Spent time swapping plugs, leads, distributor cap etc - no difference. Decided to switch investigation to fuel so took the top off the carbs, the carb supplying cylinders 1&2 has a broken float support (where the pin that holds the float goes through - see picture), so I swapped this with the one from the carb supplying 3&4 and their good one to cylinders 1&2 carb and the running issue moved to cylinders 3&4 ie pulling HT lead off making no difference, but it now does with 1&2 misfire like I was getting on 3/4 previously. I put everything back so this was my only change - could this be the issue it points to it and I am sure the break would mean the float/carb is not working properly? I have found someone who can repair it, but it takes 2 weeks so on the hunt for a Dellorto DHLA45 carb top piece.
I have ended up sending the carb top off for repair - apparently this breaking is quite common. Will update once its back - pleanty of other jobs to do in the meantime.
