Carb adjustment with the altitude???

[ryanZR7S]

Hey I just brought my bike, (2003 Kawasaki ZR-7S) up to Denver from Kansas and it is a little more hesitant to start on the first start of the day. As compared to down at the lower elevation in Kansas.

I am wondering if anyone knows if the carbs may need to be adjusted now that I am up in Denver. Once it is warmed up it runs strong and seems ok. But when compared to before at lower elevations, first starts are not as good.

And on a side note if anyone has any advice on some good tricks for increasing the HP I am all ears. It does seem to have lost some power moving up to the thinner air. I already have a Jardine slip on, and a K&N. I just don’t have the $ to get a more powerful bike. Still paying back college loans. Any advice would be great!

[dchd1130]

You can get a jet kit for it. You are probably running a little rich. You wont have as much power up here as you do at lower altitudes but if you tune it for the altitude it should be beter. Talk to the guys at TFOG they can get you set up with what you need.

[ryanZR7S]

Hey thanks I have looked into a jet kit. I just don’t know if I want to do the work myself. I am sure it must be expensive to have your carbs taken apart.

Anyways any idea on what kind of mpg hit I might take if I jet it? I did a ride a week ago from Fort Collins, horse tooth, EstesPark, Peak to Peak and down left hand canyon. I want to say it was about 150 miles long and I got 67miles per gallon when I filled up after.

[lightning126zp]

Check this guy out: http://www.sr-tuning.us/
His name is Frank and tell him Kenn (Red SV650S) sent you.

[PleasureTool]

I've also been interested in how to get my bike to run well at high altitudes since i just moved here from sea level. From what i've read, and my understanding of carbs, carbs automatically adjust for altitude (they sense atmosphereic pressure.) So the only instance you'll need to change carb jets/needles is if they are too big... which maybe the problem, because stock jetting isn't designed for 10,000 ft. To explain, the carbs automatically adjust by moving the needle further into the jet, constricting fuel (since there is less air), but at a certain point have adjusted as much as possible and there is still too much fuel delivery.

Another problem is not that the air becomes too thin, causing a rich condition... it's that at high altitudes there is less back pressure in the exhaust sytem, causing too much unburned fuel to exit during valve overlap, resulting in a lean condition. I think this only becomes an issue at really high altitudes.

Can anyone back me up on this?

[Bueller]

FI will adjust mostly for altitude, carbs don't. They are jetted were they are at and if you change altitude, and to a lesser degree temperature and barometric pressure you mixture will change. If you have a carbureted bike that came from a lower altitude (sea level for sure) It needs to be re jetted. You are running too rich.

[ryanZR7S]

Yeah I also thought that carbs do adjust to altitude by there natural design. I was more interested in jetting to make of or HP loss as I climbed in altitude from sea level. And I may just need to have my carbs re-tuned to each other from the pervious summer?

[Bueller]

You can't make up for HP loss at altitude, you can only make the engine as efficient as possible, at this altitude. When you get up on the passes it will still run rich if you are jetted for Denver.

[PleasureTool]

FI will adjust mostly for altitude, carbs don't. They are jetted were they are at and if you change altitude, and to a lesser degree temperature and barometric pressure you mixture will change. If you have a carbureted bike that came from a lower altitude (sea level for sure) It needs to be re jetted. You are running too rich.

I'm not saying that jetting isn't going to need adjustment at high altitudes, but according to this site: http://www.floheadworks.com/Products/keihin_carburetors.htm

"All constant velocity (CV) carburetors are designed to compensate for changes in altitude and barometric pressure."

also from another site:
"Because these are CV carbs which meter fuel based on flow, they are somewhat altitude compensating. As you go up in elevation the air thins out, the flow through the carb slows, and the needle doesn't rise as high yielding the necessary leaner mixture. My own tests have shown good results from the same settings when ridden in Borrego Springs, Ca. (below sea level), as well as in the Colorado Rockies above 11,000 feet."

which leads me to believe the main culprit is the decrease in back pressure that lets extra fuel escape. I've been toying with the idea to make some sort of plate that will partially cover my exhaust hole to increase back pressure, that i can easily screw on when i get to some place over 10,000 feet or so.