 Techline-Nov09-A TACH, A STUMBLE, AND A CLICK-Part II
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Emissions laws and equipment
This month we continue with an excerpt from Donny’s Unauthorized Technical Guide to Harley-Davidson 1936-Present, Volume III: The Evolution 1984-2000. (Some content has been altered to fit AIM’s style and format.)
CARB is the acronym for California Air Resources Board, a government entity that issues a stricter, California-only version of the federally mandated Environmental Protection Agency (EPA) laws. CARB is necessitated by two overriding factors specific to California: there is a heavy population concentration and its emissions have nowhere to go. The car culture produces harmful emissions, particularly in the densely populated greater Los Angeles area. The East Coast also has heavy population concentrations; however, its emissions can dissipate somewhat via prevailing winds. On the West Cost, the mountain ranges that run north and south trap the emissions over the populated areas so that smog concentrates. Therefore, California generally leads the way in dealing with harmful emissions. Emissions controls introduced in this environmentally progressive state will generally spread to the rest of the country and Canada later.
Emissions Controls
Before reed valve technology was introduced in 1988 to deal with fuel vapors inside the air cleaner after the bike stops running, H-D began following CARB dictates to redirect fuel tank fumes via the evaporative hose, through the vapor valve, and into a charcoal canister for purging. This is known as an evaporative emissions controls system (EVAP), which controls hydrocarbon vapors escaping into the atmosphere. Three hoses are attached to the canister: the evaporative hose carrying fuel vapor from the gas tank, via the vapor valve, into the canister; the clean air hose that connects the air cleaner to the canister to supply clean air; and the purge hose running from the canister to the air cleaner, allowing vapors to flow up into the air cleaner for ingestion into the combustion chamber on startup. The descending pistons on the intake stroke provide this impetus by creating low pressure inside the engine’s combustion chambers.
The CARB reed valve system, which was in use from 1988 to 1991, added another feature to the earlier one. Vapor hanging around inside the sealed air cleaner can move down into the charcoal canister or remain sealed inside the air box until startup, when the vapors are sucked into the combustion chamber. Reed valves are most commonly used internally in two-stroke engines. A reed valve is basically a one-way valve, sometimes descriptively called a flapper valve that operates on negative (vacuum) and positive air pressures. There are two reed valves that are located equidistant (on the bottom left and bottom right) on the rear of the carburetor air filter housing backing plate. They are not noticeable because this backing plate faces the engine. Each reed unit consists of a bottom reed and top reed with a reed stop. The reeds close, sealing the carburetor air box (cleaner) when the engine turns off. When the engine is running, negative air box pressure opens the reed valves and allows purged gas vapor to return from the carbon canister into the air cleaner for reentry into the combustion chamber. Damaged or broken reed valve components may reduce top speed and result in poor acceleration. The reason for this is that the reed valve may stay closed or partially closed when the engine is running, thus starving an air source. Furthermore, if a damaged reed valve stays open when the engine stops, gas vapors will escape into the atmosphere, causing pollution.
VOVS is the acronym for a repetitive but descriptive phrase: vacuum-operated vacuum switch. These were in use on 1988-89 (#27028-88) and 1990 and most 1991 (#27028-90) Big Twins. When the engine is not running, the VOVS closes off the carburetor float bowl vent passage. This will curb another source of gasoline vapor pollution. The VOVS also disrupts the vent’s function as an overflow hole if the float is maladjusted or malfunctions when the engine is not running. A rubber tube connects the vent overflow tube to the small VOVS located behind the carburetor under the manifold. Another short line continues from the VOVS up to a fitting that feeds into the top rear of the carburetor or manifold, depending on year and model. Intersecting the VOVS and carburetor-manifold fitting is the vacuum-operated electrical switch (VOES). I know, this is all beginning to sound very complicated but it isn’t. The VOES is mechanical. A vacuum causes it to operate, and atmospheric or positive air pressure ceases VOES operation.
So, where does the vacuum originate that operates this mechanical switch? It comes from the rear of the carburetor body’s throat or the manifold. The same vacuum caused by air rushing by through the manifold into the cylinders and combustion chambers that operate the VOES also does double duty and operates the VOVS emissions control switch. The VOES has an electrical component in addition to its mechanical one. It feeds vacuum information via electrical pulses to the engine management system’s electronic control module (ECM). The ECM then instructs the spark plugs when to fire via electrical pulses. Manifold vacuum indicates whether the engine is under load (acceleration) or not. If accelerating, piston speed increases, thus the spark plug must go off sooner (advanced ignition timing) to allow complete or near-complete combustion to occur at the correct time. Combustion takes a certain amount of time. The heat created due to combustion causes the gases in the cylinder to expand, driving the piston down, creating the turning force called torque. Combustion must be near completion as the piston begins descending on the power stroke. Therefore, in monitoring the vacuum, the VOES not only aids in emissions controls, but performs its primary function of advancing and retarding the ignition, also known as ignition timing.
On Harley-Davidsons produced for California, the secondary vent tube from the vapor valve is fed into a charcoal canister where the fumes are stored if the bike is not running. When running, the vacuum created in the carburetor venturi produced by the descending pistons on their intake stroke will draw the gasoline vapors from the charcoal canister up the purge hose back to the carburetor where they are ingested and burned in the combustion chamber. The charcoal canister on California bikes must deal with another source of gasoline vapors. These vapors originating inside the carburetor throat are drawn through a hose connected to a fitting in the inboard or backside of the air cleaner backing plate called the clean air inlet. This hose also feeds down to the charcoal canister container, drawing fresh air down the clean air outlet to assist in forcing the vapors in the canister back up to be burned.
On 1992 and later models, an electrical solenoid opens a butterfly seal on the backing plate when the ignition turns on, opening the clean air inlet. When the ignition is off, the solenoid butterfly plate closes in order to seal any hydrocarbon vapors in the air cleaner. A faulty solenoid-operated butterfly valve will definitely reduce top speed and result in poor acceleration. Again, it will allow the valve to close when the engine is running, thus starving an air source. If, for whatever reason, the rider or customizer wants to relocate the carbon canister, it must be below the carburetor to function properly.
The Vapor Valve
Vapors venting from the crankcase also feed into the air cleaner to be burned in the engine. The vapors not sucked in via venturi vacuum when the engine is stopped can also be fed into the clean air outlet in the carburetor backing plate and down the connecting clean air inlet hose to flow into the charcoal carbon canister reservoir. These purged fumes will then flow up the purge hose from the carbon canister when the engine starts. The vacuum created by the descending pistons on their intake stroke will create the negative pressure needed to draw up the fumes. Thus, both crankcase and carburetor throat fuel vapors that feed into the carbon canister are drawn through the purge hose, into the carburetor, and into the combustion chamber where they are burned. EFI models do not have the same level of vapor wandering that carburetors do. With EFI, the injectors spray gasoline in controlled bursts into the combustion chambers. Excess gas is pumped back into the gas tank. Due to a defect in the Evo Magneti Marelli fuel injection, the injectors point in the wrong direction, but the much-improved Delphi system introduced circa 2001 corrected and enhanced fuel delivery. Air flowing through the intake runner of the EFI system ably escorts errant vapors into the cylinders for burning.
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