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Over rev when first started.

Discussion in 'XJ Technical Chat' started by Jonathon Kent, Jun 2, 2019.

  1. Jonathon Kent

    Jonathon Kent New Member

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    So i have a 1982 yamaha xj 650 with the YICS motor. I have been slowly working on this bike over the last winter. I rebuilt the carburetor and changed the oil and the rear end oil. However I cant seem to figure out why the bike immediately goes to 5k or better rpm when I first start it. I am getting very frustrated with this. I am part of the generation that didnt grow up with carburetor or even the likes of it. When I rebuilt the carburetor I did so with the Haynes manual. All of my float levels are perfect, my slides are in great shape, all jets are looking great cleaned them multiple times over. All of my idle air screws are set to 2.5 turns out due impart to having all stock jets. I broke the rack apart all of my seals are brand new. I am really at a complete loss as far as why this is happening. If someone could please help me. I would greatly appreciate it.
     
  2. StahlMaster

    StahlMaster Member

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    You do know about the mis-print in the Hayes manual don't you?
     
  3. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    1. The idle are screws are not idle air screws. They are idle mixture screws (yes it's pedantic, but common language is vital when working solely with the written word. I stopped trying to turn the tide years ago and call them what Yamaha calls them)
    2. Did you loosely fit the throttle plates and make sure they were centered before tightening the screws?
    3. Have you inspected the intake boots, vacuum line, and vacuum port caps for cracks or splits?
    4. Did you bench synch the carbs?
    5. When you set the fuel levels did you just set the measured height of the float, or did you check the actual fuel level using clear tubing on the drains?
    6. Did you put the air jets back exactly where they came from (the error mentioned above has to do with their location. All of the manuals have errors).
    These are the correct locations
    [​IMG]

    7. Are the choke cable and throttle cable hooked up properly (a common error is to swap them for each other at the carb).
    8. Did you check valve clearance? This isn't the cause of your problem, but if they haven't been done then getting the carbs tuned correctly will be somewhat pointless.
     
  4. Jetfixer

    Jetfixer Well-Known Member

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    When you state you rebuilt carbs, did you break the rack and replace butterfly seals? If you soak a carb with the rubber seals in place seals will start to dissolve and a vacuum leak is something that can cause erratic idle.
     
  5. Jonathon Kent

    Jonathon Kent New Member

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    No I do not know of the mis print. What is the mis print in regards to?
     
  6. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    See my reply above.

    There are other errors in the manuals (yes, all of them), but they don't pertain to your particular issue.
     
  7. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    From another thread.
    Throttle shaft seals and all other parts that are currenly available can be bought from XJ4Ever. The link to the catalog is in the upper right corner of every page. Start a conversation with @chacal for service.

    Leaving the petcock in the on position will not cause the fuel to flow into the crankcase unless the vacuum valve in the petcock is faulty, and one or more float needles is not sealing. I leave my petcock in the on position all the time as Yamaha intended. Fuel will only flow with the engine turning or an ouside vacuum source applied to the vacuum line.

    Homework time.
    Read This First
    IN THE CHURCH OF CLEAN
    THE SECRET LIFE OF CARBURETORS
    Inside your Carbs
    Replacing your Hitachi throttle shaft seals
    Setting the fuel levels


    Pleas keep us apprised of any work you are doing and what the results are. We can't help if we don't know what you've been doing.
     
  8. Jonathon Kent

    Jonathon Kent New Member

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    Small update, just got a hold of a dealership that has a fossil for a tech. He has done a little work for me, not the master mechanic that worked on it previously... They are going to take a look at the bike to let me know what they believe the problem is with this bike. That might be a good starting point for me... I am just stressed and need the help with this bike. I will continue to update everyone on the status of my bike.
     
