First let me make something clear. I learned what I know about Virago carburetors the same way I learn about all mechanical things: through research and tearing into them. I am not a certified, trained mechanic. Before I bought this Virago, I had never messed with a Hitachi carburetor, much less two joined together. If the bike had run flawlessly when I got it, I never would have opened the carbs. The bike I bought is a 1984 Yamaha Virago XV 700. One of the previous owners had changed the engine out and put a 1983 750 in its place. The carbs were not changed or modified, they were just put on the 750 as is. Thinking the running problems and the fuel consumption were because the carbs were gunked up, I opened them up to clean them. I looked up the specs for the 750 not realizing that the carbs had never been changed. Here is what I found.
In the 84 700's the front carb (#2 cylinder) had a 132 main jet and the rear carb (#1 cylinder) had a 128 main jet. In the 750 both carbs called for a 122 jet.
The 700's air mix tube is pressed in and press capped, making it inaccessible for cleaning. The 750's air mix tube is accessible behind a 6mm allen hex and it can be pulled out and cleaned.
In the 700 the pilot jets are not changeable, they are pressed in. In the 750 the pilots were scew in types and easily changed.
In the 700 the air mix screw assebly was changed from the previous set up. It was almost reversed. The spring goes on first, then the washer and last the o-ring. On the 750's it was the o-ring in a groove on the mix screw first, then the washer and last the spring. That information was not included in the clymer and Hanes manuals. Shame on them.
Armed with this new information, I cleaned the carbs and changed out the main jets. Unfortunately that is not quite enough to run the 750 smoothly. The air jets and the main needle are also different sizes and the pilot is supposed to be one step different. That's where I am now. Gotta find the money to buy more parts or different carbs. Thinking different carbs is the way to go. I want to find the newer Mikuni flat slides with pumpers, but I will probobly have to settle for 81-83 Hitachi CV BSC40's. Oh well.
As an update to the above, I went with a single carb setup. You can see the specs on the "Virago" page.