The whole purpose of this thread was to stir something up. I kept thinking of the barrel carbs in terms of a butterfly valve carb and was having trouble getting my head around the change of air direction over the nozzle depending on throttle position, mainly the metering slit. Keep the pot stirring.

I read an interesting point in the Walbro Diaphragm Carburetor Service Manual on Barrel Valve Carbs. I quote

"Since the barrel restricts the air flow both before and after the main nozzle, the main nozzle is always acted upon by the same amount of vacuum, essentially a variable venturi."

and

"The advantage of the barrel valve design is the it provides for a very even air/fuel mixture thoughout all throttle settings."

Iam in the process of setting up an experiment to see if I can determine if the spray bar alignment has any effect, and if so how much. I probably should have used one of my ST for the project, but I'm going to use my Saito 150, which if you have worked on, you know it is very difficult to get the spray bar to stay in one position while tightening the nut holding it. My goal is to setup the carb for best overall performance, then get RPMreadings for a number of fixed throttle positions. I'm am going to make a ratchet arm setup for changing the throttle so it will be in the same angle for each change of the spray bar, so the spray bar change is the only thing changing. That's the plan, but reality may get in the way of the plan once I have my test bed setup.

As for your point about the spray bar being the way to compensate for altitude, I think that is the job of the high speed needle. The ST instructions are to adjust the angle for best mid range performance after the high and low end are set. This is one of the confusing things that started this thread. Why did ST go to such efforts to make the spray bar easily adjustable in relation to air flow, when other makers are content with a fixed position, or don't seem to care about the orientation?

Don