If the machine is looping it indicates that the upper thread is being picked up & that the timing is ok.Next step is to try turning the upper tension up to No9,if this doesn't eliminate the probem it indicates that there is something obstructing the upper tension unit.If this is the case turn the tension dial to zero,raise the foot & blow or use an old needle to clean between the tension discs situated to the left of the tension adjustment dial.Hope this helps.
Sewman
May 2011