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Hawk stumbles at partial throttle

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    Hawk stumbles at partial throttle

    My 1991 NT650 is completely stock with 28K miles. Especially when cold, but even a little when warm, this bike stumbles when the throttle is opened slowly. I don't detect a cylinder miss-fire but the power just isn't there. As I gradually increase the throttle there comes a point where the bike tries to spring out from under me. I guess I start getting good power from both cylinders at this moment and this is the expected performance for the now advanced throttle setting.

    This is some sort of carburetion problem in my opinion. Any guesses?
    Thanks!

    #2
    It's probably your needle height. Does it have a jet kit with adjustable needles? If so, raise them.

    If not what intake/exhaust are you running and what jetting?
    Don't spend money and buy, spend time and learn.

    Comment


      #3
      How long have you had it?
      How long has this been happening?
      How long has it been stored?
      Describe the gas in it.

      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

      Comment


        #4
        clean the idle jets first to see if that fixes it.

        post-3647-0-85981000-1347898810.jpg
        Last edited by squirrelman; 04-03-2022, 11:54 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


          #5
          Similar thing happened to me recently turned out that one of the pilot jets was blocked solid. Some idiot had used gasket sealant to fix a leaking float bowl gasket.
          You do not have permission to view this gallery.
          This gallery has 1 photos.

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            #6
            Originally Posted by mhead View Post
            My 1991 NT650 is completely stock
            I missed that part the first time......


            You have something else going on.
            Don't spend money and buy, spend time and learn.

            Comment


              #7
              Hi All, and thanks for the replies. Here are answers:

              How long have you had it?
              I purchased it new in 1991

              How long has this been happening?
              Years

              How long has it been stored?
              Never actually stored. But there was a period of possibly a year that I didn't ride it. Gas went bad and the bike barely ran, emitting the bad gas smell. I shoulda done it myself, but I let a local MC shop rebuild the carbs. The problem existed both before the bad gas event and after the carb rebuild. Since that time I've been careful about gas and always ride it enough to keep good gas moving through it. But the problem persists.

              Describe the gas in it.
              Gas is recent and premium 93 octane available here in California.

              The bike is stock and so the jets are whatever Honda installed back in 1991. Intake and exhaust are also stock.

              I do not know my way around these carbs, but I note there are pistons that raise the needles. If one of these stuck then the mixture to that cylinder would be lean and I might experience reduced power. At advanced throttle perhaps the piston lurches loose and ...zoom!

              Any of this make any sense?

              Comment


                #8
                If the slide (piston) doesn't move then it can't let in air... it doesn't really 'go lean...' it just doesn't open the throttle fully.

                It doesn't sound like your local shop did a decent job of cleaning your carbs.

                Being a 1991, it's got all the California emissions equipment on it and one of the vacuum lines could be blocked but my money is dirty slow jets like squirrelman said.

                You can check the slide operation by using your finger to move them manually (engine off) and making sure that they move freely.

                I like to blow in the " slide vent lines to make them raise. If you can't 'blow' the slides into a fully open position then I'd look at the slide diaphragms for cracks/leaks.

                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

                Comment


                  #9
                  Originally Posted by ParcNHawk View Post

                  I like to blow
                  He for some reason can't help but to say this in every post..


                  We know Bob, and love you all the more for it.
                  Don't spend money and buy, spend time and learn.

                  Comment


                    #10
                    Sorry for letting this thread go a little cold.

                    I have the feeling that as I open the throttle one of the sliders sticks until sufficient vacuum builds up to pull it past some tight spot.

                    I don't understand how the carbs work though. Can someone give me the proper name for the Hawk type carb. Maybe if I know what type of carb it is I can use this name in a search of the 'net for a description of operation. If I know how they operate then maybe I can find the problem. Thanks!

                    Comment


                      #11
                      They're called CV type carburetors.

                      And you can check smooth operation with your finger. You can also test them by blowing and sucking on the vent hose.

                      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

                      Comment


                        #12
                        Originally Posted by mhead View Post
                        Sorry for letting this thread go a little cold.

                        I have the feeling that as I open the throttle one of the sliders sticks until sufficient vacuum builds up to pull it past some tight spot.

                        I don't understand how the carbs work though. Can someone give me the proper name for the Hawk type carb. Maybe if I know what type of carb it is I can use this name in a search of the 'net for a description of operation. If I know how they operate then maybe I can find the problem. Thanks!
                        https://youtu.be/IBFq9nHVmks
                        ASMA #139

                        Comment


                          #13
                          Originally Posted by ParcNHawk View Post
                          by blowing and sucking
                          Ok. Now things are getting a little graphic.
                          Don't spend money and buy, spend time and learn.

                          Comment


                            #14
                            But I haven't even mentioned adjusting the knob

                            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

                            Comment


                              #15
                              i'd suggest you start with a routine carb cleaning
                              "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.

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