Lawn Mower won't start

I have a murray lawn mower with a Briggs and stratton 6.0HP engine (model #12H882-3173-E1) that im having a problem with. I had already changed the gas and cleaned the carb and got a new filter, but i could not keep it running. so i got the carb rebuild kit and changed everything out, but it still stalls after a few seconds, so i took the carb apart again and doubled checked that there are no obstructions. everything seems fine but it wont run. so after a lot of experimenting i found that it will run if i replace the air filter with a piece of cardboard, restricting 90% of the air, and it seems to run ok. If i keep pushing the primer i can also keep it running. why suddenly does it need such a strong vacuum to pull the gas from the carb? Does anyone have a idea whats going on here?

I had the drive coupler between the engine and the transmission break in my f935 mower and I replaced it. I drove the mower for less than 3 minutes and all the hydraulics quit, steering ,lift and drive. The transmission never made any abnormal noise , the drive shaft is turning unimpeded , nothing works. Any ideas ? By the way I was driving on level ground and the mower was not under any strain. I don't believe in coincidences is there something I may have done installing the rubber u-joint? It seemed pretty straight forward to me. Any help would be greatly appreciated Tim