GTH2448T Sputtering/Miss issues

Just picked up a new used mower, 2006 Husqvarna GTH2448T, it's a Briggs 24hp Intek V-Twin. It's got a random miss or sputter when its running. Pull the choke out a little ways and its better, but as soon as you engage the blades the extra load makes it sputter again, and with the choke out a ways, after a couple minutes it gets too rich and belches some black smoke. Mostly just a miss every 10 seconds or so, sometimes more often sometimes less.

So, I got a new fuel filter, installed it, took the fuel pump off and cleaned it, put in new spark plugs, drained what I could out of the gas tank and put some Seafoam, MMO and some premium gas in it. Started it up, ran great, I put some Seafoam right down the intake, then let it sit for five minutes, started it up and sat back and watched the smoke show. It was running nice and smooth so I let it sit and run for a while.

After about 10 minutes, the sputtering returned. :banghead3 So, my plan is this; I'm going to run a line from the filter into my gas can, thus not using the mower's tank, and see if it runs good on fresh clean gas, if it does, then I know that there is something in the tank that isn't supposed to be. If it doesn't run any better, the next step is to take the carb off and clean it out.

Thoughts?