Honda used a variety of carburetors on its passenger car engines, one of which was the oval top "three barrel" Keihin. This carburetor was used on Honda "CVCC" (Compound Vortex Controlled Combustion) engines in 1975 through 1983 Civics, 1976-1983 Accords, 1979-1983 Preludes and 1984-1986 Hondas with the 1.8L CVCC engine. A second variant of Keihin was used on 1984-1987 Civics. It was a round-top version and also had the third auxiliary venturi and throttle bore for the CVCC engine.
The Keihin three barrel is unique because it has a third "auxiliary" throttle bore in addition to its primary and secondary throttle bores. The primary throttle bore delivers fuel for low speed and part throttle operation while the secondary throttle bore opens to feed extra air and fuel to the engine at high speed or when the engine is under load. The auxiliary throttle bore, however, is a fuel delivery system unto itself. It supplies a relatively rich air/fuel mixture through a separate metering circuit which is then routed into precombustion chambers in the cylinder head via separate runners in the intake manifold.
The auxiliary throttle bore is much smaller than either the primary or secondary throttle bores, and is completely separate from the other two. It has its own fuel bowl, float, needle valve, air bleed, vent hose and cut-off diaphragm, idle mixture adjustment and throttle linkage adjustment (referred to by Honda as the "Lambda" adjustment). So for all practical purposes the auxiliary throttle bore is essentially a mini-carburetor within the main carburetor.
The CVCC engine uses a stratified charge principle of combustion to reduce emissions and improve fuel economy. A rich fuel mixture is used to ignite a much leaner fuel mixture. That's where the auxiliary throttle bore in the Keihin carburetor comes into play. Each cylinder has a small precombustion chamber in the head. The precombustion chamber receives a rich air/fuel mixture through its own auxiliary intake valve and auxiliary intake port, which is supplied by the third throttle bore in the Keihin carburetor. The rich mixture in the precombustion chamber is ignited by the spark plug, which then explodes outward igniting the leaner fuel mixture in the main combustion chamber. It's a complicated setup, but it significantly reduces unburned hydrocarbons (HC) and carbon monoxide (CO). This allowed Honda to meet the federal emission requirements without having to use a catalytic converter up until 1978.
Like most Japanese carburetors, the Keihin has a number of external vacuum hose connections. These include a dashpot solenoid valve, throttle control valve, throttle opener diaphragm and dashpot diaphragm which all work together to control the rate at which the throttle closes during deceleration to reduce emissions. On certain California models, there is also a gizmo called an Air Jet Controller which is an altitude compensation device that alters airflow into the slow and main jets of the auxiliary and secondary system. This allows the carb to maintain the optimum air/fuel ratio with changing elevation and air pressure.
Plumbing is also provided to vent the fuel bowls to the charcoal canister, and vacuum connections to a purge control valve to siphon fuel vapors back out of the canister when the engine is warm. A start control solenoid valve is another device that is included in the vacuum plumbing which allows extra air into the intake manifold to improve hot starting.
Carburetion problems with a Keihin usually fall into one of five basic categories:
* Starting Problems. Usually choke related, but can also be caused by vacuum leaks or a faulty fuel cut-off solenoid.
* Stalling. The choke and/or idle speed may need adjusting, but stalling can also be caused by vacuum leaks, a defective fuel cut-off solenoid or a defective idle boost control solenoid or diaphragm.
* Lean misfire (rough idle, high HC emissions). A lean fuel condition due to a low fuel level in the carburetor bowl (incorrect float adjustment, plugged filter, weak fuel pump), dirty jets, vacuum leaks (bad carb gasket or insulator) or incorrect idle mixture adjustment.
* Rich mix problems (poor fuel economy, excessive CO emissions). High fuel level in the fuel bowl (incorrect float adjustment or defective float, leaky needle valve), clogged or dirty air bleeds, internal fuel leaks or incorrect idle mixture adjustment. This condition can also sometimes be caused by a defective charcoal canister purge valve that allows raw fuel to be siphoned into the carburetor.
* Transition problems (hesitation). Accelerator pump and/or power valve problems. If the engine bucks and stumbles during cold weather, check the operation of the heated air inlet diaphragm on the air cleaner. Carburetor icing may be the problem.
Finding a good used or rebuilt Keihin carburetor is nearly impossible, so if the carburetor gaskets and seals have dried out with age, or the accelerator pump is only delivering a weak stream (or no stream at all), an overhaul is about all you can do to restore the carb. You can attempt it yourself, or you can find a local repair shop to overhaul your old carburetor for you. There are also some online resources that can overhaul or rebuild import carburetors. You send them your old carburetor, they rebuild it and send it back to you. However, your older carburetor may not be rebuildable if it has problem such as worn throttle shafts, cracks in the casting or other major physical damage or missing parts.
