I have a 99 Encore z48 with something like 2500 hours. I think it has a 18 horse brigs. The problem I'm having is when the motor is cold and I engage the blades it boggs down and isn't worth a darn. If I let it warm up you can hear the engine find a higher full throttle. then the mower works great for like 5 min. then I'll be mowing along and it boggs down back to where it was when it was cold. even with the blades off it can't get back into that sweet spot. what should I look at first? I havn't checked the plugs and the air filter could probably use replacing. I'm just worried this might be a head gasket. I can't mow with it when it is acting like this. Thanks for everything guys.