Small Engines (Lawn Mowers, etc.): Backfire thru Carburetor, briggs stratton, thickness gauge

Question
Hi,I have a briggs & stratton platinum 15.5 HP OHV Turbo Cool. model 28N707,- type 0173-01- code 960614 ZD, the trouble is when I get it started it backfires thru the carb, I mean with the cover off I can see the fire when it backfires, also can you tell me what the magneto should be set at,? I think ,020, see I had to remove it to clean it and not sure if I have it set right and think that may be why it is backfireing? also on the carb, there is two screws one almost on top,one lower down, can you tell me what one is air and what one is gas,? Thank You So Much for your time and trouble. Bill

Answer
Bill

What type of carb is this? they use 2 different types one is a walbro the other is a Nikki.Yes it sounds like the Magneto needs to be adjusted. between .012 and .020 is correct The screw on top is for the idle speed the one on the side is the idle mixture set this about 1-1/2 to 2 turns, do not bottom it out just tighten down till it stops then adjust up.

Problem: Engine backfires.
Possible causes:

  1. Carburetor set too lean.
  2. Magneto is not timed correctly.
  3. Valves are sticking.

Armature Air Gap Adjustments
1. Rotate flywheel until magnets are away
from armature.
2. Loosen both armature mounting screws
(A) and pull armature away from flywheel.
3. Snug one screw.
4. Insert the proper thickness gauge
between armature and flywheel.
SEE SECTION 14- ENGINE SPECIFICATIONS.
5. Turn flywheel. Loosen screws and allow
the magnets to pull armature against
thickness gauge .
6. Tighten screws to listed values