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'90 600 Rev'ing Like an M-EFFER

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  • #1

    '90 600 Rev'ing Like an M-EFFER

    So my 1990 Kat carbs were taken off and thoroughly cleaned out. All rubbers were good, (SAT). Carb #1 butterfly was stuck, so like an @$$ I decided to put a screwdriver on the butterfly and whack it with a hammer....in an effort to free it up. I know. The dumbest thing I could have done. Some Areokroil finally freed it up in the end. I pulled that butterfly to true it back up, though it wasn't that bad.

    (A little history. I got the bike from my next door neighbor. The bike has 17K original miles on it. Supposedly ran when it was last parked. Was last registered in 1999. All jets are stock and I drilled the brass plug to access the air screw circuit to clean. They were all set to 1/2 turn out and I set them to 2.5 out, as recommended.)

    So I re installed the carbs and air box w/o filter. (This bike has two petcock outlets and a nipple for the vacuum port. The tank needs to be lined yet with the POR15 tank sealer three step product since it has rust.) I used an overhead motion pro tank into a brass T which fed each of the two carb supply hoses. I started it up and as soon as it started, it rev'ed HIGH and we immediately shut it down. I tried turning the idle all the way down, CCW and that did nothing. So I turned it CW all the way and that actually seemed to lower the RPM's to about 4500 which was still screaming at 4500. I didn't mess with the sync screws and assumed that they should be alright, at least to start the bike. What kind of gap should the butterflies have at the bottom of each butterfly?

    I am a little stumped. Is it rev'ing like that because the air filter was off? Is it rev'ing like that because of the head pressure of the overhead tank? Is it rev'ing like that because of the air screw at 2.5 out? Is it rev'ing like that because of the butterflies? I am at a loss any help would be appreciated.

    Thanks
    Last edited by dnelli; 02-21-2017, 08:24 AM.
    2004 KTM 525 EXC
    2004 KTM 200 EXC
    1990 Suzuki GSX600F
    1978-1/2 AMF/H-D Low Rider 1200
    Tags: None
  • #2
    Most likely the butterflies are being held open.

    IF you have a bent butterfly, that would do it. sticking or stuck ones also. does not take much for the rpms to jump.

    out of sync carbs will do it also.

    Finally, when you mount the carbs make sure the 2 center clamps are not anywhere near the throttle cable assembly. It's common for the clamps to catch/hang up the throttle and that would also cause the issue.

    Krey
    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 http://katriders.com/vb/showthread.php?t=96736

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

    Comment

    • #3
      Originally posted by Kreylyn View Post
      Most likely the butterflies are being held open.

      IF you have a bent butterfly, that would do it. sticking or stuck ones also. does not take much for the rpms to jump.

      out of sync carbs will do it also.

      Finally, when you mount the carbs make sure the 2 center clamps are not anywhere near the throttle cable assembly. It's common for the clamps to catch/hang up the throttle and that would also cause the issue.

      Krey

      Thank you. I think I flattened it back out. It wasn't bent in the sense of a 90 degree...more marring on the beaten-ed butterfly. What kind of gap should be underneath each butterfly? I do remember a small gap. Also, I probably should have started the butterfly screws and then released the butterflies as to seat them, so that they "close" and #1 seats, then tighten #1's screws....does that make sense? Maybe the sync is off, I'll need to look at that as well as the cable and clamps potential interference.
      2004 KTM 525 EXC
      2004 KTM 200 EXC
      1990 Suzuki GSX600F
      1978-1/2 AMF/H-D Low Rider 1200

      Comment

      • #4
        Originally posted by Kreylyn View Post
        Most likely the butterflies are being held open.

        IF you have a bent butterfly, that would do it. sticking or stuck ones also. does not take much for the rpms to jump.

        out of sync carbs will do it also.

        Finally, when you mount the carbs make sure the 2 center clamps are not anywhere near the throttle cable assembly. It's common for the clamps to catch/hang up the throttle and that would also cause the issue.

        Krey
        So last night I rechecked the cable and I found that I pulled the rubber boot off of the adjusting nut to get cable out and back in. I slid it back down and then tried starting it (w/o air box) and it just cranked and cranked....no start...haha. Before it started and rev'ed its balls off now it won't start. I charged the battery this morning before work and will try again when i get home.
        2004 KTM 525 EXC
        2004 KTM 200 EXC
        1990 Suzuki GSX600F
        1978-1/2 AMF/H-D Low Rider 1200

        Comment

        • #5
          Originally posted by Kreylyn View Post
          Most likely the butterflies are being held open.

          IF you have a bent butterfly, that would do it. sticking or stuck ones also. does not take much for the rpms to jump.

          out of sync carbs will do it also.

          Finally, when you mount the carbs make sure the 2 center clamps are not anywhere near the throttle cable assembly. It's common for the clamps to catch/hang up the throttle and that would also cause the issue.

          Krey
          I wound up gettin a set of pre carbs for $65 of eBay. No top caps, needles and only two diaphragms. Ironically they were cleaner on the inside than mine...however the outsides looked a little rattier than my original carbs. I used all my upper carb parts and cleaned out the bowl and its parts. Runs just right now. I think i may have bent the butterfly axle shaft on the original carbs... Idles just fine now. Runs great. Thinking about drilling these air screws and maybe installing a jet kit this winter.
          2004 KTM 525 EXC
          2004 KTM 200 EXC
          1990 Suzuki GSX600F
          1978-1/2 AMF/H-D Low Rider 1200

          Comment

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