An HS4 will work well on a mildly modified 850. In fact on the Mini Automatics they used to fit a 1.5" SU. They are not expensive to buy used and are easy to recondition as necessary. With a better flowing head, a Cooper exhaust manifold and exhaust system, and a few other minor improvements it would go very well.
It will be done... Spoke to a mini tuner/specialist today... will get him to do it. Let's see about the results.
