I found on the CMB 90/91 and 45RS and 45VAC that the OS 9B works well. Good power and nothing breaks on the carb. I did find a few improvements though. Firstly I found that when the midrange needle is adjusted for good throttling, the carb can be too lean at tickover and very low throttle openings. This is caused by the shape of the needle. I modified the needle as shown below by changing it to a straight taper. The needle can then be adjusted for the best throttling..  The top drawing is the original needle, the lower drawing is the modified needle with straight taper













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