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Carburetors part 13 |
Altitude
First, few machines' performance is affected by the slight richening that results from small increases in altitude, at least up to about 6,000 feet. Only very skilled people can detect the loss of engine sharpness due to richness, it is so slight. And really, only a few machines exhibit issues that anyone notice. Engines 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 mixture change, which though it happens, is largely insignificant. But the loss of oxygen in the cylinder is not a small thing. 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 reached 100 mph on that 8,000+ foot high track. Why? No one was capable of properly jetting their machines’s carburetors? No. Bikes were slow because less air in means less cylinder pressure which means less power. It has nothing to do with jetting. Another example is snowmobiles. It is common for snowmobilers to retune their CVT transmissions when their sleds are used at high altitude, which of course they often are. This is done so that the engine gains more rpm before the clutches engage, basically gearing the transmission down so the engine can get up onto its torque peak sooner. It helps, and although it hasn’t made up for the power loss--it merely spins the engine harder--jetting can never result in the same benefit. It’s a cylinder presssure problem, not a mixture problem.
But there is something even more important going on with carburetors and altitude. Assuming your street machine has the CV carburetors that have defined streetbike carbuetion for several decades, there is even less reason to think about jetting when going into the mountains. Altitude's richening effect is largely absent in CV carbs. In fact, CVs are affected by altitude at only two points in their operation--idle and wide open throttle. Everywhere in-between they are not affected. This is because the CV carb's slide rises through pressure differences. And guess what? Though the minimum and maximum pressures are natually lessened, the carb "sees" the exact same pressure difference at 10,000 feet that 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 Hondas used above 6,000 feet, but this only for emissions purposes. I have ridden at 14,000 feet on my CV-equipped Honda CBX and experienced only the expected cylinder pressure related power losses, and no fueling issues whatsoever. All Honda carbs can in fact stand to have a main jet change for very high altitudes, but this is critical in very few CV-carbed Hondas. The unusually-sensitive DOHC 450 carburetor--with its wide range of factory-suggested main jets--is probably one of them. But there aren’t many CVs that are that fussy.
Cold air
Zinc |
Last updated January 2025 Email me © 1996-2025 Mike Nixon |