How to Tune Carb on Altitude change

[vickytulla]

Is there a thread already where this issue is being discussed, we being near to Himalaya are very very prone and face Carb / low oxygen missing at High Altitude every year when we head to mountains...

I went through couple of YouTube videos and forums What I am able to under stand is that one needs to lean the carb from needle if the bike start to suffocate on high altitudes ?

Is that correct?

[vickytulla]

I use RX3 on tours which do not have this issue (its EFI) but there are places where taking RX3 is actually abusing a Adventure bike on Heavy dirt tracks.. for that I will use enduro bike which is carb operated and I am 100% sure ti will scream and cry for oxygen so I want to get hands on experience on setting it up...

[ben2go]

As altitude increases, oxygen decreases causing the carb to run rich. The mixture screw will only compensate up to a certain altitude before the carb needs to be re-jetted. Usually, one drop in the size of the low-speed jet and two drops in the size of the main jet. I have known some people to carry an extra carb and jets with them to make changes as they ride higher in the mountains.

[vickytulla]

carrying a seprate carb can be an excellent idea...

so rejeted seems ok.. rejetting to smaller jets you mean ?

Say my Bike is running BST40 right now... I am not sure about its stock size of jets... say its 165 what would be ideal number for my "second" carb to have jets for mountians... ?? any rough guess... I am novice at this...

Originally Posted by ben2go

As altitude increases, oxygen decreases causing the carb to run rich. The mixture screw will only compensate up to a certain altitude before the carb needs to be re-jetted. Usually, one drop in the size of the low-speed jet and two drops in the size of the main jet. I have known some people to carry an extra carb and jets with them to make changes as they ride higher in the mountains.

[Torgo]

I happened to run across this on another forum a while back when I was looking for something to calculate jets vs. altitude. Although this wasn't particularly useful info for me, since I was looking for main jet changes between 800ft to around 3000ft, the info in the top post may apply to your area of the world a little better.

http://www.team-bhp.com/forum/motorb...impulse-2.html

I've seen the wire mod used in a couple of documentaries out on Youtube, but it also discusses dropping the main jet to compensate. If you plan ahead you can have a couple jets ready to swap in/out, but a thin wire seems like an extra backup if you go too high.

I certainly don't have any first hand experience myself at those altitudes, but it looks like an awesome place to explore. Any type of breakdown would be an adventure in itself.

[Azhule]

How many feet does the altitude change? Also what is the temperature change?

Here is something that might help...

http://www.jetsrus.com/FAQs/FAQ_reje...emperature.htm

If the link doesn't work for you it basically has:

for every 1,000 feet difference in altitude you will need to change by 1 jet size (go up 1,000 feet you go down 1 jet size... go down 1,000 feet you go up 1 jet size)... for every 10 degrees Fahrenheit you will need to change by 1 jet size (temp goes down 10 degrees F you go up 1 jet size... temp goes up 10 degrees F you go down 1 jet size)

[ben2go]

Originally Posted by vickytulla

carrying a seprate carb can be an excellent idea...

so rejeted seems ok.. rejetting to smaller jets you mean ?

Say my Bike is running BST40 right now... I am not sure about its stock size of jets... say its 165 what would be ideal number for my "second" carb to have jets for mountians... ?? any rough guess... I am novice at this...

Yes, down in size. 165 main jet drop two sizes 160 and say 40 slow jet drop one size 37.5. One size down is taking 2.5 off the jet's size. At least this is usually tru for Mikuni and Keihin.

[rjmorel]

I heard of one guy riding a Ural at altitude that couldn't get it to run . He called the shop he bought it from back in the States and was told to remove the air cleaner till he got back down in elevation. That did the trick and got him back down the mountain but I would at least put a stinky sock over the intake to keep stuff out of carb rj

[Megadan]

Originally Posted by Azhule

Here is something that might help...

http://www.jetsrus.com/FAQs/FAQ_reje...emperature.htm

If the link doesn't work for you it basically has:

for every 1,000 feet difference in altitude you will need to change by 1 jet size (go up 1,000 feet you go down 1 jet size... go down 1,000 feet you go up 1 jet size)... for every 10 degrees Fahrenheit you will need to change by 1 jet size (temp goes down 10 degrees F you go up 1 jet size... temp goes up 10 degrees F you go down 1 jet size)

Actually, for the N100.604, which is the main jet for the Mikuni Clones we all run, every 1000ft in change or 10 degree change in temperature, it is plus or minus one jet number point on that chart.

ie. Let us say I have a 110 main jet that I tuned at sea level, and I go up to 2500ft. That would be -2.5 jet points. The next size down would then be 107.5, which is an actual main jet size.

