Hi,



I have a YS 63 that is four years old. It has always run perfectly until the past couple of months. It now has problems running in the mid-range. Full-throttle, idle, and transition from either is perfect. However, when running at roughly 2/3 throttle, the engine goes rich after 8-10 seconds and eventually quits. If I throttle back after it goes rich it will quit and if I move to full throttle, it sags and eventually quits. I have checked the valves, changed glowplugs, and put in a new regulator diaphragm and it still does the same thing. I have tried screwing the regulator both in and out and did not notice any improvement. I am using Powermaster YS 20/20 fuel, an OS Type F plug, and flying at sea level. The engine has been in the same airplane its whole life.



Do you have any ideas as to what I should try next? Also, if needed, how do I send the engine in for servicing/overhaul.



Thanks,