I would guess the distributor you have has an advance curve of around 25 degrees (something to check) , 5 degrees initial will have total timing all in around 30 degrees , considering the low compression & slowwwww burn rate total spark lead could be as much as 40 degrees , this will put initial timing around 15 degrees which should clean the motor up @ idle , obviously increase timing in small increments , go for a blast & pull a plug.