1. Hello Guest. You have limited privileges and you can't "SEARCH" the forums. Please "Log In" or "Sign Up" for additional functionality. Click HERE to proceed.

Over Revving

Discussion in 'XJ Technical Chat' started by Towens, Jul 3, 2020.

  1. Towens

    Towens New Member

    Messages:
    19
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Great Falls, MT
    1981 Yamaha XJ550 Seca. Was running ok considering the idle screws are stripped so it had to idle at half choke to keep the rpms up enough. I let the bike sit for about 2 weeks while repainting some pieces and when I went to start it up again, it acted like I was going full throttle once the engine heated up a little. I turned it off and made sure the throttle cable wasn't hung up somewhere, which it wasn't, and tried again. This time as soon as it fired up it started revving at 5000 rpms and the only way to slow it down was to flood the carbs which only brought it down to about 3000 rpms. But then oil must've shot up the release hose because it started leaking out of the airbox.

    So what I've tried:
    1. Took carbs off and went to church with cleaning. Replaced gaskets and made sure all parts moved freely.
    2. Replaced petcock in case it was dumping gas.
    3. Replaced carb boots.
    4. Made sure airbox lid was tight and replaced soiled filter.
    5. Disconnected choke cable from the carbs to limit variable.

    Last night I tried starting it again and after 2 attempts my battery died without it starting. So I charged the battery and disconnected the fuel line just in case.

    Went out today and bike was hot from sitting in the sun with a cover on. Left fuel line disconnected as I assumed there was some left in the carbs from the night before. Started it and it fired up the first time but went straight up to 5000 rpms again and oil started coming out of the airbox again.

    I think the oil was a little over filled which might explain it coming up and out through the airbox but I can't for the life of me find what is causing the bike to shoot up the rpms right away. Anyone experience similar issues? Thanks for the help.
     
  2. Rusty81

    Rusty81 Member

    Messages:
    81
    Likes Received:
    23
    Trophy Points:
    8
    Location:
    Sarnia On, Canada
    Is the throttle adjustment screw under the carbs turned in and keeping the butterflies open? Try backing it off after you fire it up. (Or before). But look into new pilot screws if they’re stripped too
     
  3. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    sounds like your carbs are leaking into motor and oil check level sniff for gas smell. float needle may be bad or sticking.

    or , when first filling bowls on a 550 you may have to rap on carb bowls to keep floats from sticking and flooding oil sump , overflowing from air box

    if your using choke to start when bike is warm rpms will climb high
     
  4. Towens

    Towens New Member

    Messages:
    19
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Great Falls, MT
    Rusty: the butterfly adjustment screws look fine, they turn freely and open the flaps smoothly. They are fairly level straight across when wide open.

    Xj: I might just have to buy a kit for the carbs to rule the needles out. I tried knocking them slighty to make sure nothing was getting hung up. I was using the choke previously since I couldn't set the idle screws. They probably need to be sent to a machine shop to get pulled out. But I was basically using the choke to set my idle when it was running fine. But now with the choke cable not connected so it's fully off, the bike redlines right from the start of the engine is already hot.
     
  5. Jetfixer

    Jetfixer Well-Known Member

    Messages:
    3,173
    Likes Received:
    1,485
    Trophy Points:
    113
    Location:
    Nothern Indiana
    Have you synched your carbs, bench and then a running synch , with vacuum gauges, or a Morgan pro , or home made mamomter?
     
  6. Towens

    Towens New Member

    Messages:
    19
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Great Falls, MT
    I have not yet. I didn't really see a point when it was running since the idle screws were stuck in them so they aren't able to be adjusted.
     
  7. Jetfixer

    Jetfixer Well-Known Member

    Messages:
    3,173
    Likes Received:
    1,485
    Trophy Points:
    113
    Location:
    Nothern Indiana
    There is only one "idle" screw ..you used plural were you referring to the synch screws? Taking your carbs to "church" means breaking the rack full tear down , removing all the rubber seals , soaking carb body and reassemble with all new seals ....see my post on why you should replace butterfly seals.
     
  8. Towens

    Towens New Member

    Messages:
    19
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Great Falls, MT
    Ok so I had torn them almost all the way apart and soak the parts in carb cleaner. The first photo shows the top of the carb where there is a hole with a flathead screw. I guess I figured that was the adjuster to sync the carbs. These are Mikuni carbs but they do not have the large single idle screw I've seen on other xjs. Also just noticed that for some reason the last carb on the rack doesn't quite make it all the way into the boot between carb and engine.
     

    Attached Files:

  9. k-moe

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

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    Those are the idlee mixture adjustment screws, and they should be removed for cleaning,and the o-rings replaced. If they are stuck inplace they can be drilled out. I'll have to lookback and see whas size drill is requrired to pop the serewheads off without damaging the threads in the bore.
     
  10. Towens

    Towens New Member

    Messages:
    19
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Great Falls, MT
    That would be very helpful. I've tried soaking wd40 and heating them with a torch and can't get them to budge. But I also know that if I mess it up there goes a potentially saveable rack of carbs which is rather not have to replace
     
  11. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    machine shop can do it , I would take a new mixture screw with you for them to see.

    XJ550 carbs have the same Idle knob set up as the hatchi carbs. if you do not have one it is missing or you have wrong carbs
     
  12. Towens

    Towens New Member

    Messages:
    19
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Great Falls, MT
    K, when I get back to it tomorrow, I'll pull the carbs and take all around photos of it to see what you guys can come up with.
     
  13. Towens

    Towens New Member

    Messages:
    19
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Great Falls, MT
    Here are the front and back of the carbs
     

    Attached Files:

  14. k-moe

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

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    The tap for the idle mixture screws is M6 x 0.5, which is normally drilled with a 5mm bit. I think I used a 4.5mm bit to give me some wiggle room in case I didn't get on center.

    Now this was for a 750 carb, but I'm pretty sure that the thread diameter is the same on your 550.

    [​IMG]
     
  15. Towens

    Towens New Member

    Messages:
    19
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Great Falls, MT
    Awesome, thanks for that K-moe.

    So I took the new carb to engine boots off and put my old cracked ones back on and the revving stopped and its back to idling at normal. I'm guessing the new boots were just a hair shorter and there was some air leaking. However it's fixed! I guess I'll have to go through chacal to get new boots instead of eBay.

    Thanks for your guy's info though, will definitely help get the carbs to tip top shape.
     
    Rusty81 likes this.
  16. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    Last edited: Jul 5, 2020
  17. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    did you buy those inexpensive ones from ebay?
     
  18. Towens

    Towens New Member

    Messages:
    19
    Likes Received:
    2
    Trophy Points:
    3
    Location:
    Great Falls, MT
  19. k-moe

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

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    There is no seperate assembly for the mixture screws; each carb has its own.
    The synch screws are part of the throttle plate linkages.
    Thee idle speed screw is a seperate part that mounts to the body of the 3 carb.
     
    Towens likes this.
  20. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    the nipples leak on those
     
    Towens likes this.

Share This Page