The rocker arm clearance tends to loosen in these engines.
This causes too much compression at the wrong time resulting in hard starting.
Most people think the battery is weak but a relatively simple adjustment of the
valve/rocker arm clearance should solve your problem.
Check your manual and YouTube for instructions or consult your local mower shop.
For a Briggs & Stratton Intek engine "PH8170"
scotts intek 6.5 starter cord replace
48 oz oil capacity
try .030 unless it is an Intek OHV and then 0.020"
SAE 30
0.030
With a model number , try the link below.
Valves 21 hp Briggs and Stratton Intek V Twin can be adjusted by increasing the torque on screw band with a right rotation. This can eliminate some leaking problems.
0.030"
Even though an Intek is not shown, it would be the YouTube video below.
Try this site: http://outdoorpowerinfo.com/repairs/
It needs at least 70 psi to run.