Charlie,
You most likely have carburetor problems.
My 190C did the same thing last year. It only had 165 hours on it but it started to pop and spit and sputter at WOT and was just running really ragged.
I tried lots of things, from changing spark plugs to checking the flywheel key and even brightening the magnets on the flywheel, all to no avail. Then I started to notice it was only happening on one cylider, not both.
I learned that the carb on these Briggs engines is really two carbs in one housing. One carb for each cylinder. There is a plastic tube inside the carv with some plastic jets in the bottom. The jets have o-rings around them. Over time, the o-rings get hard and don't seal any more. In some cases, the o-ring will actually let the jet fall out of it's hole.
JD offers a kit, you can get a kit from a Briggs dealer cheaper, but I just replaced the whole carb. No one had a kit in stock, it was a Saturday and I HAD to finish mowing. JD didn't have a new carb. (thank goodness since they wanted $200 for one) Bought the same part # new carb from my local Briggs dealer for $125.
Hope this helps.
Mark
