To set the float level on a 1970 Yamaha 250 Enduro, you need to keep the needles near the middle. Set the float height at exactly 15mm. You can use a 30 or 40 pilot jet and play with the air screw. However, you can also consult your Yamaha mechanic for more professional help. Read more from http://lawrenceotoole.com/r5/labels/Technical.htm.