I also have an R90/6. Mine is 1975. It's not as pretty as a lot of these machines you guys have but beauty is in the eye of the beholder, ya'know. Those are the stock carbs you have and I also am running stock Bing carbs. Most everybody says adding the slide top springs is an improvement in carb response. I have them also. And it may be an improvement.
There are two versions of the carb top springs. The earlier version is a stiffer spring and the one you want is the later. The later one is thinner wire and not so stiff. Here s the number of the stiffer, early, spring; 13 11 1 335 324
The one you want is ; 13 11 1 338 134
They are not really very expensive, less than $20 for a pair. While there consider new screws for the carb tops, those get buggered up over the years and Stainless Steel is available. Use a little AntiSeize on them.