Carbs in Colorado (vs. Florida)

[dpep]

Living in Florida I do almost all of my riding at sea level. I notice that when I occasionally ride at higher elevations my performance and mileage are down a little and I have to be more mindful of passing and the amount of grade I am pulling. I can tell some difference in as little as 1000-2000 feet. My 850 has never had anything but stock pipes and jetting which makes me wonder about the models of that bike that were sold in Colorado or some other mountainous state. Are all carburetted bikes sold with the same set up and if so, what barometric pressure would they be optimised for?

This is probably the main appeal a more modern bike would have for me right now, fuel injection that would automatically adjust to changes in altitude/air pressure. But then I got to thinking. Since the problem is less air in the mixture, could one compensate by letting more flow into the airbox? Perhaps even getting trick with an improvised set of vents that could be cracked more open as the air got thinner?

 

[earlfor]

Thats a particularly interesting question.
On a bike with a CV carburetor, CV stands for constant vacuum. The amount of fuel that flows through the carburetor is controlled by vacuum level. The throttle does not directly control the movement of the slides. Vacuum level depends on two things, velocity and density. If we combine velocity and density, we have massflow and it is massflow that operates our carburetor. If we have lowered density, then the same flow acceleration we would have at sea level would result in low massflow. But, since density has decreased, velocity will increase until the same mass is acquired because the carburetor regulates slide position with an internal spring on the slide and a diaphram. At high altitudes, the slides would not raise fully and that would result in less HP at full throttle. It should not result in a change in mixture ratio.

The situation is something akin to an airplane at sea level and then at 20,000 ft. At sea level the airspeed indicator reads 60 mph when the aircraft is moving at 60 mph (assuming zero wind condition) At 20,000 feet when the airspeed indicates 60 mph, it is indicating that massflow across the wings is now the same as it was at sea level at that indicated speed. But, with density altitude correction, that plane is now actually moving at 250 mph across the ground.(still assuming a no wind condition)
In this case, 60 mph and 250 mph are the same thing relative to massflow. Aircraft also lose HP as they gain altitude.

VM carburetors are throttle priority and not self compensating.

Earl

 

[SqDancerLynn1]

The air gets thinner= less compression, richer mixture. I remember one trip on a sohc 750 Honda got up to 10-11K feet on Tioga pass Was all I could do to keep the bike running 25-30 mph

 

[earlfor]

I agree with your observed result Lynn, particularly if the bike had mechanical slide throttle priority carburetors as throughflow area can be changed independent of fuel flow. In a CV carburetor fuel flow and throughflow are linked and the relationship does not change. Thinner air equals less compression, because thinner air has less mass, so there is less venturi effect also and hence less fuel draw. The mixture will remain within the normal range of around 12-14 to 1. The most efficient/economical fuel air ratio is 14:1.
The fuel air ratio for most power is about 12:1. Dragsters may use mixtures in the range of 10:1 in the interest of developing more power. If altitude resulted in a richer mixture, the bike would develop more HP rather than less. A reduction in intake volume is the reason for less HP being produced. In short, yeah, there's less compression.

Earl

The air gets thinner= less compression, richer mixture. I remember one trip on a sohc 750 Honda got up to 10-11K feet on Tioga pass Was all I could do to keep the bike running 25-30 mph

 

[Griffin]

Your engine is essentially a positive displacement pump. Every stroke compresses a fixed volume of air. At higher altitudes, the mass of a fixed volume of air is less. The volume you take in and exhaust will remain constant, but the number of air molecules you compress, and the number of fuel molecules you ignite, will drop corresponding to altitude. That means less energy released thorugh combustion, therefore less horsepower at any given rpm.

 

[Mr. Jiggles]

I had a similar experience last year while riding in the santa cruz mountains (maybe 1500-2000' ?). I felt a relative loss of power that was bothersome enough that I cut my ride short. When I got back down to sea level all was fine. In fact within 10 minutes of coming down off the mountains I was able to humiliate a honda S2000 and a porsche boxster easily from a standing start.

Yet another reason to go with RS flataslides??

 

[Bolder Biker]

Thats a particularly interesting question......

Earl

And your answer is particularly interesting also, Earl.

Boulder is at about 5,300 feet. Using the old rule of thumb of 3% power loss per 1,000 feet, my GS probably puts out about 16% less power than its equivalent at sealevel. If I go up the canyons about 17 miles to the Peak-to-Peak Highway, which is at about 8,000 feet, the bike's power output is then about 24% less than at sealevel. I must admit that I don't notice the extra 9% powerloss overmuch. However, if I go over Trailridge Road,which gets to an elevation above 12,000 feet (in the Rocky Mountain National Park), the power loss becomes very obvious: about -36% less that at sealevel, and 20% less than that in Boulder.

