I have a riding mower with a Kohler SV715 engine. The engine has 32 hours running time. For the last couple of uses it has run a little rough at high RPM and when under load. Today it was much worse. The engine misses and backfires (thick black smoke) and the exhaust fumes are eye watering. It feels like both RPMs and power are very low. The engine seems to idle OK and it runs fairly smooth at low speeds unloaded. At the start of the season I changed the oil and replaced the air filter. I checked the plugs and they were clean and not worn. I did not replace the in line fuel filter. Is dirty gas the most probable cause of the problem? Anything else I should look for? I just filled the tank and I don't want to empty it unless I have to.

How can I remove the float bowl from a briggs and stratton model # 28U707 engine> It has the fuel shut off solenoid on it. I can remove that but then am not sure what to do next. Is there a nut that also comes off or does the bowl come off with out any more part removal ? I am asking because I think the float needs to be cleaned. When trying to start engine the combustion chamber loads up with so much fuel that the starter cannot spin the engine. Thanks for any insight.