How time flys?
That DZ170 CDI of mine was running great until one day (early 2023) it started coughing and misfiring again. Did a few quick checks and a few more flights but nothing improved and then noticed the intake valve spring felt "soft" and discovered it was broken right at the bottom. Replaced it and it made no difference. There was noticeable wear in the fuel pump bore with a small lip at the bottom of the piston stroke which I thought could have been causing the pump piston to hang up under certain conditions and stop the intake valve from fully closing but I put a new pump on it and again, no change. I messed around with a few things but I was totally sick and tired of chasing problems and I was enjoying flying helicopters with an OS 105HZ-R that's never missed a beat so I stopped flying it.

Fast forward to early 2025, and I decided to replace the throttle body on the brand new DZ200CDI I had that had never worked properly from new and Hey Presto!!, I had an engine that I could put in my Aries 07 and go flying with again. The 200CDI isn't as "nice" in the Aries as the 170CDI but for sport flying I'll take a running YS over a non-running YS any day of the week. Did I mention the third set of servo's I put in in 2022 (nice Futaba HV ones) the pot of one aileron servo has a worn spot at neutral in it already from the vibration. I swear if I ever replace them I'll mix in flapperons just to keep the ailerons away from neutral more often to reduce that spot wear.

So I guess it's safe to say after 15+ years of flying with two YS engines that my sum knowledge of them boils down to "if it's not running right there's something wrong with it" and your guess of what that is, is as good as mine . But , please let us know what you find if you figure it out.....