Exmark Lazer HP 19 HP 48" Deck Problem started today or I should say we noticed it today. When we start up the mower and push throttle to full power it seems like it is only going about half. ( This is what is sounds like anyway.) When we engage the blades it seems very slow and under powered. Air Filter NEW I checked spark plugs. One had a little carbon buildup not bad the other hardly any however it did seem to be wet. I couldn't tell if it was oil or gas. Throttle is mechanically opening all the way so not a linkage problem. I replaced both plugs. I did notice a little white smoke before the plugs were changed but not noticeable after plug change. Power did not increase at all with the change. Engine maintained regularly and I have no problems with it at all. 598 Hours on meter. What should l look for next?