Small Engines (Lawn Mowers, etc.): lawn mower idles drop, briggs and stratton, shaft move

Question
i have a Briggs and Stratton push mower and when i start it it revs are ok for a while. but after i start mowing the revs just start revving high and low/up and down. what is wrong and what can i do to fix the problem

Answer
hi josh

sounds like you have a fuel  flow problem or governor needs adjustment  what you described is hunting / surging

be sure to check governor springs as well  make sure not deformed or missing / un-hooked from links

try  cleaning carb out - check main jet  and openings , check idle air bleeds and progession port holes ( allowing fuel in @ top no load speed ) clean main nozzle and bowl vent as well
reasseble carb and refit  with new o ring an primer gasket if it has one  

main jet supplys fuel @ full speed  with load only   be sure to adjust governor and check fuel flow from tank to carb and also tank cap breather ,   i presume   mechanical governor ( is under  flywheel cover  

to adjust governor ,remove  pull start cover   with engine stopped  move the throttle plate and link from closed to full open  , note direction governor arm moved ( this should be anti-clockwise )

loosen nut on governor shaft move throttle to full open and hold @ this position
turn the governor shaft anti-clockwise as far as it will go  and tighten  governor shaft nut
making sure you dont move the link as you do - refit starter cover

all going well should go well  provided good spark and compression

cheers paul