jetting and altitude question

[daron]

Hola a todos,

I have just a quick question about the stock jetting for an 06 GS500f.  I've read on this forum that the US market bikes are jetted lean.   I'm living in Mexico at about 6000 feet above sea level.  Should I consider rejetting the bike anyway?

Basically, I guess I'm confused about the meaning of "lean" and "rich".  Lean means that the bike has more air than fuel right?  Wouldn't the altitude sort of "counteract" the leaness of the bike?  I mean that if you're at a higher altitude the bike needs a higher air to fuel ratio, right? That is to say, leaner?

I could be totally out to lunch on this but please enlighten me!

thanks a lot
Daron

[trumpetguy]

Hola Daron,

Yes, altitude does affect jetting.  I have read that each 1,000 feet is equivalent to enriching the mixture (more fuel because of less density in the air) one jet size.  I am quoting from memory, so don't take my word as truth.

My suggestion would be to ask some performance motorcycle shops in Mexico City what they do.  They should be able to help you.  Your bike may already even have been rejetted by your dealer to compensate for the altitude.

[daron]

Thanks for the info!  That's a good point about asking a performance shop here.

However, the bike was originally purchased in Texas, so it should have standard US jetting.  I'm guessing on that because I didn't buy it new, but the original owner said that he just did the normal routine maintainence to it.

Thanks for your help,
Daron

[Wrecent_Wryder]

[6

[ducati_nolan]

Bikes are less suceptable to mixture changes due to altitude than old cars with carbs. The diaphram that lifts the slide that has the tapered needle delivers fuel in response to the air pressure flowing through the carb and it corrects for altitude to a large degree.

My Dad's airplane has Bing carbs that are Identical to the ones that were on my BMW bike, (they have a diaphram and tapered needle just like the Mikunis) they compensate just fine to over 10,000 feet.

So basically, I would just leave the jetting alone unless you start experiencing fouled plugs or poor fuel economy.

[Wrecent_Wryder]

[3

[trumpetguy]

Here's a link to a web article on jet correction for Bing carbs on BMWs (similar to ours):
http://www.horizonsunlimited.com/tech/carbjet.shtml

Another detailed carburetor article with jetting compensation info on page 4-5:
http://www.motorcyclecarbs.com/carbs101.pdf

My memory was definitely not correct about one jet size per 1,000 feet, but there is a change and it can be significant.  Check with the charts and formulas in the pages above.

My quick glance shows a possible correction of 2 sizes smaller for 6,000 feet as compared to sea level.  You probably won't foul plugs with the jets that are in there, but you won't get the best mileage either.

FWIW, I just went back to stock jets on my 98 GS -- 37.5 pilots and 122.5 mains with stock airbox and exhaust.  I live at 1600 feet and it works well.  Most of my riding is at that elevation or higher.  I haven't been able to check mileage yet (just made the change on Saturday), but it should be an improvement over the 45 pilots and 128 mains I took out.

[daron]

wow,

excellent response!  Thanks to all for your time and help. I'm not experiencing anything horrible with the carbs right now except for poor mileage.  However, I'm attributing that to the fact that I live in an old colonial town and most of my driving is very short distance in 1st and 2nd gear.  I'm going to get out of town this week (vacation) so I'll get a real feel for gas mileage under normal driving conditions.

Again, all of your help is well received! 

Daron