® Carburetors part 13


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. I’m familiar with every one of the Big Four’s dealer mechanics training material--due to a career in votech training--on carburetion and yet I have no idea where that came from. Unfortunately, this is almost completely myth, especially when considering a large percentage of the vintage Hondas out there, that is, those having constant velocity (CV) carburetors.

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
It is commonly believed that a bike taken out for a cold, even foggy ride tends to run better due to the more oxygen-laden air. The bike seems more responsive, better performing overall. Unfortunately, though a brisk day often results in a uniquely enjoyable ride, one that I appreciate myself, this is one of those very subjective things that is rooted more in perception than in realty. Actually, the canopy of moisture in the foggy atmosphere that creates a denser atmosphere and thus a kind of pressure wall around the rider, powerfully reflects sound and thus the rider hears his engine more distinctly. It is natural to feel that the engine is running better than usual. But it is, alas, only a dream.

Zinc
Remember the candy Cracker Jack? Each box came with a toy inside. In the candy’s earlier years, this toy was made of what many people would call "pot metal." It was in fact zinc. Being a relatively heavy metal and one that flowed more easily than most, zinc was once used by manufacturers who did not possess the best casting technology--the permanent multi-part metal mold and sophisticated vacuum-based casting methods of today just were out of their reach. Zinc pours well and is heavy. A pre-1980 Honda carburetor body, often made of a very zinc-rich alloy, stripped of everything save the throttle shaft weighs two pounds. This is exactly double the weight of a later post-1980 similarly-sized and prepared but improved alloy casting Honda carburetor body. Something else: Zinc is a highly reactive metal, so reactive that in the marine world it has traditionally been employed as a sacrificial corrosive annode, giving itself up to save the aluminum and other metals of marine engine and powerdrive assemblies, for example. Pre-1980 zinc-based Honda carbs that sit too long exposed to the weather tend to corrode internally quite rapidly and irreparably, so much so that it is this characteristic that has given rise to the belief that motorcycle carburetors in general corrode badly. Additionally, the zinc based alloy in these early carbs is so chemically reactive that special cleaning methods need to be practiced to avoid darkening or even destroying the metal. Zinc type carburetors are also more difficult than most to repair by welding.

Part 14


Last updated January 2025
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