The most important thing about the Mikuni's is that they do not have the bridge that SU's have and the butterfly spindles are very slim as per the way David Vizard recommends in his book 'Tuning BL's A-series Engines' to improve airflow in SU's. When the butterflies are open there is a straight 40mm hole which is only obstructed by the needle and the slim butterfly. The other aspect of them is that you are getting a carb per cylinder - the R1's had 4 carbs and this is what I have. I am going to make up a manifold for them to fit the B20.

The HIF's won't work because the butterflies work the opposite way to the HS6's. I did not realise this until after I had bought them. It is not possible to get them sorted short of reconfiguring the accelerator mechanism to work in the opposite direction to what it does now.

The head is now back on the car so I will persist with it for the moment. The valve sizes from memory are 43mm and 36mm. I suspect the work on the head was done many years ago. When I had new valve guides put in, the ridge leading to the spark plugs was rounded off from being sharp edged the rest left as it was. The head is defintiely not off a B20E.