Small Engine Troubleshooting Problems? Ask a Mechanic Now.
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There is a possibility that your choke is not fully opening, after your engine starts. This is likely the reason for the rich running condition, and the black smoke.
Remove your air filter cover, and your air filter. Move your mower to a concrete surface, then start your engine. Check to see if your choke is fully opening.
Your choke is thermostatically controlled by a thermostat on your muffler. As the temperature of your thermostat rises, your choke should automatically open.
Check this, and we can continue from here. Thanks!