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'79 550E intermittently weak at 6K rpm

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    '79 550E intermittently weak at 6K rpm

    Hey Forum,

    I can't shake the feeling that my daily rider developed an issue. It most likely is an intake O-Ring issue - if it's even an issue at all -, but nonetheless, I'd like to pick your brains for other possibilities, so I don't need to take apart my bike multiple times.

    The reason why I wrote "If it's even an issue" is because my senses might be off, as I had to ride a loaner bike for some time, a 90ies Yamaha TDM 850. That's way way more power than I'm used to from my GS550E. Nonetheless, I can't shake the feeling that she used to pull better, and that the power band/curve fluctuates.

    From a standstill, the bike begins to really pull at around 2500 to 3k, and then kicks in between 5 to 7k - but hitting a wall at 8500 to 9000, it's time to shift up to get more pull.

    On the highway, it's worse; depending on the incline, in 6th gear, I just really can't get beyond 6k (120kmh / ~75mph). Shift down, I manage to push beyond 7k, but that's about it.

    However, I also had rides where she almost effortlessly got into redline, which puzzles me a bit.

    This is all somewhat unreliable information:
    - I notice this stuff during normal riding/commuting, and I cannot afford to troubleshoot & test my bike in traffic too much; also, to really get a picture, I would need to go faster than the law
    - A said, warped perception because of loaner bike
    - Newbie rider; not really wound her out until recently
    - No Idea how much power the 550 brings to the table anyway; air resistance, i.e. riding position really makes a difference I feel
    - Never checked systematically against engine temperature (i.e. at which point after taking off the issues develop)

    That being said:
    - The valves have been adjusted just recently by a shop - didn't have time to do it on my own, but I trust them, and had them report all measurements to me
    - The points have been replaced by the same shop (because of an accident, more on that later), so these should be good (also, I noticed this issue earlier)
    - The spark plugs and caps have been replaced a while ago
    - The R/R has been replaced with a SH775
    - When requiring choke to start up, RPM do increase as the engine heats up (strong hint at intake O-Rings, right?)
    - When I measured temperatures around exhaust collars, cylinder 1 measured hotter than all others (around 130°C, #4 at the same time measured 119°C), indicating a leaner mix than the others

    I'm not sure wether I'd need to sync up the carbs as well, or check the timing advance.

    I'm not looking forward to this work (the dreaded 550 airbox....), so I'd check as much as possible when I have her apart. Any hints?
    #1: 1979 GS 550 EC "Red" – Very first Bike / Overhaul thread        New here? ☛ Read the Top 10 Newbie mistakes thread
    #2: 1978 GS 550 EC "Blue" – Can't make it a donor / "Rebuild" thread     Manuals (and much more): See Cliff's homepage here
    #3: 2014 Moto Guzzi V7 II Racer – One needs a runner while wrenching
    #4: 1980 Moto Guzzi V65C – Something to chill

    #2
    "- When requiring choke to start up, RPM do increase as the engine heats up (strong hint at intake O-Rings, right?)"

    yes, this would make me suspicious. You need all 4 cylinders being fed correctly (and together) .
    1981 gs650L

    "We are all born ignorant, but you have to work hard to stay stupid" Ben Franklin

    Comment


      #3
      Update: After the issue went away, appeared again (which I mulled over a bit in another thread), went away again, it finally came back today, and stayed.
      So at least I'm sure now that there is an issue - the ping-pong above made it very hard for me to determine if there's something going wrong.

      Currently, everything is fine up until 4000, but between 4500~5000 rpm the bogging begins. The engine also sounds different, as if the mixture is off - similar, but not exactly the same when starting up with choke and playing with the throttle tad bit too much.

      - I did run briefly with choke - boy did she protest when I wanted to open the throttle.
      - Switching the petcock to reserve made no difference, so I assume a proper fuel flow and the petcock in good enough working order.

      My hypothesis is that around these RPM, the vacuum becomes strong enough to suck false air through an aged intake O-Ring.

      From what I know and read so far about carbs; if I were to have an issue, the issues would be mainly correlate with throttle position, not RPM - please correct me if I'm wrong here.

      Blech. Not looking forward to removing the airbox.
      Last edited by roeme; 09-02-2016, 04:45 AM.
      #1: 1979 GS 550 EC "Red" – Very first Bike / Overhaul thread        New here? ☛ Read the Top 10 Newbie mistakes thread
      #2: 1978 GS 550 EC "Blue" – Can't make it a donor / "Rebuild" thread     Manuals (and much more): See Cliff's homepage here
      #3: 2014 Moto Guzzi V7 II Racer – One needs a runner while wrenching
      #4: 1980 Moto Guzzi V65C – Something to chill

      Comment


        #4
        Oh, completely forgot to update this thread.

        Did take off the carburetors last Friday, and as soon as I had them off, noticed a inlet vacuum screw loose.

        Ugh...oh well, since I had them off, proceeded to replace the intake O-Rings. However, since I only detached the airbox, not disassembled it, I was unable to reach all screws with the impact driver. And the chedar is strong with these screws. As it was getting late, and I had no other means of transport available, I just replaced #4, and put everything back.

        Fired right up, with no choke required whatsoever.
        #1: 1979 GS 550 EC "Red" – Very first Bike / Overhaul thread        New here? ☛ Read the Top 10 Newbie mistakes thread
        #2: 1978 GS 550 EC "Blue" – Can't make it a donor / "Rebuild" thread     Manuals (and much more): See Cliff's homepage here
        #3: 2014 Moto Guzzi V7 II Racer – One needs a runner while wrenching
        #4: 1980 Moto Guzzi V65C – Something to chill

        Comment

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