if i understand correctly it is the bottom tension that is to tight if you say thread run in straight line or top tension discs have dirt between plates,way to check is lift pressure foot,put some coarse material between discs and move to and fro to try clean dirt,while moving material pressure foot can be lowered again to get more tension ,also blow air between discs to check,the smallest thread can cause tension to be non existing,also ensure lower tension is correct by threading and feel for slight resistance when thread is pulled.good luck
johan
August 2014
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