So I put the new fuel filter on & the mower ran fine for 20 hours or so. Now I can mow fine on a flat or straght up & down a hill. But when my discharge side (right side) is on the uphill side, it backfires & sputters. If I turn around & mow the other direction so my discharge is blowing down the hill, then no problem. My fuel tank is full, pretty sure my gas is clean. Anybody have any ideas?