Dirty fuel metering circuits, leaky O-rings or gaskets, leaky or misadjusted floats, choke problems, a bad accelerator pump, etc., are all problems that can be fixed provided you can kind an overhaul kit. A basic service kit usually includes new needle valves, diaphragms, O-rings and an airhorn gasket. But the Honda OEM on-car service kits do not include all the parts one might need to do a complete overhaul. The accelerator pump kit and base gasket must be purchased separately (if you can find these parts). Same for the floats if they are bad. Some aftermarket carb rebuild kits are more complete, but also do not include floats or other external items like choke hardware and solenoids.
Another item that may have to be replaced when overhauling or replacing a Keihin carburetor is the molded rubber insulator that sits under the carb. The insulator has a reputation for being a source of troublesome vacuum leaks.
One way to check the insulator on a car that idles rough is to spray carburetor cleaner around the insulator (don't do this while the vehicle is connected to an infrared exhaust analyzer as doing so may damage the analyzer!). If the idle suddenly smooths out, it means the solvent is being siphoned past a vacuum leak. If tightening up the base bolts on the carb doesn't cure the problem, remove the carburetor and inspect the O-rings that seal it against the insulator.
As for floats, there are two (one for the auxiliary throttle bore, and one for the other two barrels). The Keihin carburetors have hollow plastic injection molded floats, which means they fill with fuel and sink if they develop a leak. The result is fuel flooding. Most replacement floats are identical to the OEM Honda design, but some aftermarket floats may be made of molded nitrophyl (a foam-like plastic that can't leak because it is a solid material).
Keihin carburetors have two float level adjustments, one for the main fuel delivery system and one for the auxiliary system. If the carburetor has been disassembled, both floats first have to be set dry, then fine tuned once the carburetor is on the car and filled with fuel (a running wet adjustment).
The dry adjustment is performed while holding the bowl cover in a vertical (not inverted) position. The float arm should rest lightly against the spring-loaded needle valve, but not compress it. The float is then adjusted by turning the needle valve with a screwdriver until the distance between the base of the bowl cover and the outside edge of the float is within specs.
The on-car wet adjustment is made with the engine idling at normal operating temperature. Snap the throttle several times, then check the level when it stabilizes. The correct float height is obtained when the fuel level touches the dot on the inspection window. If it needs to be adjusted, turn the needle valve 1/8 turn left or right to raise or lower the float. Wait 15 seconds, and readjust as needed until the correct level is obtained. Don't turn the needle in too far or you can unseat and damage the lower O-ring on the needle valve.
On vehicles where no sight window is provided, Honda had a special float gauge for these carburetors (P/N 07501-6950100) and catch tray (P/N 07501-6950201) for making the float adjustment. To use the gauge, the primary main fuel cut-off solenoid has to be removed along with auxiliary main jet plug.
The auxiliary throttle linkage, which is called the "Lambda" linkage, is a critical adjustment and is set at the factory. So unless someone has messed with it, no adjustment is needed or advised. If the carburetor has been tampered with or disassembled for rebuilding, here's the procedure for adjusting the Lambda linkage:
First you need a dial indicator. Place the tip of the indicator against the auxiliary throttle blade. Then remove the cap from the Lambda adjustment screw, loosen the locknut and tighten the linkage until the primary throttle blade just opens. Then zero the dial indicator and back off the Lambda adjustment screw about 2-1/2 turns until the dial reads the required distance. On a '79 Prelude, for example, the manual calls for 0.1 mm plus or minus 0.01 mm (0.004 plus or minus 0.0004 in.). Once the linkage is set, retighten the locknut being careful not to disturb the setting. The throttle stop screw can be readjusted once the carburetor is reinstalled on the engine.
CARBURETOR ACCELERATOR PUMP ADJUSTMENT
Another critical adjustment on the Keihin carb is the accelerator pump linkage. The adjustment is made by bending the tang on the pump lever so the distance between the stop bracket and lever tang is within specs (which vary from 11.0 to 15.0 mm depending on the application).
AUXILIARY IDLE MIXTURE ADJUSTMENT
The auxiliary idle mixture adjustment should be done with the engine at normal operating temperature (make sure the choke is fully open). If you're starting from scratch, turn the screw in lightly until it seats, then back it out about 1-3/4 turns as a starting point. You can use either propane enrichment or a lean drop technique to set the mix.