I am missing the oring out of the idle mixture screw cab seat. the idle mixture screw is red and numbered 1-4. It has washer and spring. Do i need the oring or can i just adjust the needle valve 1/2 way between rich and lean rpm drops. What size is the oring so i do not have to buy a complete carb kit and where can you get them in Canada or British Columbia? thank you for your experience and time.