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rich idle & black plugs 1981 GS750EX

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    #1

    rich idle & black plugs 1981 GS750EX

    I recently finished restoring bike, and performed all upgrades including etching & Kreeming tank, Dyno ignition/coils/wires & rewire, NGK 5K ohm plug caps, Electrex stator & RR, K & L carb & petcock kits, K & N filter element. Bike has stock airbox and exhaust. Great charging voltage, and full voltage to coils (fed direct off battery via relay and 14 gauge wiring). Igniting timing/advance set right on, as is float level (measured with caliper, and then externally confirmed with sight tube on each bowl).

    Bike ran great, but had a mid-range flat spot, so I got a Dynojet kit so I could set the needles. I initially set to position 3, but since flat spot still there, set to 5 which eliminated flat spot. (I bypassed 4 since flat spot so distinct! Dynojet's trick of putting duct tape on 1/3 of airfilter vs. no filter indicates mixture right on!) Bike idles good, and pulls like nobodys business from 2000 rpm though redline. I was very happy until looking at the plugs...

    The problem is that all four plugs are turning fluffy black after a short period of city riding, and the exhaust smells rich at idle. I have turned in the top air screws (engine side of BS32SS CV carbs) from 3 turns to 2 turns (still black), to 1-1/2 turns (still black), to 3/4 turn (still black). I just tried turning the screws all the way in, and bike still runs. (Various positions do affect idle speed, and do seem to reduce the rich smell to a certain degree.)

    I am wondering how accurate the jets are that came with the K & L kits, specifically the pilot and air jets. (I'll be derned if the chewed up original jets I removed didn't get round filed...) The air jets are marked 170, and I am using the 116 Dynojet mains. The pilot jet and needle jet have no markings (thank you very much K & L).

    Since the bike runs so well at mid and high speeds, I feel I should be playing with the idle circuit jets (unless someone has a good suggestion for something I might be overlooking). Since I have no experience with idle jets, I could use some help. HELP!!!!!

    --Brian
    Tags: None
    #2
    Sounds like you're siphoning fuel off from another source. Could be your needles are too high and its pulling from the mains? Which kit did you get? I have the stage 3 installed on mine and my needles are at 3 1/2 from the top and mixture is just under 2 turns. Plugs are alot cleaner than they used to be at 4 1/2 setting.

    Comment

      #3
      rich idle & black plugs

      Jon,

      Thanks for info regarding jet sizes. I called Sudco and they advised dropping the pilot jet one or two sizes. So since you were kind enough to provide the stock size as being 42.5 (Sudco 004-059), I ordered the 40 (004-058) and the 37.5 (004-057) pilot jets for my BS32SS Mikuni carbs.

      I asked the tech (Chad) what his thoughts were on gas pulling from the needle jet since the needle so high -- he thought the contribution would be next to nil. He also felt that even though K & L's pilot jet was not marked, that they ran real close to factory specs. So I will try the smaller pilot jets to see if this clears up the situation.

      I have the stage 1 Dynojet kit, since running the stock exhaust, cams, airbox.

      BTW, for people switching to pods simply because they can't get the airbox back on right, try ordering new airbox inlet boots -- available from your dealer or Bikebandit!

      Brian

      Comment

        #4
        rich idle & black plugs

        Leaning pilot jet to 40 (004-058) from stock (42.5) took care of the problem! Good range of mixture screw adjustment now, with optimum setting at 2-1/2 turns out. No more rich smelling exhaust -- plugs nice color after city driving, and again after on the road.

        By the way, I lost a float pin while the carbs were apart, and found the most unusual source of an exact replacement -- one of my kid's Awana pinebox derby car axle!! This proves once again that God likes GS's!!!

        I am ready for nice weather and some ridin'! If anyone in Columbus would like to ride, drop me a line!

        Brian

        Comment

          #5
          Re: rich idle & black plugs 1981 GS750EX

          Turning airscrews in richens mixture. Out leans it.

          Earl

          [quote="BDLubell"]
          The problem is that all four plugs are turning fluffy black after a short period of city riding, and the exhaust smells rich at idle. I have turned in the top air screws (engine side of BS32SS CV carbs) from 3 turns to 2 turns (still black), to 1-1/2 turns (still black), to 3/4 turn (still black).
          Komorebi-The light filtering through the trees.

          I would rather sit on a pumpkin and have it all to myself than be crowded on a velvet cushion. H.D.T.

          Comment

            #6
            I have an 81 550. I have the air/fuel screw on the engine side. Earlfor your saying turning that screw all the way in richens it??? I thought if the screws were on the engine side then turning all the way in leans it? I'm just trying to
            understand.

            Comment

              #7
              The CV carbs on these bikes have fuel mixture screw on top of carb engine side. Turning mixture screw in leans mixture, out richens mixture. (Earl is referring to non-CV carbs having air mixture screws, which have the opposite effect.)

              Comment

                #8
                Sorry, (I have to start paying attention :-) ) An air/fuel screw on a CV is a mixture screw. Turning it in will lean it. An air screw on a VM carb is an air only screw. Turning it out will lean.

                Earl


                Originally posted by Wigglespank
                I have an 81 550. I have the air/fuel screw on the engine side. Earlfor your saying turning that screw all the way in richens it??? I thought if the screws were on the engine side then turning all the way in leans it? I'm just trying to
                understand.
                Komorebi-The light filtering through the trees.

                I would rather sit on a pumpkin and have it all to myself than be crowded on a velvet cushion. H.D.T.

                Comment

                  #9
                  Whew!!!
                  Thanks for clearing that up for me. I was starting to get nervous!!!!

                  Comment

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