Jim, I received the PDF and thank you for your trouble. That helped answer my questions on the plumbing layout. I'm still trying to wrap my head around the operation of the engine however, and I wont presume to know more about engine design than YS, after all , this engine sold for about $550 in the late 90's so I have to reason it was well worth it, but why is the check valve between the airbox and the backplate there anyway? Looking at some of the other models with the airbox, like the 91 and I think i saw a 110 too, they didn't look to have the same setup as the 120. Is it some simple form of a wastgate so dropping to idle from full throttle doesn't load it up? Well, anyway i'm glad to finally own a YS, and I feel I stole it on ebay. I've been looking to get a 120 for the Bipe project and was looking at buying a good used Saito or O.S. but both in good use condition were selling for $250-$300 and a guy had a BIN on this YS way less than that so I figured there wouldn't be a better opportunity to give one a try. As the engine and I get to know one another, I'll drop by every now and again to ask a question and to let you know what I got figured out. Now, on this Ace 4-120 Bipe I'm starting, I'm looking for moderate speed but max pull on up lines. I most generally use 6* pitch on all my props no matter engine size, I just adjust the dia. to suit the rpm range the engine like to pull best in. It seems to be the best compromise between pulling power on the up lines, over all speed range I'm looking for and best braking on down lines. I'd like to use a MA 18x6 with this plane and engine combo. How well does this engine like to lug a big prop? I'll try a few different dia. in flight but I'm looking for min rpm numbers I should keep the engine above to use its torque most effectively. Right now, I figure no less than 8k but this engine might be happy down to the mid 7's for all I know at this point. Any of you pattern guys play with numbers this low?. Mike