Small Engines (Lawn Mowers, etc.): briggs & strattin 12.5hp, lean condition, condensor

Question
moter is from 83 model weedeater[usa] mower.
backfires through carby when trying to start.
will not start at all.
plenty of spark at plug.have pulled down and seems fine.not sure on valves how do i test.

Answer
Backfiring through the carb generallay indicates either a lean condition or an issue with timing.  There are only two things that can affect timing on a B&S.  A sheared flywheel key and, if equipped, a problem with the points and condensor.  Based on your model year (83), this engine could still have points (the switch, as I understand it, was 1982).  If it will start with a squirt of fuel down the carb, you have a lean condition (carb not supplying fuel).  If this doesn't help, you will need to look at the points and condensor, if equipped, as well as the flywheel key.  It's unusual for a valve (especially an intake valve) to go bad quickly.  It is difficult to perform a compression test because the engine has a built-in decompressor and the reading will be low at cranking speed.  Next to listening for valve leakage through the intake or exhaust during cranking (whisping noise), visual inspection is the best way to determine valve condition.  Hope this helps.