Grumpy, If I reading the IPL correctly it does not have the ready start choke but could the manual choke or primer system. As you probably already aware of the primer system a bad habit of becoming warped or deformed causing the primer not to work. Sometimes doubling up on its sealing gasket can work.
The reason I suggested checking the flywheel key is that sometimes at low start rpms a partially sheared key can cause backfiring as the spark may advanced a little too much.
AVB