BB

 

[KEITH KRAUSE]

Don, I forgot what year your 850 is. If it has the VM carbs, you can try turning the side air screws OUT an additional 1/2 turn or so. That's what they're for, elevation compensation. Adjusting them is not a cure all, but it definitely makes re-starting and lower cruising speeds better. If your side air screws have never been moved, be careful because they can be stiff and the heads stripped eaily.
The CV carbs may benefit from adjusting their mixture screws IN a 1/2 turn or so. The CV's don't seem to respond as well as the VM carbs.
Your air box mod idea could work. Maybe a sliding door type vent. If your bike has CV carbs, I wouldn't modify the air box.
But I would first try the air screw adjustment next time you're in higher elevations.

 

[dpep]

My 850 is an '80 and has CV carbs. I believe '80 was the first year for them. I know that my bike is extremely sensitive to the airbox. I would never create any holes I couldn't seal up.

I'm learning a lot here but I am still unclear on something. Do CV carbs try to compensate for less dense air by sucking in more air, or by sucking in less gas at a given throttle setting? It sounds like it is the latter. It that is the case the restriction on horsepower should be at or near full open throttle. . You essentially run out of throttle setting. Is it the case the carb would not pull in more air even if the source of that air flow were made less restrictive; it is not trying to achieve a specific psi?

I know the amount of restriction plays some role in the operation of the carbs. A bad seal anywhere around the airbox can keep the bike from running.

I have not done my own carb work in the past, but I am going to start. I noticed that someone in the past has taped off the breather hose fitting on the bottom of the airbox, and the breather hose itself is attached to nothing. Can I assume this is pollution consideration only and would have no impact on engine performance?

 

[earlfor]

For ease of explanation, I'll throw in some numbers.
Lets say your bike has a compression ratio of 9:1, and lets assume a cylinder displaces 9 cu ft and 1 cu ft of air weighs 1 lb. At sea level we can say the cylinder will compress 9 cu ft of air into a space of 1 cu ft.
The compressed charge then has a weight of 9 lb cu ft.

At 10,000 feet altitude, the air is thinner, assume the air has a density of 1/2 lb per cu ft. Compressing that 9 times gives a charge with a weight of 4 1/2 lb cu ft. Putting bigger holes in the airbox will not solve the problem.

A CV carburetor automatically maintains a fuel air ratio as a constant.
Intake air flow is measured as a product of density and velocity. That is called massflow. For any given massflow, the amount of fuel metered into the airstream will be unchanged. Mixture ratio will be the same. The mixture 14:1 or whatever, is the same at sea level as it is at 10,000 ft.
The reason there is more power at seal level than there is at 10,000 feet is that at sea level, there is a weight of charge in the cylinder of 9lbs, but there is only a weight of charge of 4 1/2 lbs at 10,000 feet.

Since the carburetors draw fuel in relation to massflow, if the air has less weight, then a smaller amount of fuel will be mixed with the incoming air at altitude because venturi effect depends entirely on massflow. Fluid dynamics dictates the situation. The carburetor is only maintaining a predetermined amount of fuel for a given weight of air.

For a normally aspirated engine, there is no carburetor or airbox adjustment that will restore sea level HP at alititude. The only way to restore HP is to restore air density, or more precisely, restore charge density.

Turbocharger or supercharger.

Earl

My 850 is an '80 and has CV carbs. I believe '80 was the first year for them. I know that my bike is extremely sensitive to the airbox. I would never create any holes I couldn't seal up.

I'm learning a lot here but I am still unclear on something. Do CV carbs try to compensate for less dense air by sucking in more air, or by sucking in less gas at a given throttle setting? It sounds like it is the latter. It that is the case the restriction on horsepower should be at or near full open throttle. . You essentially run out of throttle setting. Is it the case the carb would not pull in more air even if the source of that air flow were made less restrictive; it is not trying to achieve a specific psi?

I know the amount of restriction plays some role in the operation of the carbs. A bad seal anywhere around the airbox can keep the bike from running.

I have not done my own carb work in the past, but I am going to start. I noticed that someone in the past has taped off the breather hose fitting on the bottom of the airbox, and the breather hose itself is attached to nothing. Can I assume this is pollution consideration only and would have no impact on engine performance?

 

[Sandy]

I hate to disagree with you Earl because you're usually right and maybe you are in this case too. I am with the understanding that CV carbs used vacuum to adjust slide position according to engine vacuum and that was all. For instance if you have a VM carb with the slide open at 1/2 throttle and a CV carb with the slide open by vacuum at 1/2 throttle the air/fuel mixture function of both carbs would actually be the same as that portion of the carbs circuitry is in fact the same or very similiar. The volume of air flowing through the venturi area is what determines the amount of fuel pulled into carb throats. The oxygen component of the air has some effect on this, but not a lot. Thus when you get higher in elevation your engine will run richer due to lack of oxygen in the air. A few years ago while riding at 13,000 feet in Colorado everyone I was riding with, CV carbs or not, was running so rich we hardly had enough power to get out of our own way. Unless you are running something with an O2 sensor and fuel injection or carburation with proper altitude compensation you're going to be running rich at higher elevations.

