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Need help with XV250 carb settings

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Need help with XV250 carb settings

PostAuthor: Dark Angel » Tue Sep 12, 2017 5:41 pm

Hi folks,
I'm working on a '94 Virago 250 and have run into an issue I've never faced before. I just cannot get the fuel height set right and it's running very rich.
Normally I'd just bend the tang on the float until the fuel matches the required height, a little fiddly but no drama. This time something's different. If I bend the tang far enough to get close to the required fuel height (16.5 to 17.5mm down from the cast in mark on the outside of the float bowl) the float contacts the inside of the bowl and pushes the needle completely closed. If I bend it back to where there's enough needle opening to get sufficient fuel flow to run the bike (about 2-mm off the seat) the fuel height is nearly 10mm too high. This is with brand new OEM float and needle and seat assembly.

A little background. The bike itself sat for several years before the previous owner got it. He was a butcher and the guy before him seems to have been a mug as well. They did things like hacksaw the intake funnel off just above the carby and drill holes in the frame (not drill and tap, he just wound the stock screws into the holes) to relocate the choke lever, and no maintenance was done on it for years (the brake fluid had turned to white slime). I've been replacing worn or damaged parts since I got it. I tried to clean the air filter yesterday only to have it disintegrate. Thankfully I already ordered a replacement. There's every chance one of them did something stupid to the carby that's caused this but I'm at a loss as to what it might be.

Does anybody have any ideas on what needs to be done to get this right? A new carby is pretty much out of the question at the moment, but surely there has to be something that can be done.
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Re: Need help with XV250 carb settings

PostAuthor: CX Rancher » Tue Sep 12, 2017 7:52 pm

Dark Angel wrote:Hi folks,
I'm working on a '94 Virago 250 and have run into an issue I've never faced before. I just cannot get the fuel height set right and it's running very rich.
Normally I'd just bend the tang on the float until the fuel matches the required height, a little fiddly but no drama. This time something's different. If I bend the tang far enough to get close to the required fuel height (16.5 to 17.5mm down from the cast in mark on the outside of the float bowl) the float contacts the inside of the bowl and pushes the needle completely closed. If I bend it back to where there's enough needle opening to get sufficient fuel flow to run the bike (about 2-mm off the seat) the fuel height is nearly 10mm too high. This is with brand new OEM float and needle and seat assembly.

A little background. The bike itself sat for several years before the previous owner got it. He was a butcher and the guy before him seems to have been a mug as well. They did things like hacksaw the intake funnel off just above the carby and drill holes in the frame (not drill and tap, he just wound the stock screws into the holes) to relocate the choke lever, and no maintenance was done on it for years (the brake fluid had turned to white slime). I've been replacing worn or damaged parts since I got it. I tried to clean the air filter yesterday only to have it disintegrate. Thankfully I already ordered a replacement. There's every chance one of them did something stupid to the carby that's caused this but I'm at a loss as to what it might be.

Does anybody have any ideas on what needs to be done to get this right? A new carby is pretty much out of the question at the moment, but surely there has to be something that can be done.


How do you measure your float height?
How do you measure your fuel height?

How do you determine how much you change the float setting?

If I bend the tang far enough to get close to the required fuel height (16.5 to 17.5mm down from the cast in mark on the outside of the float bowl) the float contacts the inside of the bowl and pushes the needle completely closed. yes. is this not correct? Older Hondas had a spring dampened needle and you were to set initial float height as the needle contacted the seat without loading the spring. The Virago has no spring and will seat fully.

Sure could be other issues but the fuel height, not the float height, is the primary setting to start at.
G'Luk !
---Rancher


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(Corrections to my Speeds and Mileage posted before 5/2014 (-10% above 50mph) and (-1.8%mpg) Your Stock Speedo Variation may vary) Corrected by MoDot and TrailTech on Utah Tour. Truth in Posting Law
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Re: Need help with XV250 carb settings

PostAuthor: Dark Angel » Tue Sep 12, 2017 8:05 pm

1- How do you measure your float height?
2- How do you measure your fuel height?

3- How do you determine how much you change the float setting?

4- yes. is this not correct? Older Hondas had a spring dampened needle and you were to set initial float height as the needle contacted the seat without loading the spring. The Virago has no spring and will seat fully.

5- Sure could be other issues but the fuel height, not the float height, is the primary setting to start at.

I've numbered the points to answer in order.

1- There is no float height setting for the 250 carbie in the service manual. Typically I use a steel rule with 0.5mm gradients but that's no use in this case.

2- I use a clear plastic hose attached to the drain spigot, per the service manual. I have scribed a mark at 17mm down from the mark on the float bowl that aligns with the centre line of the main needle, as per the service manual. When I open the drain valve I make sure there are no air bubbles in the line which would give false readings before letting the level come up.

3- Digital vernier calipers. I measure from the flat of the metal float arm to the point of the tang that is getting bent.

4- The needle for the Mikuni carb in the 250 Virago also has the spring loaded pin.

5- I am specifically trying to set the fuel height as that is the only spec I have available. Once I have that correct I can be more concerned with idle mixture etc but not before.
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Re: Need help with XV250 carb settings

PostAuthor: robimisery » Wed Sep 13, 2017 12:00 pm

One question
how to properly measure the fuel level
when the engine is on or while it's off
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Re: Need help with XV250 carb settings

PostAuthor: Dark Angel » Wed Sep 13, 2017 10:57 pm

robimisery wrote:One question
how to properly measure the fuel level
when the engine is on or while it's off

The service manual specifies that the motor is to be running.
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Re: Need help with XV250 carb settings

PostAuthor: Dark Angel » Sun Sep 17, 2017 6:35 am

Had another go at it today. It's as though the pump is pushing the needle open until the bowl is over filled. Lifting the float by hand stops the fuel flow (bowl bottom off, turning the motor over with the fuel tap set to 'on') but it would be very easy to put more force on that than the float would manage on it's own.
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Re: Need help with XV250 carb settings

PostAuthor: Dark Angel » Tue Sep 19, 2017 1:51 am

Nope. There is no pressure relief in the fuel pump. It's just not part of the design.
Really at a loss here now.
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Re: Need help with XV250 carb settings

PostAuthor: robimisery » Tue Sep 19, 2017 10:54 am

I removed the carburetor and without cover I dive it into a bowl of water
I simulated the natural movement of the float and needle
Through the fuel hose I blew air
In that way I checked the sealing of the whole valve set
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Re: Need help with XV250 carb settings

PostAuthor: Dark Angel » Wed Sep 20, 2017 8:15 pm

I lifted the float up till it pushed lightly on the spring with the fuel tap on 'prime'. This stopped the flow of fuel.

The float will sit higher in water than in fuel because water has a much higher density.
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