® Carburetors and Altitude

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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