I have a troy-bilt 5550 model 01919-1 generator that I left gas in tank for more then a year. Of course it won't start now. I plan on putting in new gas and adding a gas treatment brand name seafoam. Any other advise?

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?