Its 4 seasons old, bought new, on a LittleWonder loader. Engine Model 305442 / 0121-B1. B&S Vanguard Twin 16hp. Been running like a champ up until day before yesterday. I regularly change oil, oil filter, etc. I dont think the ambient temp has anything to do with it but it was sort of cold, around 24 degrees. Started right up on first pull, ran for 5 min sucking up leaves then the engine went down to a crawl with some backfiring some smoking not too bad smoking but it does. I can still start it up but that's all I can get out of it a chug chug chug crawl pace at full throttle with an intermittent rattling sound, not very loud at all, not very long at all. Checked all linkages and everything appears to be intact. I replaced the fuel pump, still same problem. I took apart the carb today thinking maybe the float, needle something went wrong there, carb to me looked pristine (shockingly). Float and needle appeared fine. Strange set up as I could not get to the seat and couldn't figure out how to remove the lower body of the carb off of the manifold. I mean I could easily see where the needle went into the seat but couldn't figure out a way to get to it if I wanted to get a carb rebuild kit. Checked both plugs and both were pretty black from carbon, one on oil fill side was dry but the one on the other seemed to have unburned gas on it from the smell and appeared slightly damp. I'm stumped and not mechanically inclined enough to tear the engine down myself. Guess it's going to the shop tomorrow unless anyone can offer ideas of things to check (greatly appreciated). We are slammed busy with jobs, about 10 people waiting and the phone still ringing, and this is my only loader. Thanks in advance.