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Terrible carburetion between 3500 and 6000 rpm

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    Terrible carburetion between 3500 and 6000 rpm

    My bike was making "pffft" noises near idle and I didn't like it. It carbureted quite well and started easily and was predictable.
    I replaced the spark plugs with new. I adjusted the valves and 3/6 were too tight.
    My carb isolator boots are less than 5 years old, both to the engine and to the stock airbox and are still soft and compliant and I assume are sealing correctly. The carbs likely have fewer than 15K miles on them.

    I took the carbs apart completely (I didn't remove the butterflies or pull out the shafts) and ultrasonic cleaned them in hot water with Dawn detergent in it for 10 minutes and then then blew 90psi through every passage for a long time. I kept all the parts separated into Left and Right. I soaked all the brass bits in carb cleaner and then placed that bowl in my ultrasonic cleaner where I could see the cleaner vibrate and bubble for 5min. I used oxy-acetylene torch tip cleaners small enough to fit in every small hole in all the jets except for the bottom of the slow jet, so I made sure not to enlarge any hole but made sure they're all open without any crud. The needle is clean and straight and both look exactly the same.

    I replaced the two air cutoff valves with new ones (originals looked fine).
    My diaphragms have good springy rubber and I see no cracks or holes in it. My vacuum cleaner lifts both diaphragms about halfway up when the carbs are fully assembled.
    I replaced the pilot valve screw and set it to 2.2 and 2.5 turns out from all-the-way-in as it had been before.
    I synced them visually and then with my mercury sticks to get them equal.

    After taking it completely apart two times consecutively, now I have to hold the starter for about 15 seconds for it to start. It idles pretty good.
    But it falls flat on itself at 3500 to about 6000 rpm. It barely accelerates and spits and makes no power until it clears out at about 6K and then has power that seems normal.
    If I give it extremely slow additional throttle it lessens the symptoms a little.

    What did I miss?


    FWIW, at eBay CARB REPAIR REBUILD KIT FOR GY6 125CC Aprilia PD24J BMW ATV Go Kart won't work much for us, but it has a pilot screw with several orings, bowl drain screw and gasket, and 4 screws which work with the Hawk, but the diaphragm and fuel float and bowl gasket don't work AT ALL. I have Hord pilots on the way and will replace with his.
    Hawk with many differently shaped fuel tanks.

    #2
    does using a bit of choke help or hurt the problem ? blowing air into the 1/2" diameter black holes should lift each slide all the way up.

    wudda been best to just ignore the "pffft".
    Last edited by squirrelman; 07-04-2021, 09:46 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


      #3
      Try running off a bottle to eliminate any fuel contamination.....what did the floatbowls look like? clean? Mung?
      Do the carbcleaner trick for air leaks......I'm assuming yours is a Cali bike......smog block-offs OK?
      Lastly.....combo of pods and pipe? Jetting prolly not the problem.....but it's air or fuel....

      Comment


        #4
        This time I didn't put on the entire intake system and watched the carbs. One of the diaphragms wasn't moving correctly. It seems smaller than it needs to be so I put it in the sun for 20 minutes and reinstalled. It's better but not right yet. Need to find diaphragm solution. I wish I could also get the piston, but I doubt that's possible. I see this as the most recent link for semi-DIY fixing: http://jbmindustries.com/K-72-S.html
        Hawk with many differently shaped fuel tanks.

        Comment


          #5
          Originally Posted by Henway View Post
          This time I didn't put on the entire intake system and watched the carbs. One of the diaphragms wasn't moving correctly. It seems smaller than it needs to be so I put it in the sun for 20 minutes and reinstalled. It's better but not right yet. Need to find diaphragm solution. I wish I could also get the piston, but I doubt that's possible. I see this as the most recent link for semi-DIY fixing: http://jbmindustries.com/K-72-S.html
          There are some Chinese slides on ebay.
          Brian - Richland, WA
          1991 Hawk GT
          1997 VFR

          Comment


            #6
            Originally Posted by Henway View Post
            This time I didn't put on the entire intake system and watched the carbs. One of the diaphragms wasn't moving correctly. It seems smaller than it needs to be so I put it in the sun for 20 minutes and reinstalled. It's better but not right yet. Need to find diaphragm solution. I wish I could also get the piston, but I doubt that's possible. I see this as the most recent link for semi-DIY fixing: http://jbmindustries.com/K-72-S.html
            for a routine basic carb cleaning it's not necessary or advisable to remove the vacuum slide since getting the diaphragm re-seated properly is a common problem, just as the op discovered. if the slide lifts and returns ok while blowing air into the side tube, don't remove the top cap unless the slide needle is worn or damaged, very uncommon.
            Last edited by squirrelman; 07-31-2021, 10:40 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


              #7
              Still the same terrible fueling issue after completely reassembling after syncing the carbs.

              Re-re-reconfirmed valve adjustment is correct (at FT and RT marks, 6thou for intakes, 8thou for exhaust).
              Removed all the removable bits and soaked carbs in carb cleaner again and blew out everything with 90psi air. Again.
              Confirmed float level is very close to 9mm.
              No muck in the carbs--absolutely clean, have new fuel filter, and the two tiny filters inside the carbs are spotlessly clean but brown and old.
              CA version bike and carbs with everything blocked off (no change from when it ran OK)
              New eBay slide diaphragms which are sealing correctly. If I rapidly lift them and let them go, they don't return quickly because of the air shock so I'm sure they've seated. Originals had slides which had some of the black anodizing abraded off through years of use and one of the diaphragms had shrunk a tiny amount so it wouldn't seat properly.
              New OEM slow jets.
              New air-fuel mixture screws with new springs, washers, and rubber o-ring adjusted to 2.5 out.
              New air cutoff diaphragms.
              Front 130MJ, Rear 126MJ, all holes in its tube sparkling clean and not enlarged. Stock exhaust. Stock intake with new air filter and relatively new rubber isolators on both sides of carbs.
              Both exhausts get hot but I haven't measured the temps with a pyrometer.

