I've noticed something really strange with the ST 51. Mine is a T&L version mounted inverted in a Legacy. Choking, as usual until it seems "wet", it will never give a "bump". Flipping, it seems quite dead. suddenly, it takes off! There is one other T&L 51 here that seems to act the same way.
Every other engine I've used, mounted inverted, will give a bump, and starting after that is quick and easy.
Something strange is going on in Italy, or wherever they are made.
Floyd