Hoping for some expert advice. I have a lawn tractor with a Briggs 12.5 model 288707-0110-01 engine. The engine starts fine and runs great for a minute or two. As soon as it begins to warm up, the engine starts to race and can't even be pulled down with throttle lever. I've cleaned the carb with carb cleaner as well as all the linkage. The throttle lever and linkage is moving freely and I see no binding. I have also checked the static adjustment of the governor arm. The rpms do come down some under load, but still seem awfully high to me. What's the next logical step in troubleshooting this problem? I'm mechanically inclined, but not very experienced with small engine repair.