I've found a problem yesterday that I wanted to share in case someone else is going through the same thing. I have a YS110 installed in a UCD 60 and starting from square one it wouldn't idle down without petering out within a very short time. It would never come close to idling down properly. Many club members attempted to help. No one was familiar with YS including myself. At first I thought it was just our lack of knowledge. Eventually, it became clear something was wrong. I came home with it Saturday thinking it was an engine problem because of fuel noticed on the joint between the rear air chamber and the carburetor body. Thats the one where you see the o-ring seal. There was always fuel around that joint and one guy said he even saw it dripping while the engine was running. That got me thinking that at rpm with the system under pressure, a leak there would allow air to pull in during the vacuum created at idle. Sunday I pulled the motor and removed the rear air chamber and saw what appeared to be imperfections in the o-ring seating surface on it. I took pictures of it and reassembled it. Then I put it on a test bench and started it up. It ran great! First time for that. I was so elated that things were turning for the better. I thought of what had changed. It wasn't inverted and it was hooked to a different tank with different lines. OK. Next step. I reinstalled the motor and hooked everything back up as original. Started it and it was idling poorly again. I removed the fuel lines from the tank and patched in the tank from the test bench. Laid it on the ground underneath the plane. Started the engine again. Ran great. Tank problem right? I pulled the tank and inspected it. I knew it didn't have a leak because YS pressure would have exposed that on day one. Clunk line? I held it to a bright light and everything looked OK. Couldn't think of what could be wrong with it so I hooked it back up and ran the engine with the original tank only I left the tank on the ground. Tried it at a few different attitudes and it ran great. Not making any sense? I know. That's what I was beginning to think. After revisiting that it ran great at rpm (pressure mode) but poorly at idle (vacuum), it dawned on me that a pinched fuel line may cause my problem since at pressure the fuel would flow but at vacuum the line would collapse and starve the engine. Finding very little pressure in the tank after successful extended idles helped me to realize this was a possibility. The picture below will show where I installed foam around the fuel lines to help isolate them from vibration. This foam extended underneath the throttle servo. I trimmed it back and reinstalled everything. Today I ran it again and everything is perfect. Starts great even by hand. All of these starts were at 1-1/2 turns on both needles. Idles lowwwwww. I don't have a tach but I will check it next time I can borrow one at the field. I'm sure it is at or below 2000 rpm. Apparently this servo and foam was putting enough pressure on the line to cause this problem and it has cost me a full day of flying. [>:] Thought I'd pass this information along hoping to help someone. The fuel around the o-ring? Since it's inverted I believe any fuel that drips from the carb when the engine is off will creep to that area and wet the oring. That day at the field there were so many starts and stops that this was happening a lot. I believe that a normal shutdown with a properly tuned motor won't yield so much fuel drip.