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.
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/
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These are the two lines that are referred to in various service manuals.
Ducati
 This float height of 11mm was measured on a brand new set of carburettors for a 900 Monster. They had never been used. |
Yamaha
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 FUEL 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 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. |
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 FUEL 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 FLOAT HEIGHT One of the members on the250cc forum measured the float height on his FZR250 3LN3. |
 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. |
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