I used to fix lawn mowers in my high school mechanics class almost every day, this one stumps me though. The engine is on my neighbor's riding lawn tractor. 12.5 hp vertical shaft Briggs & Stratton I/C, probably from the late 1980s. When he brought it to me it wouldn't start. I replaced the plug with the correct Champion and set the gap. It does have spark. I also changed the oil with Rotella T5 10w30, and put on a new air filter. I took the Walbro carb apart and cleaned it. Next I put in fresh gas and changed the fuel filter. The engine ran roughly and was hard to start. I've cleaned the carb 2 more times, even soaked it in carb cleaner overnight. It runs a bit better but still doesn't start very easy and runs slightly rough at full throttle. I even ran a thin wire through all the jets when I had it apart. The carb looks very clean. I tried adjusting both the jet screw and the idle and it didn't do much. Any ideas? It runs perfectly smooth at idle and 3/4 throttle.