              New spark plugs which look quite rich (ceramic is covered with black) for all 4. I cannot easily do a proper plug-chop under load at WOT and chop and still get the bike in my garage. I replaced the rear spark plug cap (the one with the cover) that was loose with new OEM.

              Using choke helps it start cold. Choke needs to be turned off within 20 seconds or it won't continue to run. The sound of the carbs is different with the choke on vs. off. The choke cables are a tight fit with very little slack but I believe with it off the choke cylinders are returning completely to their off position because of the sound of the carbs. The choke cylinders and needle are clean. I made sure the two choke cables are properly seated into where they converge into 1 cable.

              When I bench assembled it I used foam Qtips (no fibers) to spread very thin silicone spray on all rubber bits which seal before assembly. So the very outside of the slide diaphragm, the outside of the air cutoff, and on the bowl seal o-ring.

              It doesn't start as easily as before. It takes 10+ seconds while twisting the throttle a little. It's more difficult when the bike is warmer. After it starts it idles quite well. Sometimes it's in very high idle for 20 seconds before reducing to lower idle. The LiIon battery is good and sometimes I completely charge it beforehand and it makes no difference.

              Maybe it's leaking from the butterfly shaft or something like that that's not a fixable thing? My carbs never had external corrosion and look cosmetically clean.

              I feel as though I've exhausted every possible avenue to diagnose the problem. These problems with the jetting are the same problems before changing the slide diaphragms, air cutoff, slow jets, air-fuel screws as after changing them.

              At this point, can I send the carbs to someone to confirm they're not the problem?

              Do I buy a used set and rebuild them? Seems like a bit of a waste, but what else is there to do?

              I'm flummoxed and don't know how to fix it or properly diagnose the exact source of this problem.
              Hawk with many differently shaped fuel tanks.

              Comment


                #8
                deep soaking carbs may lead to frozen throttle shafts after a few months unless shafts are lubed with oil.

                you know the plugs look too black, so are they the correct plugs, and is the air filter clean ?

                deposits can build up so it's good to run a fine (.012") steel wire through the side holes on the emulsion tubes when cleaning.

                will the bike rev past 6000 and up to redline ?
                Last edited by squirrelman; 08-15-2021, 05:22 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


                  #9
                  I lubed the shafts after dipping and drying them with air.
                  They're the Iridium correct plugs.
                  New air filter.
                  I can see light perfectly well through the emulsion tubes and all holes look the same size with no buildup of anything. They're absolutely clean.
                  It "clears its throat" at 6000+ rpm and seems to have nearly the power it should.

                  It's borderline dangerous to ride now. Unpredictable power that finally comes back at high revs.
                  Hawk with many differently shaped fuel tanks.

                  Comment


                    #10
                    Food for thought, found on one of the vintage Triumph web sites: "Ninety percent of my carburetor problems turn out to be electrical problems."
                    Coils? Plug wires? Other wiring? It sounds like you have the carbs sorted out.

                    (Ages ago, I had a similar problem with a Greeves Challenger. Screamed, then cut out, cooled down, then screamed. It was a $1 condenser.)

                    Comment


                      #11
                      That's for British electrics

                      I doubt the electrical problem would consistently resolve itself above 6,000 RPM.

                      ?

                      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
                        As the problem is in the mid range , I'd pull the slides and just make sure the needles are sitting correctly. ( all the way down }

                        I never have to use choke starting , but I do live in a warmer climate.

                        Did you put new slide and diaphragm or just diaphragm ?? The reason I ask is if you have put new Chinese slides in , I would be inclined to measure how far out the needle protruded on the old slide to the new slide as there may be a difference.

                        Comment


                          #13
                          I'll measure how far the needle protrudes from the new slide+diaphragm vs. the original OEMs tonight. Using stock needle with one thin shim washer.
                          Hawk with many differently shaped fuel tanks.

                          Comment


                            #14
                            are the emulsion tubes the same for front and back ? another test: slowly turn in (one at a time) each idle mixture screw until that cylinder looses power and count the number of turns out and report here.

                            would be good to try a different blackbox if you can borrow one.
                            Last edited by squirrelman; 08-17-2021, 08:22 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


                              #15
                              The Chinese carb cylinders are 1mm taller than stock and that length is below where the diaphragm attaches to it. OEM cylinder is 51.5mm from tip where the needle comes out to where the diaphragm attaches, the new one is 52.5. Whether this means the slide at WOT opens at max 1mm less is a question I have no answer for, but it's possible it opens the same regardless.
                              My stock needle with a 0.5mm washer spacer protrudes 42mm. The new one is 42.5mm with the washer, so I can use 2 washers and make it equivalent (if I use the new one).
                              Going back to OEM slide diaphragms and will start it tomorrow and see if there's any difference. I silicone lubed and pressed the old ones into place with a glass plate and visually made sure it was seating properly, then quickly put the cover on like Indiana Jones exchanging the bag of sand for the idol.

                              I will check for the emulsion tubes being the same later. They are the originals and should be the same for front and rear, right?

                              By black box do you mean CDI? All of these issues happened after I changed the spark plugs, adjusted the valves, and dipped and cleaned the carbs. It's possible that a completely different system went bad at the same time.
                              Hawk with many differently shaped fuel tanks.

                              Comment

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