If the soft start was obligatory, Yamada would have recommended the usage of such devices in the user's manual. Kickback is not common only for the DZ series but on the pressurized ones.

Troy Newman has recommended the usage of a very powerfull starter to avoid kickback, practice shows that he is correct. Also in warm days kickback is more possible as pressure is developed in the tank and engines is flooded. The usage of a heli tank clip may be useful here. Another way of avoiding this is hand start, by turning the engine clockwise.

The piston degradation is a long term process it does not happen at once, here is a piston failure of a YS-140 FZ I used to run in the past. The initial symptoms were loss of power, and not as stable idle. This lasted for a few gallons till the engine died in a upline. I replaced only the piston and engine ran like a pro.

I have seen a similar DZ piston failure in a Japanese F3A web site with cracks on top. These kinds of failure may be random and rare, I can not believe that the Japanese are unaware of running YS DZ engines.

That's my 2 cents...

Regards,
Nikos