I have done extensive searches on this subject and found that my problem has they exact same similarites as many other 26 EFI owners.Runs great most of the time but has a hesitation when not under a load then when under a load sputters like its starving for fuel never can recover.I have fouled more plugs than you can shake stick at. My dealer just takes it in lets it run for hours to duplicate cenerio never seem to fix problem.So far they have changed muffler (cracked) new O2 sensor (waited two weeks) this was last fall now its doing it again and my dealer just acts like they have no clue what to do. Personally i think they are trying to run out my warranty. Now that i know there is a serious problem with this engine from my searches i definately feel i am getting the run around from my dealer.Because it is either diagnose the heck out of it for hours or replace numerous parts to solve problem. I am loyal to exmark (entie fleet) and thought i was buying the best mower available (which it is when its running) but this is starting to cost me major money in lost revenue and major frustrations beyond belief.Either exmark or kohler needs to step up to the plate and help me through this with my dealer.Either fix problem right, replace motor, or replace unit because when you spend $9,000 on a Lawn mower and it sits at dealer for days on end somthing has to be done.Lets put it this way, the mower that comes off the back of trailor first right now is ORANGEORANGE (loaner).