  9. dkavanagh

    dkavanagh Well-Known Member Premium Member

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    Hey @Jonathon Kent I noticed you're in Rochester as well. I'm heading to the Carb Clinic tomorrow, but am generally around if needed.
    I had my carbs re-built by @hogfiddles in 2016. They've been flawless since!
    I can say that if you have have a sync or mixture issue, it's not impossible for one carb to bring the revs up for the entire engine since once one cylinder starts getting more gas/air, it will cause the others to pump more air through the carbs, which will happily adjust to deliver more fuel/air as well. (because of the slides that respond to air movement).
     
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  10. Jonathon Kent

    Jonathon Kent New Member

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    That's really good to know. I may look up @hogfiddles. I since found the butterfly valve on carbs 2 and 3 were just slightly open compared to carbs 1 and 4, I adjusted them and now I am around 2k for rpm, but that fluctuates. So clearly not the entire problem. But all the same, thanks for the input; this weekend I am going to be building a jig to set the carbs on to get them wet set, and making absolutely certain that everything is exactly as it should be.
     
  11. dkavanagh

    dkavanagh Well-Known Member Premium Member

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    cool. I did a simple jig with 4 deck screws into a block of wood, supporting 4 corners of the carb rack. Nothing too fancy.
     
  12. Jonathon Kent

    Jonathon Kent New Member

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    Do you still have it? Perhaps you could send some photos... for some food for thought.
     
  13. dkavanagh

    dkavanagh Well-Known Member Premium Member

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    Found it! I had a chunk of 4x6 post I cut off from another project. A 2x6 would be plenty big, or some plywood.
    Screen Shot 2019-06-07 at 2.38.45 PM.jpg
     
  14. Jonathon Kent

    Jonathon Kent New Member

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    That's pretty genius! Sometimes the simplicity should not be overlooked.
     
  15. Jonathon Kent

    Jonathon Kent New Member

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    This is what i ca 0608192122.jpg 0608191932.jpg 0608191932a.jpg 0608192122.jpg 0608191932.jpg 0608191932a.jpg me up with.
     
  16. Jonathon Kent

    Jonathon Kent New Member

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    I however keep having a particular problem. After I did a float height set and 17.00 mm and then went after setting the fuel level in the bowls. I started with one then worked my way down the rack. Upon the 3 rd carb I overadjusted and ended up with no fuel in the line when checking the fuel level. And carb number one began overflowing. This has happened on 4 separate occasions. Each time I would readjust the float not getting fuel and then another bowl would overflow. This grew very tiresome. I finally called it a might after working on this for 4 hours after work. Any suggestions would be appreciated.
     
  17. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    1.If you're going to measure with calipers you have to physically register them on the lower lip of the carb body or paralax will throw the measurement off.

    2. Have you replaced any of the original parts yet? Particualrly the float needles.
     
  18. Jonathon Kent

    Jonathon Kent New Member

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    Yes i replaced the original needles and seats. With the steel after market options that are available. The size on the seat and needle claim to be the correct size. And what is paralax?
     
  19. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    Paralax is the difference between what you see (based on the angle of view) and the actual measurement. Think it as if you were looking down a line of fence posts (no fence). Standing direclty in front of the fitst post you would only see a singel post, but step just a tad to either side and you begin to see more of them.
    IMO it's better to scribe marks onto the carb and bowl and use that as your scale.

    You might have a float that's hanging up, or a needle that isn't perfectly mated to the seat. You can use a bit of jewler's rouge or valve lapping compound to mate the needles to the seats. Just a tiny bit of the aforementioned polishing compound on the needle, and just a few spins of the needle against the seat (by hand).

    I'd wager that the float is more likely the culprit than the needles and seats. Inspect the #1 float, float tower, and float pin carefully. Polish out any roughness that you might find on the pin, the float hanger, and the inner portion of the towers where the float hanger can contact them. Also make certain that you have the floats in rightside up.
     
  20. k-moe

    k-moe Pie, Bacon, Burbon. Moderator Premium Member

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    And just to be sure, new washers were used when you installed the seats?
     

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