Altitude cough
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Road Warrior
Joined: Apr 2018
Posts: 1,347
Likes: 820
From: Moneta, Virginia
Altitude cough
I read a really interesting article on www.motorcycleproject.com regarding carburetor jetting and altitude.
I live at 800 elevation; my 97 RK runs great and gets 47-50 mpg most of the time. When I ride above 2500 my bike occasionally gives me a cough and once or twice Ive experienced a small backfire. My bike is also less powerful at these altitudes. My MPG declines a bit too, I attribute the decline in mileage to the mountainous up and down. (I like to ride hard up the grade!)
I have been attributing the cough and lower power to a rich condition. After reading this I dont know what to think!
Heres the text, but visit the site... lots of good info.
www.motorcycleproject.com
Many riders believe that their carburetted street bikes have to be rejetted, that is, their carburetor jets decreased in size, when they are going to ride for an extended period at altitudes over sea level. A fairly popular carburetor tech site even declares a one main jet size formula for every 2000 feet. Unfortunately, this is almost completely myth, at least when considering 90 percent of the carburetted street bikes out there.
First, few machines' performance is affected by the slight richening that results from small increases in altitude, up to 4,000 to 5,000 feet.. Only very skilled people can detect the loss of sharpness due to richness, it is so slight. And really, only a few machibes exhibit issues at all, because they have as-manufactured carbureting issues that are aggravated by altitude's richening effect. Engines just are this way. They tolerate a lot of richness pretty much invisibly, but conversely don't do well with leanness. This is common knowledge among tuners. But doesn't altitude affect performance, you might ask? Sure, altitude reduces power, but that comes from the drop in cylinder pressure that occurs at altitude, not from the tiny mixture change. Nothing can be done about the lack of air. Case in point. There was once a roadrace track near Aspen, Colorado that had an interesting record. As of the mid-1980s, a period when plenty of street bikes were easily capable of triple-digit velocities, no machine had ever gone faster than 100 mph on that track. Why? No one was capable of properly jetting their machines? No. Less air in means less cylinder compression means less power. It has nothing to do with jetting. Another example is snowmobiles. It is common for snowmobilers to retune their v-belt (CVT) transmissions when the sleds are used at high altitude. This is done so that the engine gains more rpm before the clutches do their thing, basically gearing the transmission down so the engine can get up on its torque peak. More engine rpm partly compensates for lost engine cylinder filling. Jetting can never solve it.
But there is something even more important going on with carburetors and altitude. Assuming your street machine has the CV carburetors that have defined street bike carbuetion for the past 40 years or more, there is even less reason to think about jetting when going into the mountains. Altitude's richening effect is almost completely absent in CV carbs. In fact, CVs are affected by altitude only at idle and full throttle. Everywhere else they are not affected. Nada. This is because the CV carb's slide rises through pressure differences. And guess what? The carb "sees" the exact same pressure differences at 6,000 feet as it does at sea level. CVs are famous therefore for being altitude-compensating, meaning that jetting changes for altitude just aren't necessary for purposes of performance. A Honda factory bulletin from the 1980s recommends altitude-related jetting changes for CV-equipped street bikes used above 6,000 feet, but this only for emissions purposes, because emissions specs back then centered around the idle circuit. I have ridden at up to 14,000 feet on my CV-equipped Honda CBX and experienced only the expected cylinder pressure related power losses, and no fueling issues whatsoever. With non-CVs I wouldn't worry about rejetting for altitude until some pretty serious changes are made, say above 6,000 feet. And with CVs, it's already taken care of, as high as you want to go.
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Š 1996-2017 Mike Nixon
I live at 800 elevation; my 97 RK runs great and gets 47-50 mpg most of the time. When I ride above 2500 my bike occasionally gives me a cough and once or twice Ive experienced a small backfire. My bike is also less powerful at these altitudes. My MPG declines a bit too, I attribute the decline in mileage to the mountainous up and down. (I like to ride hard up the grade!)
I have been attributing the cough and lower power to a rich condition. After reading this I dont know what to think!
Heres the text, but visit the site... lots of good info.
www.motorcycleproject.com
Many riders believe that their carburetted street bikes have to be rejetted, that is, their carburetor jets decreased in size, when they are going to ride for an extended period at altitudes over sea level. A fairly popular carburetor tech site even declares a one main jet size formula for every 2000 feet. Unfortunately, this is almost completely myth, at least when considering 90 percent of the carburetted street bikes out there.