My general rule of thumb with the Mikuni carbs basically simplifies that chart.

For every 2500ft of elevation change and/or 25 degree change in temp. You need to go up or down a "half" step in jet size.


If you are running a carb that uses Keihin jets, then you can use this chart to get a feel for what the equivalent steps would be. https://drriders.com/jets-are-evil-m...et-t21713.html

Alternatively, you can also use a correction factor chart/calculation to help determine any changes. This just requires a bit more math.

Keep in mind that this whole rule of thumb is to keep the bike in a perfect state of tune. There is a little "forgiveness" to be found that doesn't require constant jetting changes.

All of that said, it really depends on how you have your biked tuned at your normal elevation to begin with.

For example. I tend to tune my air cooled bikes a hair on the rich side during hot weather. This helps with keeping the motor cool. Since I live somewhere where elevation changes are measured in a couple hundred feet, I don't have to deal with elevation changes, but I do deal with drastic temp swings. By making it a tad rich in say 90 degree heat, the tune is about spot on in 65-70 degree weather, and will start to act a bit lean around 40 degrees, at which point I will go up half a step from my normal 117.5 to a 120. That gives me a very wide window to work with before I have to even consider a jetting change.

Depending on your starting elevation, you shouldn't have to carry more than 2 or 3 main jets, and maybe one pilot jet if the altitude increase is drastic enough. The needle jet is directly affected by the main jet size, so more often then not you shouldn't have to adjust the needle clip position, but you can use it to fine tune if necessary. The same thing is true of the pilot jet and the idle mixture, hence the suggestion to carry at least one pilot jet.

The extra carb idea is a good one, but 3 or 4 jets and a couple of screwdrivers takes up a lot less room than a whole carb, screwdriver, and wrench needed for a carb swap. Plus, you would need to store said carb in a way that keeps the dirt out. Time wise, swapping jets and swapping carbs will end up taking about the same amount of time, give or take a few minutes.

[vickytulla]

Originally Posted by rjmorel

........was told to remove the air cleaner till he got back down in elevation. ............

absolutely that is the quick hack we do right now.. but that also has a certain limit... like as soon as we go beyond 13000/14000 feet above filter removal fully opening Air.. nothing works...

Even head ache starts some time and breathing issues...

Originally Posted by ben2go

Yes, down in size. 165 main jet drop two sizes 160 and say 40 slow jet drop one size 37.5. One size down is taking 2.5 off the jet's size. At least this is usually tru for Mikuni and Keihin.

This is all making sense now..

Originally Posted by Azhule

http://www.jetsrus.com/FAQs/FAQ_reje...emperature.htm
)

This my friend is a very very useful link... I wonder why I wasted so much time on you tube search...

Originally Posted by Torgo

I happened to run across this on another forum a ........

http://www.team-bhp.com/forum/motorb...impulse-2.html

..... Any type of breakdown would be an adventure in itself.

Adventure? you bet bro you bet.... we had a first bike brok at 6000, and finally when we were at 14000feet, 7 out of 16 bikes were already down and returned back...

on Top we camped in snow and next day we went back to recover the riders along their bikes and finally have to return them back as they failed to climb in second attempt too..

Originally Posted by Megadan

.... .

All of that said, it really depends on how you have your biked tuned at your normal elevation to begin with.
....

I am going to make sure I know this stuff before I head to mountains...

[Azhule]

Megadan again has my brain and wording problems fixed for the N100.604 it goes by points not by jets sorry for my brain fog

[timcosby]

Originally Posted by Torgo

I happened to run across this on another forum a while back when I was looking for something to calculate jets vs. altitude. Although this wasn't particularly useful info for me, since I was looking for main jet changes between 800ft to around 3000ft, the info in the top post may apply to your area of the world a little better.

http://www.team-bhp.com/forum/motorb...impulse-2.html

I've seen the wire mod used in a couple of documentaries out on Youtube, but it also discusses dropping the main jet to compensate. If you plan ahead you can have a couple jets ready to swap in/out, but a thin wire seems like an extra backup if you go too high.

I certainly don't have any first hand experience myself at those altitudes, but it looks like an awesome place to explore. Any type of breakdown would be an adventure in itself.

that whole thread is a great read with pics!