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Carbs in Colorado (vs. Florida)

Griffin said:
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. :)
Click to expand...
High altitude! :lol: I live at over 6000'. 8)

They should have re-jetted slightly lean.
 
Fuel/Air

Fuel/Air

Ok,
Maybe SOME of you have heard of Carl Shipman. His book's have been the bible for tuning for decades. Here are some notes from one of his book's on this subject.
Sea level (all carbureted bike's are jetted at sea level, stock) has a barometric reading of about 30" of mercury which is about 14.7 psi.
Pressure will drop about 1 inch for every 1000 feet you go up.
This is very close to 1 pound of air pressure for every 2000 feet you go up. When air becomes less dense, a carb will meter fuel so there is excess fuel and a rich mixture is the result. Temperature also has a affect as well. 5000 feet above sea level at O degrees F will be just under 95 percent air pressure of a 60 degree day at sea level. If the temperature is 60 degrees at 5000 feet, air pressure is down to just over 85 percent. No carb, cv or otherwise, can re-meter the fuel ratio to compensate for this pressure loss. You have to lower the amount of fuel available, re-jet.
A note on CV carbs, because a CV carb operates the slide according to engine vacuum and not by the riders right hand, they tend to be more accurate at adjusting the venturi area and air flow. This is obvious by the fact that any bike with a given displacement will have a larger carb if it is a CV than a carb with a rider controlled slide. Another problem is the float bowl. At sea level with 14.7 psi, this pressure is acting on the fuel in the bowl through the vent tube keeping everything ballanced. As you go higher in alltitude there is less pressure and the level in your main nozzle will rise. This will make it easier for the venturi pressure drop to "suck" out fuel, and you get a richer mixture.
I have way to much info on this subject to put into this post, so if anyone is really interested in the math and the calculations for this topic, feel free to e-mail me and I can scan and send whole pages.
Keith
 
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