Small Engines (Lawn Mowers, etc.): engine sputters, briggs and stratton, fuel tank cap

Question
Lawn mower engine: Briggs and Stratton model 10A902 type 2189B3, 4.5 HP (I think).  Just recently had coil unit and plug wire replaced (motor wasn't firing) and also drained oil and resealed gasket on oil pan (oil was leaking).  Cleaned carburetor and drained fuel tank. Refilled motor oil and fuel. Now the motor sputters and runs very rough and weaker than before.  Could I have inadvertantly adjusted the timing or is that possible with small engines?  Any suggestions on how to get the engine purring again?

Answer
Jay
Here are some possible solutions

Engine lacks power (possibly after warmup).
Possible causes:

# Old gas, bad spark plug, very thick/dirty oil.
# Choke is partially closed.
# Carburetor needs adjustment or service.
# Ignition not timed correctly.
# Air filter is clogged.
# There is a lack of lubrication.
# Valves are not sealing properly.
# Piston rings are not sealing properly.
# Head loose or head gasket blown or damaged.

Engine operates erratically, surges, and runs unevenly.
Possible causes:

  1. Fuel line or fuel tank cap vent is clogged.
  2. There is water in the fuel.
  3. Fuel pump is defective.
  4. Governor is not set properly, sticking, or binding.
  5. Carburetor needs adjustment or service.
  6. Loose carburetor/intake pipe resulting in vacuum leak.

# Spark plug is fouled, improperly gapped, or damaged.
# Breaker points are pitted or improperly gapped, breaker arm is sluggish, or condenser is bad.
# Carburetor needs adjustment or service.
# Fuel line, fuel filter, or fuel tank cap vent is clogged, or fuel shutoff valve partially closed.
# Valves not adjusted properly or valve springs weak.