chineese carb good freaking luck. Knew a guy with one on a 350x had nothing but issues. Went to OEM 400ex jetted correctly ran perfect.

  15. #15
    Join Date
    Dec 2019
    Location
    Jackson MI, USA
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    54
    I am looking into simply replacing the carb completely with a real kehein carb, I’m just not sure if I wanna bite the bullet yet. Price tag is a little steep to me ��.

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