 

[earlfor]

Nah, Sandy, you're not wrong. CV carbs do use engine vacuum to set slide position. As the slides raise the needles, fuel is drawn and sprayed into the intake stream. Since raising the slides changes the throughflow area of the carb, the slide also changes the intake velocity of the airstream. (slide raises, opening gets larger, if volume remains constant, velocity decreases)
The velocity and its density control how much fuel is metered, so in addition to the CV carbs using vacuum to set slide position, that vacuum also controls fuel flow.

"The volume of air flowing through the venturi area is what determines the amount of fuel pulled into carb throats " ************ Well, yes and uh mmmmm more. Volume and density, or massflow determines how much fuel is drawn into the carb throats. If velocity is constant and density is reduced, then fuel draw will be reduced.


"The oxygen component of the air has some effect on this, but not a lot. Thus when you get higher in elevation your engine will run richer due to lack of oxygen in the air. " ************** The RATIO of the weight of fuel combined with the weight of air of each intake charge remains the same regardless of altitude. Loss of efficiency is due to two things. As altitude increases, the weight of the intake charge decreases and the oxygen component decreases(reduction in combustibility). The engine is not metering the mixture richer, leaner, or any different that it always has.
You are right, lack of oxygen is a good/major/some? ... part of the problem.

Earl

I hate to disagree with you Earl because you're usually right and maybe you are in this case too. I am with the understanding that CV carbs used vacuum to adjust slide position according to engine vacuum and that was all. For instance if you have a VM carb with the slide open at 1/2 throttle and a CV carb with the slide open by vacuum at 1/2 throttle the air/fuel mixture function of both carbs would actually be the same as that portion of the carbs circuitry is in fact the same or very similiar. The volume of air flowing through the venturi area is what determines the amount of fuel pulled into carb throats. The oxygen component of the air has some effect on this, but not a lot. Thus when you get higher in elevation your engine will run richer due to lack of oxygen in the air. A few years ago while riding at 13,000 feet in Colorado everyone I was riding with, CV carbs or not, was running so rich we hardly had enough power to get out of our own way. Unless you are running something with an O2 sensor and fuel injection or carburation with proper altitude compensation you're going to be running rich at higher elevations.

 

[dpep]

Could some sort of turbo arrangement compensate by forcing more air into the cylinder?

 

[Anonymous]

pod filters

pod filters

i too have a 80 gs 850 you said that

My 850 is an '80 and has CV carbs. I believe '80 was the first year for them. I know that my bike is extremely sensitive to the airbox. I would never create any holes I couldn't seal up.
I have 4 individual pod filters on my bike, i'm fixing this bike up, is this going to cause me a problem like i have found everything else on the bike will?

 

[KEITH KRAUSE]

Re: pod filters

Re: pod filters

i too have a 80 gs 850 you said that

My 850 is an '80 and has CV carbs. I believe '80 was the first year for them. I know that my bike is extremely sensitive to the airbox. I would never create any holes I couldn't seal up.
I have 4 individual pod filters on my bike, i'm fixing this bike up, is this going to cause me a problem like i have found everything else on the bike will?

You'll be able to re-jet correctly if you take the time to do things right. Jetting will require patience and some trial and error. You should get a jet kit and use only quality filters (K&N) and a quality pipe, or the re-jetting will be more complicated. You'll also need to vacuum synch the carbs.

 

[Sandy]

Could some sort of turbo arrangement compenstate by forcing more air into the cylinder?

Yes it would. The turbo would compensate for the lack of air pressure by forcing air into the cylinder at a rate higher than the current atmospheric pressure present at a given altitude.

 

[earlfor]

Yes, and if we are considering only enough of a turbo to compensate for altitude, I suspect it could be relatively simple and compact.

Earl

Could some sort of turbo arrangement compenstate by forcing more air into the cylinder?

 

[dpep]

Yes, and if we are considering only enough of a turbo to compensate for altitude, I suspect it could be relatively simple and compact.

Earl

Easy for you to say

 

[earlfor]

Another project for YOU.

Earl

Easy for you to say

 

[Griffin]

I lived in Idaho Falls (elev. 4,700 feet) for about a year. The GS700 was my principle means of transport during most of the time. I didn't notice the lack of power compared to Iowa (elev. 800 feet) where I moved from while riding around town or in the general area, but I did notice it when I went up into the high valley passes above 6,000 feet. The difference was quite pronounced then. The local YamaHondaZukiSaki dealer told me that they rejet quite a few bikes slightly rich, then put hotter plugs in them, for customers who demand that they "do something".

I'm sure they then received complaints from customers because their bikes were indeed running rich.