17-07-2018, 08:09 PM
I had simlar problem with my Mk1 Ruby in that it would not slow run and altering the slow running jet made no difference at all wherever I turned it to. I also had the feathering condition you described together with the choke making things slightly easier and not cutting out when trying to increase revs with the accelorater. I solved this problem by cleaning out the jet that is behind the blanking screw you describe. The purpose of this jet is to allow a small amount of air to bypass the throttle butterfly when closed and allow the engine to slow run. I then found that I could adjust things with the slow running jet. This small jet is very difficult to remove and it is better to clean in place rather than removing it. By the look of your photo someone has tried to remove it and damaged it.
John Mason
John Mason