The Mikuni BSR36 found on the S and SM commonly has the needle seat o ring start leaking.... its not normally the needle or seat. the O ring is an odd ball size 7.75 I.D. x 1.53mm only seen available from the OEM More commonly found is a 1/16 by 5/16 Chemical-Resistant Viton® Fluoroelastomer O-Rings It is a little tight but with some O ring lube it will push in and work... the Dash Number is 2-011 Or Chemical-Resistant O-Ring Viton® Fluoroelastomer, 1.5mm Wide, 8mm ID If using Viton try the durometer 75A (Medium) or durometer 60A (soft) If you can find the right size in Viton material, all the better. it will resist ethanol in the gas for longer periods of time then the rubber o ring commonly used. Keep a few on hand and if removed for cleaning replace each time. as if they get even a littler stiffen then new, it is very difficult to reinstall a used one without damage.