![]() |
#1 |
Join Date: Jan 2017
Location: Lahore
Posts: 186
|
How to Tune Carb on Altitude change
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?
__________________
![]() ![]() - Adventure Touring# RX3 - 2017 - Dirt Machine# FSE-450 - 2022 - Classic# Honda CD175 - 1977 - Daily Runner# 1992 - CG125 (engine swap CB250 Sleeper) - Project#1 : 1995 - Honda C70 w/ 180cc engine swap |
![]() |
![]() |
#2 |
Join Date: Jan 2017
Location: Lahore
Posts: 186
|
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...
__________________
![]() ![]() - Adventure Touring# RX3 - 2017 - Dirt Machine# FSE-450 - 2022 - Classic# Honda CD175 - 1977 - Daily Runner# 1992 - CG125 (engine swap CB250 Sleeper) - Project#1 : 1995 - Honda C70 w/ 180cc engine swap |
![]() |
![]() |
#3 |
Join Date: Jun 2011
Posts: 1,335
|
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.
|
![]() |
![]() |
#4 | |
Join Date: Jan 2017
Location: Lahore
Posts: 186
|
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... ![]() Quote:
__________________
![]() ![]() - Adventure Touring# RX3 - 2017 - Dirt Machine# FSE-450 - 2022 - Classic# Honda CD175 - 1977 - Daily Runner# 1992 - CG125 (engine swap CB250 Sleeper) - Project#1 : 1995 - Honda C70 w/ 180cc engine swap |
|
![]() |
![]() |
#5 |
Join Date: Oct 2015
Location: Texas
Posts: 93
|
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. |
![]() |
![]() |
#6 |
Join Date: Dec 2015
Location: CO
Posts: 1,525
|
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)
__________________
"Think as you like... but this self proclaimed Professor is always right" - Buckshot ![]() "You never know what someone is hiding beneath their smile..." - NinjaTom - R.I.P. |
![]() |
![]() |
#7 | |
Join Date: Jun 2011
Posts: 1,335
|
Quote:
|
|
![]() |
![]() |
#8 |
Join Date: May 2007
Location: Warshington
Posts: 928
|
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
![]()
__________________
Live Zong, and Prosper |
![]() |
![]() |
#9 | |
Join Date: Apr 2017
Location: Omaha, NE
Posts: 8,110
|
Quote:
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.
__________________
Hawk Information and Resource guide: http://www.chinariders.net/showthread.php?t=20331 2018 Hawk 250 - Full Mod list here. http://www.chinariders.net/showpost....62&postcount=1 2024 Royal Enfield Shotgun 650 https://chinariders.net/showthread.php?t=34124 |
|
![]() |
![]() |
#10 | ||||
Join Date: Jan 2017
Location: Lahore
Posts: 186
|
Quote:
Even head ache starts some time and breathing issues... Quote:
Quote:
Quote:
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.. ![]() I am going to make sure I know this stuff before I head to mountains...
__________________
![]() ![]() - Adventure Touring# RX3 - 2017 - Dirt Machine# FSE-450 - 2022 - Classic# Honda CD175 - 1977 - Daily Runner# 1992 - CG125 (engine swap CB250 Sleeper) - Project#1 : 1995 - Honda C70 w/ 180cc engine swap |
||||
![]() |
![]() |
#11 |
Join Date: Dec 2015
Location: CO
Posts: 1,525
|
Megadan again has my brain and wording problems fixed for the N100.604 it goes by points not by jets
![]() ![]()
__________________
"Think as you like... but this self proclaimed Professor is always right" - Buckshot ![]() "You never know what someone is hiding beneath their smile..." - NinjaTom - R.I.P. |
![]() |
![]() |
#12 | |
Join Date: Nov 2016
Location: robertsdale alabama usa
Posts: 1,007
|
Quote:
![]()
__________________
1991 harley heritage softail classic 2016 hawk kaw h1 400 tripple 13' long chopper in progress 2009 johnny pag bar hog 2004 hd electra glide ultra classic shriner edition 1997 harley davidson 1200 sportster 1972 harley davidson amf sx250 |
|
![]() |
|
![]() |
![]() |
|
|