Yeah, wht DH and acdii have said makes total sense. I was 'assuming' that there was an open circuit in the crankcase, which is not the reall case, in fact the breather nipple at the bottom, is between the bearings as well, and in combination of the varying crankcase pressures on the up and down strokes, its not likely easy for air to influence the manifold pressures. I was overthinking it, and along with your explanations, it makes sense.
I will get my engine hopefully at the end of the month, and would be good to know the RIGHT way to connect those tubes. One thing that is not clear is how the lower nipple on the manifold works. From the exterior look of where the nipple connects, it appears to be open to the pulse port on the carb, as well as being open to the manifold? Is this the case or is it only open to the pulse port?
I have never noticed any difference between 4T and 2T Walbro carbs, but then, I havent opened one up. It makes sense to have the diaphragm assisted by a spring opposing the pulses, rather than just pulses.
Frankly, I dont think Saito pay enough attention to their gas engines, they would benefit from the sort of set up you see on this engine, rather than just upping the oil ratio so that 'blow-by' is lubing the crankcase and valve mechanisms.....it means trying to burn more oil, and in my FG60R3 and FG40, running on the 20:1 mix has fouled my plugs, and the ex valve is all carboned up. I have recently converted to Stihl oil to try and reduce this. On the FG40 I dont have an option to reduce the oil ratio. On the FG60 I have modified to run fuel/air through the crankcase, and am now running it on 30:1. It seems happier this way, but not going to call it a success until some run time has bene completed.
Sadly, the FG40 this is not an option, but if this NGH38 works well enough on this design, I might look to do something similar with the FG40....its just a few nipples really...and the cost of a manifold and backplate if it fails and has to be converted back. Hell, even just plugging the holes you have made would reverse the mod. This is why I am keen to know how that lower nipple is open to manifold AND pulse port OR just the pulse port.
Thank you for the updates, info and insight....I am known to run out of talent and knowledge, so getting others input is great!
Andy