Just bought a bob vila signature series lawnmower (mod# 917.274031/ DYT4000) with 91 hours on it.pristine shape. I mow a ski slope sort of lawn that killed a husquvarna which is why i bought it. the problem I'm haning is that with the blades engaged, it bogs down and almost stalls going up. bypassed the air filter temporarily to see if it was that and it wasn't. any ideas on what it could be?