Here is comparison on how the atmospheric pressure pushes the gasoline into the engine, as the engine is creating less atmospheric pressure (suction) as it is drawing air into the engine.
Same as sucking liquid through a straw, reducing the atmospheric pressure that is actually pushing the liquid through the straw.
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Now figure in the particular aspects to your situation in the mountains and long steep roads.
First there is a lot less atmospheric pressure at 10,000 feet in the mountains to push the gas into your engine than the 15 lbs per square inch at sea level.
Now along with that heading up a steep slope with the floats out of whack, set too low to build a reserve of fuel in the fuel bowls. Along with the angle of the slope, the gas may be coming up short to even enter the jets where it gets drawn into the engine.
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So in the thin Mountain air you are loosing 5 psi of atmospheric pressure right off the bat, to push fuel into the engine in the manner that it was designed to run.
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