Virago 750 - Will Not Rev

Question:

I recently bought this bike, which had been sitting for at least a year. Cleaned the carbs, checked the filters, gave it gas and started it up. Bike starts fine and idles great, but when I throttle up it dies. One person suggested the fuel filter as the culprit. If you agree, where is it located? I can not find any diagrams to tell me. If you dont agree, what could the problem be? {Cinnemon, Colorado, USA - 1983 Yamaha Midnight Virago 750, XV750}

Answer:

I have a short list of things for you to check on your XV750. Try revving with choke on, even if you have already warmed up the engine. Is it better? I think you have some sort of fuel problem.

Fuel Filter

First of all, your Virago 750 did not originally have an inline fuel filter. A previous owner may have added one, but there really is not a lot of room to fit a filter because of the way the carbs sit in the V of the engine. If you had one, you would be able to see it.

Your fuel petcock, aka gas valve, has a plastic tube and screen that filters the fuel as it leaves the tank. It is a couple of inches tall. You can not see it with out removing the entire petcock, it sits up inside the gas tank. Older bikes and those that are not ridden year round tend to have a build up of sediment inside the fuel tank. This sediment is made up of little pieces of dirt and rust that settle at the bottom of the tank.

When your petcock is On, it pulls fuel from the top of this screen. When you put the valve on Res, it pulls fuel from the middle and bottom of the screen. This is where all the sediment collects and can clog the screen. A 27 year old bike that sits in the garage each long Colorado winter could have a significant sediment buildup.



Check your fuel valve. If you do not notice a change in how it runs when the valve is set to On versus Res than move on to the next item. However, if it is significantly worse when set to Res, you will probably benefit from removing the petcock and cleaning the screen with fresh gas. While you have it off, you can also shake out sediment deposits from the gas tank, or pour a little fresh gas on the top and swish it around before letting it drain out the petcock opening.


Not Enough Air

The Virago 750 models had what Yamaha calls a monocoque frame. A true monocoque is a type of frame in which all or most of the stress loads are taken by the skin and it is a design with no separate chassis and body, like a car with unibody construction instead of a traditional frame underneath the body parts.

According to the 1982 Yamaha Virago 750 brochure “The functional monocoque frame design makes the engine an integral frame member, eliminating the excess bulk and weight of downtubes. Air actually passes through the frame to an air cleaner beneath the seat, eliminating the need for bulky air cleaners off to the side.”

While this may be a great innovation that reduced size and weight, it created a new problem. As strange as it sounds, this bike was known to get spider, bees and small animal nests inside the airspace of the back bone of the frame. This blocks the air from getting to the air filter. No air = no combustion.

Make sure that the passageway is clear all the way to the air box. Shine a flashlight in there to check. You can hold a shop vac against the air intake while blocking the opening in the air box with a shop rag to remove any squatters that may have taken up residence while the bike was sitting. Also, you must have the air filter assembly in place to properly run this bike.

If, after evicting all frame residents, your problem is solved, you are done. If not, keep reading.




Carburetors

I know you said that you already cleaned the carbs. I would just like to point out that most people overlook the slide when they clean their carburetor. Make sure that your carb slides, or diaphragms, move freely. They should make an air whooshing should when you push them up with your finger. If they drag, or hang up inside the carb housing, check the diaphragm spring and rubber skirt around the slide for holes and dry rot cracks.

If the slides do not lift up, they are blocking the air flow to the engine. If air flow is not sufficient and the slide does not rise, it can not lift the fluid needle, which hangs down into the carb.

The fluid needle helps to control the flow of fuel from the tank into the carb. When lifted, it allows fuel to flow through the main jet. The main jet controls bike operation in the mid to high RPMs.


If none of these suggestions alleviate your predicament, you may have a larger problem.

Try this. Start the bike and let it idle for a minute. Using a damp shop towel, touch the headpipe coming out of the front of the engine, it should sizzle. If it does not, the engine may only be running on one cylinder. {Yes, bikes can idle on one cylinder.} In this case, your Midnight Virago 750 requires a trip to the shop for a proper diagnosis.

Ask Weeks Cycle a Question about your Virago 750




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