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wont go over 5000 rpm under load .

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    #1

    wont go over 5000 rpm under load .

    hi,i have a 91 bros 650 which i have recently taken to the south of france and back to the uk . never owning one before i was suprised whilst going through the gears that it wouldnt let me go in to the hi revs without spluttering but as it was smooth up until around 6000 rpm i lived with it .i could still do 80 -85 mph so was adequate for my trip ,as i continued on my trip this problem got slowly worse and even though i managed to limp home it was a case of doing no more than 50mph for the last 350 miles (it was a very long day). i have checked the carbs and cleaned jets,inspected the Diaphragms for splits,checked spark plugs,air filter and petrol filter both new and all pipes seem perfect . it has new battery and appears to be charging well.really dont know where to go from this point. it has recently in its current state also started to misfire- im to be honest still baffled whether it is a fuel or electrical problem ....any suggestion or guidance would be so appreciated
    cheers for now
    alan
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    #2
    Over the years, I've had similar problems a couple of times. Once was a vacuum leak and once the diaphragms weren't seated properly.
    Brian - Richland, WA
    1991 Hawk GT
    1997 VFR
    • Likes 1

    Comment

      #3
      ^^ yes, if vacuum slide tops were removed for inspection, it can be tough to get the diaphragm edges back in their grooves if the rubber shrank over the years. blow test is necessary to confirm.

      you might be firing on one cylinder only, so check to see that both headers are hot.

      i'd suggest a hot compression test first thing, then a check to confirm all plugs are firing when engine is up at operating temp.

      also, drain and flush the petrol tank after cleaning the petcock screens. it's possible you got a tank of bad fuel from abroad.
      Last edited by squirrelman; 4 Weeks Ago.
      "It's only getting worse."


      MY rides: '97 VFR750, '90 Red Hawk, '88 Blue/Black Hawk, '86 RWB VFR700 (3), '86 Yamaha Radian, '90 VTR250, '89 VTR250 (2), '73 CB125, '66 Yamaha YL-1

      Sold: '86 FJ1200, '92 ZX-7, '90 Radian, '73 CB750, '89 all-white Hawk, '88 blue Hawk, '86 FZ600, '86 Yam Fazer 700 , '89 VTR250, '87 VFR700F2, '86 VFR700F.
      • Likes 2

      Comment

        #4
        The above advice is great. Is add check for air leaks with a can of carb clean spraying around the intake boots. They can look fine to the eye and be un-fine in reality.
        Don't spend money and buy, spend time and learn.

        Comment

          #5
          in the worst case it could be a failing cdi box
          "It's only getting worse."


          MY rides: '97 VFR750, '90 Red Hawk, '88 Blue/Black Hawk, '86 RWB VFR700 (3), '86 Yamaha Radian, '90 VTR250, '89 VTR250 (2), '73 CB125, '66 Yamaha YL-1

          Sold: '86 FJ1200, '92 ZX-7, '90 Radian, '73 CB750, '89 all-white Hawk, '88 blue Hawk, '86 FZ600, '86 Yam Fazer 700 , '89 VTR250, '87 VFR700F2, '86 VFR700F.

          Comment

            #6
            Thanks for all the advice guys,it really is becoming a mystery ,the carbs have been taken off numerous times now and inspected and they look perfect, im going to a bike garage next week so will come back and let you know what (if) they find anything .

            p.s the cdi box has been mentioned a few times - am i right in saying there is no way of testing this than borrowing /buying another one ?
            cheers
            alan

            Comment

              #7
              Correct but I don't suspect the CDI. In my experience they either do or don't.
              See if the shop can do a coil test under load.

              Comment

                #8
                Originally Posted by rcthirty View Post
                Correct but I don't suspect the CDI. In my experience they either do or don't..
                i usta think that too, but i discovered the cdi can fail to fire one cylinder only when i was having problems with one of my 1986 VFRs.

                902185173_Picture1-12-2020236.jpg.73919c3b7ba7309c5a1b07ea986d83ae.jpg

                "It's only getting worse."


                MY rides: '97 VFR750, '90 Red Hawk, '88 Blue/Black Hawk, '86 RWB VFR700 (3), '86 Yamaha Radian, '90 VTR250, '89 VTR250 (2), '73 CB125, '66 Yamaha YL-1

                Sold: '86 FJ1200, '92 ZX-7, '90 Radian, '73 CB750, '89 all-white Hawk, '88 blue Hawk, '86 FZ600, '86 Yam Fazer 700 , '89 VTR250, '87 VFR700F2, '86 VFR700F.

                Comment

                  #9
                  Originally Posted by ratty View Post
                  Thanks for all the advice guys,it really is becoming a mystery ,the carbs have been taken off numerous times now and inspected and they look perfect, im going to a bike garage next week so will come back and let you know what (if) they find anything .

                  p.s the cdi box has been mentioned a few times - am i right in saying there is no way of testing this than borrowing /buying another one ?
                  cheers
                  alan
                  Did you try opening the drain screws on the bowls and see if both have gas in them?
                  Don't spend money and buy, spend time and learn.

                  Comment

                    #10
                    you said you never owned one before, is this a new to you bike/first time out? have you rode it before and it worked fine? could be improper carb jets if there are any mods. maybe a bigger main jet? orrrr kinked fuel line?

                    Comment

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