This is my first adventure with a YS engine and so far I'm liking it but I'm still in the dark on a few things. I've been able to find the instructions for the 120SC model and it helped some but my AC has a fitting at the top of the air box and one below on the backplate that look like they should be connected with fuel tubing right? Well, when i bench ran the engine I had them connected and it seemed that if I pulled the fuel line off at the airbox while idling, the engine went lean, but again, I only tried this at idle. What are these two fittings for at the back of the engine for anyway? Maybe I'm not using them correctly. I've got the whole one-way valve,pressure, fuel, and regulator thing figured out well enough that using WC 20/20 fuel and a Zinger 16x8 that i'm getting a steady top end of 8400rpm and the engine seems happy but I'm not one to go needle happy and try to find the last bit of rpm I can get. I feel the engine is running at normal temps and has good throttle response. My biggest problem, maybe, is just how much vibration this engine has at mid-range down to idle. The prop is balanced perfectly and it would seem that its just too rich but without a low-speed needle, I dont know how to fix it. I have plans of using this engine in an Ace 4-120 Bipe so it will see mid-range speeds most all the time. Maybe the engine wont make a good sport engine because it was designed to be at WOT most of the time in pattern ships and mid and low throttle vibration wasnt a concern. Thanks for any input you guys might have. Mike