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Spark plugs

  • 21 Sep 2019 4:28 AM
    Message # 7892196

    Here are two plugs, the one showing a tan colour around the electrode is from the front cylinder and the one showing a black sooty colour is from the rear cylinder.  The book states to screw the rear mixture screw out three turns, then screw it in until the motor starts to slow and miss.  At that point it states to screw it out a quarter turn.  My mixture screw seems to go almost all the way in before any noticeable drop in rpm results.  The high speed circuit is fine, it’s just the low idle speed circuit I’m concerned about, as like this, it eventually soots up the plugs and causes problems on the low and high side.  Has anyone got any ideas.  Thanks.  The plug type is NGK B5HS.



    Last modified: 21 Sep 2019 4:30 AM | Danny Marks
  • 22 Sep 2019 7:58 AM
    Reply # 7893038 on 7892196

    It would be  a good idea to check for air leaks at the inlet manifold nipple  nuts  before going any further  .Mixture will never balance between cylinders if   there is a leak

    regards Phil

  • 29 Sep 2019 3:43 PM
    Reply # 7908173 on 7892196

    Ok thanks ...I have tightened the nipple nuts and also removed the carb and submersed it in an ultrasonic cleaner.  I also ensured the float level was set to a 1/4 inch below the bowl lip.  I did find some gumming/corrosion on the needle and seat, which was cleaned by the ultrasonic cleaner.  On refitting the carb, I now have even mixtures in both cylinders, but both plugs are very ‘sooty’.  On adjusting low idle mixture screw, it screws all the way in (clockwise), with no decrease in engine revs or splutter.  Any suggestions welcome.  I also notice that the temp of the front cylinder/head, is much hotter than the rear, with forced draught ventilation provided at idle.  I am running a M741-1 bowl on my carb, which has 1/8 inch clearance from the front and rear barrels.  Maybe this is too close, where the fuel is boiling and offsetting the vaporisation of the low speed circuit.

  • 30 Sep 2019 2:58 AM
    Reply # 7908839 on 7892196

    Hi Danny   

     If you are running  An original Carb   with the 741 Bowl    The float level will be way too high    That is   if running  a DLX51     The 741 Bowl will work fine on early carbs   just have to set float as in the service books     Both my 101  have  741 carbs and Bowls and work fine   although i did make larger venturi for them.

    Correct Float level for a DLX  type Carb is suggested  as 15/16"  in a riders book i have .

    A high Float level for your Carb will certainly  make for very rich idle and  very little adjustment from Low speed Needle 

     Regards Phil

  • 30 Sep 2019 2:59 AM
    Reply # 7908840 on 7892196

    Once plugs are soot  Starting will be more difficult.

  • 30 Sep 2019 7:30 AM
    Reply # 7909118 on 7892196

    Hi Phil ...yes I am running the original DLX white metal carb, with a M741-1 bowl.  What would be the correct float height setting, from the bowl lip.  Thanks.

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