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I recently purchased a Honda CRF50 dirt bike for my kids. When applying the throttle black smoke that smells like gasoline comes out of the tailpipe. From what I understand this means that there is gasoline that is not being combusted (running rich?). When I purchased the bike I changed the oil and put in a new spark plug.
What could cause this and how can I go about fixing it?
The problem is most likely the carburetor. It may have been mucked up by ethanol in the fuel. At a guess I'd suggest either the float isn't floating, or the float needle isn't closing correctly.
You may be able to get this cleaned up by running some SeaFoam or BG44K treatment through the gas tank. Make it a fairly high concentration of it (double what the can says for dilution in the gasoline). This won't hurt either way.
It may also be an obstruction in the intake. Assuming there is an air filter present, check it to make sure it isn't clogged up. Also check to ensure all of the air ducting is in good shape. If something like that is crushed or deformed, it could be causing a restriction, which would cause higher than normal vacuum while running, which would cause the carb to dump excess fuel.
