Mikuni BDST Fuel / Float Level

This guide concerns the fuel and float level for Mikuni BDST carburettors used on various models. Most service manuals for motorcycles that use these carburettors give details of how to set the fuel level not the float level. There is no guarantee that this information is correct and we welcome new information from contributors.

BDST Float Bowl Line Also note that the float housing is held in position by the float bowl. To measure float height it is often advisable to reverse the bowl so you can use it to hold the float assembly fully home or remove the o-ring whilst setting the height. This image sourced from http://www.factorypro.com/

BDST Float Bowl Line BDST Float Bowl Line

These are the two lines that are referred to in various service manuals.

  1. Ducati Monster & SS
  2. Suzuki
  3. Yamaha

  • Ducati

  • Monster & SuperSport 600 - 750 - 900

    Float Height

    Ducati Mikuni Float Level
    This float height of 11mm was measured on a brand new set of carburettors for a 900 Monster. They had never been used.

  • Yamaha

  • FZR250 3LN1

    Mikuni_BDST_Fuel_Level Mikuni_BDST_Float_Level
    It is unknown whether this is from the top line or the centre line on the float bowl. The consensus on the 250cc forum was that it is measured above the lower line.

    Float Height

    One of the members on the250cc forum measured the float height on his FZR250. This was done after fitting new float valve needles and confirming that the fuel level was correct at 9.2mm. To quote:

    "I found that with the gasket surface inclined down at 39 degrees from horizontal the float was just touching the needle and not compressing the needle spring. That cutout is 16.2mm - so aim for 16mm - it kisses the peak of the float."

    Before checking the float height it is very important to ensure that the float needle seat housing is completely seated. On many models the o-ring will push against the body of the carburettor and lift the float assembly up while you are trying to confirm your measurement. It may be best to remove the o-ring first.

    FZR400RR 3TJ

    This is the page from a Yamaha FZR400SP 1991 3TJ-ME1 Service manual. It is completely ridiculous. If this diagram were correct the float bowl would be empty.
    This is the page from the genuine 3TJ1 service manual supplement. It correctly indicates that the fuel level should be 4.5 to 6.5 mm. We can assume that this is above the middle line on the float bowl.


    It would be safe to assume that this should be above the float chamber line not below.

    XJ600S 1992

    Mikuni_BDST_Fuel_Level Mikuni_BDST_Float_Level


    Zinc Plated

    TDM 850


    FZR1000 1989

    Mikuni_BDST_Fuel_Level Mikuni_BDST_Fuel_Level