On 3.5 I tested many carbs over the years and the best that I found is definitely the Super Tigre carb Used on the 10cc Supertigre aero engines. It’s the one designated G61 to 3000.   . 8.75mm bore, 15 mm spigot.. The metering system on this carb is perfect and easy to set. Once set it never needs to be adjusted.

I modify the carb by machining off the main needle and drilling and tapping the housing for the OS radio needle. The only part that fails sometimes the barrel retaining screw. I replace that by tapping the housing M4 and fitting a very short M4 screw with a 2mm needle roller fitted into it. This will never break. The throttle stop screw is not needed and so I cut it short and loktite it into the carb body. I use the original plastic throttle arm and never had a failure.

I found on the CMB 90 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