Small Engines (Lawn Mowers, etc.): Lawn Mower-Model Number Added, briggs stratton, rear discharge

Question
How to clean fuel line on lawn mower?  I have a 12 year old "Yard Machines," by MTD, 4 HP/21", powered by Briggs & Stratton. Model Number 414, Series 1B107C40705.  Rear Discharge Rotary Mower/Bagger & Mulcher.

One of my sons inadvertently put weed eater fuel (gas & oil mix) into the lawn mowers gas tank.  

I emptied the gas tank, used "Gumout ALL-N-ONE" added fresh gas, and replaced the air filter.  It ran for me, but coughed and smoked.  Now it's time to cut grass again and it won't start.  I'm thinking the fuel line is clogged with the gas and oil mix.  

What can I do to fix it?

Answer
If it was clean gas and oil mix, then it wouldn't have caused any problems, so the problem has
another cause.  Just for testing purposes, remove the air filter and cover and leave off for now.

Replace the spark plug, and see if it just not a fouled plug, cause if the air filter was dirty
the plug could be fouled.

With a new plug and the air filter off, try to start and see what happens.  If nothing, squirt some fuel or carb spray and squirt in the carb and try, if it starts then dies, then you know that the problem is in the carb.  Try these things, and let me know, and find the engine's model numbers.