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The term "running lean" means the engine is not getting enough fuel, or the engine is getting to much air. I would start with replacing the fuel filter. You can check the filter to see if it is restricted by blowing through the filter. Should be easliy blown through.
The other possibility if that there is an air leak or "unregistered" air entering the engine, either from the intake or exhaust. Make sure the air intake from the filter to the engine is good and secure.