This is the special thin oring that goes on the end of all (7746) CHOKE JETS found on PHF & PHM carbs. Photo of a choke jet shown for clarity only - the oring fits in the groove on the fat end.
NOTE: on the exploded diagram this is shown as item #53 (below the choke jet as well as on the pump jet) or part # 6173. In reality, the choke jet requires this slightly thinner cross section oring found in the seal kit but not called out as a different part #. After digging and communications with Dellorto, part # 9513 is the CORRECT part # for this choke jet oring.
