My 17 hp briggs mower starts right up and idles fine. As soon as I engage the blades it starts to sputter and bog down. However, if I let the engine idled for 4-5 minutes first I do not get the sputtering? Once its warmed up it mows fine.

New to web site just to let you know most small 4 stroke engine require a little warm up time before putting them to work... try to let it warm up before use just so all the oil has a chance to work its way around the engine. Also might be time for a oil change

Hi,
I agree with the "warm up" but would add that most engines should perform well with little or no warming period if fully functional. I would suggest an additive to your fuel as the best guess is that you have some minimal varnishing in your low speed circuit that makes it "act up" under load.
Peace,
Clarke