The chonda base engine is capable of running as well and as long as a Honda, Peter. I make no comment on the mower, though: chondas can be found on some pretty awful bases, I don't know if yours is at the better or worse end of the range.
The main reason to never overtighten a valve-adjustment locknut, is that you stretch the thread out of pitch, and it then becomes extremely difficult to adjust the clearances accurately, because right at the point where you want the adjustment to be, the thread suddenly goes out of pitch. Of course the out of pitch condition exists in both nuts as well as the central screw. When I was a kid all cars that I came in contact with had manually adjusted valve clearance (hydraulic lifters came much later) and the cars I worked on were always pretty old and had been treated pretty badly. I ended up thinking that adjusting valve clearance was a really difficult job. Of course it isn't, when you work on a well made engine that hasn't been abused. In the current case, the abuse probably happened in the factory.
In reality though, life can be more complicated than that. I once towed a caravan around Australia with a brand new diesel 4WD. I ran it around the local mountains first, just long enough to have a first service from the dealer before I left. When I got to Darwin it needed an oil change, so I crawled under and applied a cheap 3/8" drive socket to the drain plug. No success: just ripped the points off the socket, at probably 80 lb ft of applied torque. Mildly miffed, I bought a good quality long-handled ring spanner from a speed shop and tried again. No joy: spanner was fine, but I just couldn't pull hard enough. We are probably talking 100 lb ft or so. Determined to succeed, I applied a double-compound lever system, using various crow bars I had, and really swung on the end of the final lever. It came loose with a horrendous bang, and I changed the oil. When I got home I went to see the dealer's service manager about the matter - it was a cast aluminium sump, and it seemed like a miracle that it had survived the drain plug being done up to more than 200 lb ft. The service manager looked slightly embarrassed, and told me that a few days before my car was serviced, he had noticed that the sump plug on his identical diesel 4WD was leaking, and he had ripped the service tech's arms off and beaten him about the head and shoulders with the wet ends. Then my car came in for service. Guess what the service tech did to my car, as a sort of protest?
I'll close this thread.
