carb jets at elevation

[Henri]

Here is some info that will be useful to those riding Vs at higher elevations.  It comes from some experienced techs at the local Yamaha shop here in the mountains.  Carbs are jetted for sea level to around 2000'.  They claim that to convert jets for higher elevations you can either take the stock jet and multiply that # .97 for every 1500 ft. in elevation gain above sea level or you can take that # multiply by .96 for every 3000' above sea level.  
Of course you are going to come up with numbers that will have to be rounded to the nearest available jet but it gets us closer.  I bumped both of my main fuel jets down to 120 and 122.5 and it smoothed things down a lot.  Still running 130s for pilot air and am thinking about upping those to 135s.  I don't think their conversion applies to air jets unfortunately.
For what it's worth...

[h2olawyer]

Welcome Henri!  Nice to hear from another "elevated" Visionary!  I have an 82 that I bought new in 1984 from the dealer in Steamboat Springs, CO - elevation 7,000 ft.  They had changed the jets to 120 front and 122.5 rear before I bought it - they said they did it when they put the factory air bod modification on it.  They also put 135s in both pilot air spots.

It never ran great until I got it back out this summer and did all the suggested mods.  I've thoroughly cleaned and rebuilt the carbs twice now after long periods of sitting.  I think the carbs got gummed up while it sat so long at the dealer.  The jet combo works great here in Fort Collins (5000 ft.) and performance is still fairly good even at 11,000 ft. - the highest I've been so far.  Since you seem to be at a couple hundred feet lower than me, the 135 pilot air jet sizes should work well for you as well.

[Henri]

Yea I'm at approx 4800' here in Bozeman, MT.  I look forward to getting rid of all stuble someday.  I recently baught a used air mod carb and airbox and hope to get them installed although sounds like plenty of people have had luck without the mod.  I don't realy like that little air filter that hangs off the side.  I noticed that w/ the modification specs from Lucky's site that they also changed the front main jet to a 127.5.  That would have made both jets the same w/ the modification.  I wonder what they were doing.  I still don't get why one jet is different.  Maybe someone can shed some light on that for me.  This forum is the best thing since electronic ignition :)  

[Lucky]

That jetting difference seems to make up for the different length exhaust routing between front & rear. cutting open the exhaust collector also reveals that the left side is more of a straight shot out the pipes , the right side seems more restrictive in that there are mor twists & turns to the exit...strange design.  
--Lucky

[Rick G]

No body has had much luck at getting them to run as they should , without the modifications. some have had some problems even with it . Although that probably had more to wrench's, who didn't give a tinkers dam  about this "oddball" bike. When the carbs are set up right , with the mods , these things will really please you!  anyway mine does me!