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Troubleshooting an already modified carb

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  • Troubleshooting an already modified carb

    Hey doods,
    I've found myself the proud new owner of a '89 GSX 1100F
    She hadn't run in four years so I put the carbs through a sonic cleaner, installed new plugs/etc and she sputtered herself back to life.
    Even got the adjustable windscreeen working again!

    But she still has some carb issues and I found some red flags during the initial tlc session. Any input is valued since I'm still learning my way around carbs.

    She still has the origial BST34SS carb

    External mods:
    • K&N air filter pods
    • Vance & Hines four-to-one exhaust

    Internal mods:
    • Main jets are stamped 132 (not the stock 110/112.5)
    • Jet needles are multi-tapered aluminum jobs with the e-ring in position 3 of 6 (third down from top)

    Internal stock parts

    • Needle jets (will measure tonight but believe they are the stock P-2)
    • Pilot jet is the stock 32.5
    • Pilot air jet is the stock 135
    • Pilot screw setting is the stock 1.5 turns out
    • Floats are spot on at 14.6mm

    Running issues:
    • Idle can be erratic*
    • Idle will only drop slowly*
    • Backfires regularly below 4000 rpm
    • Backfires are seldom but still exist at higher rpms

    *I will be checking for vacuum leaks and will follow with a carb sync.

    Primary question:
    Does the current mix of intake/carb/exhaust mods seem correctly done by the former owner or am I better off reverting the mods back to stock?

    Secondary question
    In prep for reverting back to stock I've already acquired an original air box and correct Mikuni oem main jets, but....
    1) Can I make the aftermarket jet needles work (and in what e-ring position?)
    2) Can I get away with leaving the aftermarket exhaust on there?

    Luv U Long Time

  • #2
    A dynojet jet kit was installed (needles and main jets). The size tells me they were trying for a "stage 3" setup, but... even pods and the aftermarket exhaust do not match up to the need for the size of jet they choose.

    You can not use the needles with OEM Mikuni jets. Dynojet jets are sized by the hole size. Mikuni jets are tapered shaped holes and are sized by flow. The needle shape does not match up to that flow for the various rpms, so will never run right mixed.

    I'd keep the exhaust performance wise, unless you just don't like it.

    I'd put a factory pro jet kit in the bike replacing the dynojet kit myself with OEM airbox setup, the V&H exhaust, one size up on the jets (will come in the kit) from OEM setting, FP needle start at 3rd slot, 2.5 turns on the a/f screw.

    You say backfire... is that in the exhaust and popping on decel? We generally refer to that as afterfire, and it's a symptom of being lean. Pods = lean things way out. A/F at 1.5 sound lean (I know, OEM it used to be that way... 30 something years ago with non ethanol fuels too... ) Do 2.5 as a start and see what you think.

    You can fine tune the a/f screw setting with the drop down method (google it, vids show how) with no special tools.

    This is a US model, not CAN model bike right? The A/F settings may be different otherwise...

    93 750 Kat

    Modified Swingarm, 5.5 GSXR Rear with 180/55 and 520 Chain, 750 to 600 Tail conversion, more to come. Long Term Project build thread

    "I've done this a thousand times before. What could possibly go wron.... Ooops!"


    • #3
      I couldn't have asked for a better response Krey!

      Yes, it's a US model.
      Yes, it's the afterfire I'm referring to - thanks for the correction.

      I'll be keeping the oem airbox/filter and V&H exhaust.
      *With the above in mind, and considering I've already acquired the oem main jets and just need to buy oem needles only a final question remains:

      If my bike is only going to be my long-haul, straight-line, gear-laden highway rig (my nakid sv650 wears me out after an hour) am I best off just spending $50 on the oem needles and getting her back to stock or is the $150 Factory Pro kit really gonna make a difference?
      I read somewhere that the FP kit seemed to smooth out engine vibes somewhere around the 6K rpm range. Legit?

      Damping vibrations over long hours in the saddle would be worth the extra $ to me, but the 1100 is such a step up in horsepower (and comfort) from my 650 twin that just adding more horses on top doesn't make my dingdong feel any bigger.

      I'll let your wisdom will make my decision for me.



      • #4
        All Kats modified or not, will benefit from the FP carb tuning kit.
        "I'm sorry, I didn't mean to upset you when I called you stupid. I thought you already knew..."
        spammer police
        USAF veteran
        If your a veteran, join the KR veterans group


        • #5
          OEM needle will be lean (cause vibs)... you can poor man jet kit it "put shims under the needles) to help a little, but the FP kit is by far the best improvement over all.

          93 750 Kat

          Modified Swingarm, 5.5 GSXR Rear with 180/55 and 520 Chain, 750 to 600 Tail conversion, more to come. Long Term Project build thread

          "I've done this a thousand times before. What could possibly go wron.... Ooops!"


          • #6
            FP carb kit it is!

            You guys rock.


            • #7
              Fyi, when you go to order: it might get a bit confusing. The FP website isn't that easy to navigate. And their description might confuse you. They only list 1 kit compatible for any 1100, and it's a poorly worded description. But it is the right kit.
              1998 Katana 750
              1992 Katana 1100
              2006 Ninja 250

              2006 Katana 600 RIP - 130k miles


              • #8
                Thanks for that shpielers!
                I've ordered the S13-10 kit as sussed-out by earlier threads.

                Thanks again dudes (and dudettes where applicable).


                • #9
                  I have the same problems, I'm using 127.5 and 130 jets, needles and pilots are stock, fuel screw 3.75 turns. Exhaust is RRR Hindle 4-2-1 with a very short mid pipe (tube before the muffler), stock velocity stacks (longer in the middle) with ramair foam filters, no airbox,

                  This is the set up that run the best for now, I tried 140 and 135 main, 37.5 pilot, 3mm needle shims in different combination with the stock ones.

                  I also tried 34 from a tuned GSXR1100 with the same dyno jet kit, 132 main and 3rd slot from the bottom (pilot is 37.5) It does less afterfire but pull less at full throttle and still stuttering when full throttle at low rpm

                  With both set of carbs I have back fire when cold.