Check the valves again.
I could not even remember just how many have come into the shop with an impossible to fix mower that they had spent near $1000 on replacing everything that bolts on.
It turns out to be common valve lash.
People are used to modern cars with twin overhead cams fitted with roller rockers, bucket lifters or hydraulic lifters.
These of course never need to adjusted for the life of the vehicle
Fifty years ago monthly adjustments were considered routine maintenance, along with adjusting the points .
While it is an advancement technically speaking, it has been a giant step backwards educationally speaking.
So now 99.9% of the population have no idea what happen with their vehicles when they turn the key.
Prior to 1964 when mowers changed from points ignition to Hall Effect triggered magnetos the average Joe Blow could tell from the way the mower was running if the capacitor ( condenser to some ) had failed or the points were too wide or were losing up.
So dads would eventually devote a 1/2 to do some mower maintanance.
Remembering that back them a petrol push mower was 3 to 6 months wages so it was worth putting the effort into this, and while he was there he would grease the chassis parts and check all of the nuts & bolts, perhaps even look at the air filter although we had snorkels so, an air filter was good for 10+ years.
Post 1964 mowers no longer required constant attention to the ignition system and owners quickly forgot how to diagnose their mowers performance so now days I only see them when there is no blades left or they won't start.
This situation was made even worse when the Powertorque engine was introduced in 1984 because it requires next to no maintanance so none got done till it took 2 hour of handle pulling to get them to start.
