remove the spark plug and turn it thru (be sure the battery for the ignition is on and charged). Ground the base of the plug against the engine. Verify that it DOES spark! Timing is not relevant at this point. If it does, reinstall the plug and proceed to adjust timing (I think 28 degree advance to idle is a LOT for a 4-stroker, your ignition box is probably and auto advance unit, try an intial setting at about 10 degrees and experiment from there). If it FAILS to spark you may have -

a bad ignition unit
a bad battery/switch
a bad hall sensor
the trigger magnet mounted upside down (THIS is the most likely problem)

If you are getting spark, it may be more likely that you have a FUEL issue... Fuel (proper mixture of air) + compression + spark WILL EQUAL detonation!

Hall sensors are North/South sensitive, they only trigger one way. If you have the magnet mounted backwards, it WILL NOT fire... You can try removing the sensor and passing a magnet over it to verify this. Holding the magnet one way will fire, turn it around and it will not.... BTW - the magnet needs to pass pretty close to the sensor pickup, within 1/16" is nice... A strong magnet is desired. Radio Shack sells some little neodymium ones that are about 1/8" diameter and about 1/16" thick. The work very nicely and are cheap.