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.030 overbored 550 backfiring out carb

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  • .030 overbored 550 backfiring out carb

    My 550 has been bores .030 over. Do I now need to use a different spark plug or diff gap? It backfires through the carb. It has a Mikuni SBN 44mm carb. Anyone know what the high and low level mixture screws should be set at with this setup? ANy help is greatly appreciated.
    Thanks.

    Also, The plugs are wet and black when I pull them out. If I let the ski sit a little while and put in diff plugs it runs for a little while. any suggestions?


    It also idles rough and when given gas it bogs down and backfires through the carb. This is a new carb. I originally thought it was the carb so now I have 2 SBN's doing the exactly the same thing. That leaves me to believe that there isn't anything wrong with the carb. Plugs or Mixture screws????
    Could this also be the problem just because I might have the water presssure on too high when testing the ski? Thanks in advance!

  • #2
    I also just foud out it has br9es spark plugs in it.

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    • #3
      I don't have the manual in front of me so I can't give you the base screw settings or confirm the plug type or gap.

      Going larger in piston size does not dictate a change in plug heat range or gap, however.

      From the sound of it, you probably have damaged reeds.

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