The rubber seals and fittings between the carbs and cylinder head get hard and brittle. They can develop small cracks and also loose the seal they had when new and pliable. This means that extra air gets into the air/fuel mix as it heads for the intake valves. The air that has not gone through the carbs leans the fuel mix and causes the high revs. The seals and fittings will need to be replaced. Dealers have parts in stock for 10 years. Your bike is 22 years old. Google yamaha virago 750 parts
also Google vintage motorcycle parts.
Babbitt's would be a good source if the '87 and '88 parts match up. They list an '88 and have all the parts in stock except one hose line.
Go to the site below where you can see a parts diagram for your specific bike. You will select the actual brand, year, model, etc., once you go to the site. Part numbers and prices are also shown. You can order parts from this site. In the event no price is shown on a particular part and/or the notation "Not Available" is in the description, the part is not in stock. www.babbittsonline.com/pages/parts/viewbybrandand/parts.aspx