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    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

    #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

    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; 09-03-2022, 05:51 PM.
      "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


        #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


                      #11
                      hi guys ,sorry been a while but a lots happened since i last wrote on here .Got it back from garage after the mechanic had showed me a very worn rocker arm that had been under extreme heat making the exhaust valve not to work ..it was really localized ,luckily the mechanic managed to find a rocker case with arms etc in good condition and replaced ,he has looked for reason as to why this has happened and cant fathom it. i got it back and have test ridden a few times only to find the temperature cage after a few miles is at halfway and a few junctions and a few traffic lights later the fan is on and temp gage really high - it has new coolant and is not leaking - this weekend im swapping the thermostat around with another bros owner to see if its that but have had suggestions of water pump not working correctly,radiator blocked or even head gasket but no signs of the latter --ohh and the reason im so late to post back here is that on one of the test runs i had it stolen and amazingly got back a few days later from the help of social media and helpful bikers -im finding this bike quite testing but im determined to make it work .Although its still a mystery as to why it almost melted one of the rocker arms (front cylinder) it now runs like a dream and better than ive had it -but this temperature reading whilst riding is worrying -will get back after thermostat swap to let you know ,in the meantime any suggestions gratefully appreciated

                      Comment


                        #12
                        Originally Posted by 6 View Post

                        Did you try opening the drain screws on the bowls and see if both have gas in them?
                        yeas done this thanks

                        Comment


                          #13
                          Originally Posted by Peteroo View Post
                          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?
                          it is new to me but have ridden one before and now rides and goes as it should just a temp problem now -thanks

                          Comment


                            #14
                            How high is the temp? Is it over heating? Lots of guys install a fan switch that you can engage manually and/or a lower temperature fan switch.
                            Don't spend money and buy, spend time and learn.

                            Comment


                              #15
                              If a rocker is heat damaged like that then it's probably not a "cooling" problem but a lubrication problem.

                              If you were looking for a support group you're barking up the wrong tree. This place is fulla enablers dude. - Shooter77us

                              The bitterness of low quality lingers long after the thrill of a low price has gone. - RacerX450

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