First, few machines' performance is affected by the slight richening that results from small increases in altitude, up to 4,000 to 5,000 feet.. Only very skilled people can detect the loss of sharpness due to richness, it is so slight. And really, only a few machibes exhibit issues at all, because they have as-manufactured carbureting issues that are aggravated by altitude's richening effect. Engines just are this way. They tolerate a lot of richness pretty much invisibly, but conversely don't do well with leanness. This is common knowledge among tuners. But doesn't altitude affect performance, you might ask? Sure, altitude reduces power, but that comes from the drop in cylinder pressure that occurs at altitude, not from the tiny mixture change. Nothing can be done about the lack of air. Case in point. There was once a roadrace track near Aspen, Colorado that had an interesting record. As of the mid-1980s, a period when plenty of street bikes were easily capable of triple-digit velocities, no machine had ever gone faster than 100 mph on that track. Why? No one was capable of properly jetting their machines? No. Less air in means less cylinder compression means less power. It has nothing to do with jetting. Another example is snowmobiles. It is common for snowmobilers to retune their v-belt (CVT) transmissions when the sleds are used at high altitude. This is done so that the engine gains more rpm before the clutches do their thing, basically gearing the transmission down so the engine can get up on its torque peak. More engine rpm partly compensates for lost engine cylinder filling. Jetting can never solve it.
But there is something even more important going on with carburetors and altitude. Assuming your street machine has the CV carburetors that have defined street bike carbuetion for the past 40 years or more, there is even less reason to think about jetting when going into the mountains. Altitude's richening effect is almost completely absent in CV carbs. In fact, CVs are affected by altitude only at idle and full throttle. Everywhere else they are not affected. Nada. This is because the CV carb's slide rises through pressure differences. And guess what? The carb "sees" the exact same pressure differences at 6,000 feet as it does at sea level. CVs are famous therefore for being altitude-compensating, meaning that jetting changes for altitude just aren't necessary for purposes of performance. A Honda factory bulletin from the 1980s recommends altitude-related jetting changes for CV-equipped street bikes used above 6,000 feet, but this only for emissions purposes, because emissions specs back then centered around the idle circuit. I have ridden at up to 14,000 feet on my CV-equipped Honda CBX and experienced only the expected cylinder pressure related power losses, and no fueling issues whatsoever. With non-CVs I wouldn't worry about rejetting for altitude until some pretty serious changes are made, say above 6,000 feet. And with CVs, it's already taken care of, as high as you want to go.
Email me
Š 1996-2017 Mike Nixon
Seasoned HDF Member
Joined: Aug 2007
Posts: 8,195
Likes: 3,085
From: NW AR
Is your road king carbed or EFI? I can tell when my bike needs an adjustment for altitude changes. When we ride to Sturgis, once we hit the high plains, I lean out the idle mixture screw about 1/8th turn and I'm good. The Sportster has barometric pressure compensation, so it doesn't need it. When it had a carb on it, it was a lot more sensitive to elevation changes.
Thread Starter
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Road Warrior
Joined: Apr 2018
Posts: 1,347
Likes: 820
From: Moneta, Virginia
Is your road king carbed or EFI? I can tell when my bike needs an adjustment for altitude changes. When we ride to Sturgis, once we hit the high plains, I lean out the idle mixture screw about 1/8th turn and I'm good. The Sportster has barometric pressure compensation, so it doesn't need it. When it had a carb on it, it was a lot more sensitive to elevation changes.
Next time I get high, Ill try adjusting the idle mixture!
Seasoned HDF Member
Joined: Aug 2007
Posts: 8,195
Likes: 3,085
From: NW AR
On the S&S, the idle mixture runs most of the bottom of the fuel map. So, adjusting the screw will change the whole mixture for part throttle. I dunno about the CV carbs. I have one on my chopper but I've never messed with it.
Used to be with the twinkies, it seemed to me that people were getting them "tuned" wherever they live, then when they would run across I90 at altitude, they were running pig rich. I don't think the early ones (88's) had barometric correction. If they were just a little fat wherever they were from, they were massively fat at altitude. We would blast by them on I90. They would see some guy on an Evo blast by, then some woman on a Sportster blast by and they would try to keep up for a while. Never make it for long. Now those new ones have it, and the 10x motors will flat out run.
