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sudden idle speed increase

Discussion in 'Norton Commando Motorcycles (Classic)' started by o0norton0o, Sep 13, 2019.

  1. o0norton0o

    o0norton0o VIP MEMBER

    Joined:
    Apr 27, 2015
    Went for a test ride today. Started fine, idled fine. I went a few miles and when I slowed down and pulled the clutch in, the idle speed was racing. I thought maybe that one of the choke cables snapped, but the bike was running fine, just suddenly idling very high.

    Not one to ignore mechanical issues, I turned for home. I pulled the air filter to have a look. Slides and needles went up and down fine. Chokes were both up and the cables were working. The only thing was out of order was that on one slide stop screw seemed to be holding the slide up higher than the other side. I decided to pull the stop screw and it turned out with no friction, and the rubber O-ring looked skinny. I put a new one in it's place, started the bike, and readjusted the idle stop screw, it idled perfect again....

    Could the engine vibration over the course of riding a few miles cause a stop screw with a worn O-ring to turn more than just a little bit???

    OR, is it more likely that a cable got caught up on something under the tank and held the slides up slightly, and my working the cable back and forth freed the cable.... and the screw probably wasn't the reason for the high idle??...

    I wonder if anyone ever had this significant quick change in idle speed, and if so then what was the cause?

    thanks
     
  2. Tornado

    Tornado VIP MEMBER

    Joined:
    Dec 5, 2017
    Not exactly your situation but I discovered a slide getting jammed at wide open position shortly after shutting down after a hot day/spirited ride. Just prior to starting up i cranked the throttle grip but it didn't immediately return to closed position. Turned out my carb was too hot to touch after 10-15 min heat soak and this must have distorted the slide bore enough to cause it to stick at fully open. This was with a single Amal concentric setup. Not sure it would have happened while riding as heat not building up on the carb.
     
  3. ashman

    ashman

    Joined:
    Jul 11, 2010
    One of the big problems is that the carbie's vibrate and over time jets, needles as well O rings wear out and need replacing after 43 years with my Norton every 2 or 3 years I replace all of them with a carbie kit to keep my Norton running good as most of its life was a everyday rider, its all about maintenance if you don't want problems and even carbie bits wear out.
    The needles vibrate and cause the jets to wear oval so important to replace them every so often.

    Ashley
     
  4. maylar

    maylar VIP MEMBER

    Joined:
    May 13, 2007
    An air leak will do that. Check manifold bolts and the balance tube between them, a cracked hose will suck air and idle speed will rise. BTDT.
     
    Last edited: Sep 13, 2019
  5. maylar

    maylar VIP MEMBER

    Joined:
    May 13, 2007
    I'd wager that a poorly sealing o-ring would allow air to be sucked in. If the screw moved it wouldn't likely go inward.
     
  6. o0norton0o

    o0norton0o VIP MEMBER

    Joined:
    Apr 27, 2015
    The last time I had a similar scenario it was a cracked intake manifold that was pretty invisible. This time when I got home I looked with a flashlight and didn't see any cracks, but I didn't see any last time either. I wonder if I'm over tightening the manifolds and the head expansion is breaking them. I have the proper insulators on the manifold joint.

    It's going to rain for the next week, so I bet I don't get to ride for a while, besides having a million things to do. So, finding the cause and the solution will have to wait for a decent day to do some test rides. It would be nice if it was just needing new O-rings, but that doesn't seem like it would cause such a noticeable change.

    I was so sure that the choke cable had broken when the idle changed that I turned the bike off when I got home and took the air filter off to check that. The choke cable was intact and the choke bodies were where they belonged.

    Thanks Maylar, I'm thinking air leak (again) It's such a pain in the ass to disassemble the carbs, but I guess it could be worse...
     
  7. maylar

    maylar VIP MEMBER

    Joined:
    May 13, 2007
    Did you ever identify where the crack was? I have a random intake leak too that I can't identify the source of. Everything "looks" ok.
     
  8. Douglass Harroun

    Douglass Harroun

    Joined:
    Sep 12, 2008
    Could the engine vibration over the course of riding a few miles cause a stop screw with a worn O-ring to turn more than just a little bit???

    I wonder if anyone ever had this significant quick change in idle speed, and if so then what was the cause?

    thanks[/QUOTE]
    ---------------------------------------------------------------------------------------------------
    Yes the O- ring can lose grip and turn IN and raise the slide. I had it happen. Solved with new O-rings
     
    comnoz likes this.
  9. triumph2

    triumph2

    Joined:
    Aug 13, 2008
    Also possible, if you are running points, that the advance/retard mechanism could stick in a slightly advanced position.
     
  10. o0norton0o

    o0norton0o VIP MEMBER

    Joined:
    Apr 27, 2015
    I found the previous cracked manifold by using carb cleaner sprayed on the outside of the manifold as the bike idled in the driveway. The bike stumbled when I sprayed the cleaner at a certain spot. I still could not see the break in the manifold with it bolted up to the head. When I unbolted the left side allen bolts, the manifold split into 2 pieces. It could be that same thing all over again....

    Thanks Doug, that's what it seemed like to me when I looked into each carb at both slide's resting heights, but I thought it would be an unlikely scenario, which is why I posted this thread.

    If it did happen, it would go like this: The O-ring has worn to the point where it has no friction left and the slide's weight is lifted off the stop screw as you ride along. Add the combination of the intake's vacuum and vibration of the engine to pull on the screw enough for it to turn inward. It sounds totally crazy to me, so I had to ask if anyone ever had this happen.

    I did change the O-ring, started the bike and it idled fine. That still doesn't rule out a cracked manifold because maybe it will open up when the bike gets hot again... which is my theory if the issue reoccurs.

    T2, No points, so no AAU

    Thanks for the comments.
     

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