Used to be with the twinkies, it seemed to me that people were getting them "tuned" wherever they live, then when they would run across I90 at altitude, they were running pig rich. I don't think the early ones (88's) had barometric correction. If they were just a little fat wherever they were from, they were massively fat at altitude. We would blast by them on I90. They would see some guy on an Evo blast by, then some woman on a Sportster blast by and they would try to keep up for a while. Never make it for long. Now those new ones have it, and the 10x motors will flat out run.
Grand HDF Member
Joined: Nov 2009
Posts: 4,315
Likes: 1,663
From: Tampa
Has anybody ever actually taken a wideband oxygen sensor to increased elevations to actually see what happens to the AFR on a carbed bike?
This article is way more right than most people are led to believe. Why do you think your bike has less power at elevation? Well less oxygen, duh... But oxygen isn't the only component that's more rare, it's that there's just less AIR in general.
Imagine, if you will, take one big gulp of air at sea level and you inhale a specific volume of air that's 21% oxygen, 70% nitrogen, and some other stuff
Now take another gulp of air at the top of Pikes Peak. Those ratios are the same but the volume you inhaled was lower since there's less content of air in what you inhaled. It's hard to grasp, but think about outer space. What do you take a big gulp of in space? Nothing! Well as you increase in elevation here on earth you go from big gulps of air at sea level to big gulps of nothing in space. Atmospheric pressure is basically a stack of air measured in pressure at sea level that is the physical weight of a column of air that's 7 or so miles high. It's lighter at the top.
Since the CV carb (and carbs in general) all allow fuel based on how much air is going thru them, when at elevation since there's less air per gulp there will inherently less fuel to go with it regardless of how large your pistons are which keeps the ratios consistent.
This article is way more right than most people are led to believe. Why do you think your bike has less power at elevation? Well less oxygen, duh... But oxygen isn't the only component that's more rare, it's that there's just less AIR in general.
Imagine, if you will, take one big gulp of air at sea level and you inhale a specific volume of air that's 21% oxygen, 70% nitrogen, and some other stuff
Now take another gulp of air at the top of Pikes Peak. Those ratios are the same but the volume you inhaled was lower since there's less content of air in what you inhaled. It's hard to grasp, but think about outer space. What do you take a big gulp of in space? Nothing! Well as you increase in elevation here on earth you go from big gulps of air at sea level to big gulps of nothing in space. Atmospheric pressure is basically a stack of air measured in pressure at sea level that is the physical weight of a column of air that's 7 or so miles high. It's lighter at the top.
Since the CV carb (and carbs in general) all allow fuel based on how much air is going thru them, when at elevation since there's less air per gulp there will inherently less fuel to go with it regardless of how large your pistons are which keeps the ratios consistent.
Last edited by Mattbastard; Jul 9, 2019 at 04:16 PM.
Seasoned HDF Member
Joined: Mar 2010
Posts: 21,025
Likes: 7,549
From: Poolville
I did read that guys article though and there is some truth to what he says when it comes to cylinder fill... But you need to remember AFR's are totally dependant on cylinder fill.. Less fill less fuel... I am not sure though at what altitude it would make a true power loss issue from being too rich... I did see a show from them 3 British guys that were going over the Ande's and at 14,000' their truck would hardly run and had to turn around.. They never made it to the summit.. Maybe Mattbastard will take a ride and let us know...
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Grand HDF Member
Joined: Nov 2009
Posts: 4,315
Likes: 1,663
From: Tampa
Sure thing bro... What's the highest point in Floriduh, like 200 feet? 
I just got back from the Smokey's, but the bike I was on had FI. My 94 bucket of bolts is still being deemed street-worthy.
I just got back from the Smokey's, but the bike I was on had FI. My 94 bucket of bolts is still being deemed street-worthy.
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Road Warrior
Joined: Dec 2010
Posts: 1,268
Likes: 507
From: Florida
Been up two 14,000 plus summits in Co. Bike is a 93 Evo w/ S&S E. Never touched the carb either time. At top turned bike off, when ready to leave bike fired up no problems. Doesn't have the punch at altitude but don't crawl either.
