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Motorcycle Repair/1971 Honda CB 350 Carburetor's

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Question
I would like your advice concerning my 1971 Honda CB 350 K3. I wanted to ask you about the three holes on the top of the carburetor. I don't know what these three holes are for. I have tried blowing air through them as well as carb cleaner but I am having trouble with my left carburetor in that the spark plug is running rich. The right carburetor is a nice chocolate brown. I would appreciate it very much your advice concerning information about my motorcycle.
Robert Anderson

Answer
Robert, I assume you are referring to the three small metered holes in the carburetor body, beneath the carburetor diaphragm.

The diaphragm is vacuum-controlled and these three ports help to move the diaphragm up and down, based upon the throttle plate position and engine speed/load. The balance the manifold vacuum signals with the external ambient air pressure and allow the slide to react to the engine demands almost instantaneously.

http://www.cmsnl.com/honda-cb350-super-sport-350-k4-1972-usa_model433/partslist/  to the left of #20, which is not marked for unknown reasons is the emulsion tube for the primary main jet and must be removed and cleaned, as does the one for the secondary main jet    #3

What looks like a slot for a screwdriver on the primary emulsion tube is NOT threaded and is only pressed in by the primary main jet. However, they can become corroded in place. Soak the emulsion tubes with carb spray and then use a plastic or wooden rod (Chopsticks are great for this job) to work the emulsion tubes out of the carburetor body. If you use a screwdriver on the end of the primary emulsion tube, it will probably break the little ears off, damaging the tube permanently.

The tubes are cross-drilled and the holes must be clean, as does the inside of the carburetor body which feeds air and fuel into these passages. If you have access to an ultrasonic cleaner, that would be a great way to ensure that the carb body is completely clean inside.

When you are troubleshooting metering issues, be aware that there are many factors that will cause a plug to foul...

Fuel fouling can come from either wrong/damaged jets, incorrect sized jets from aftermarket carb kits, damaged carb body where the jet o-rings seal the jets to the body, blocked air passages in the throat of the carburetor inlet, incorrect float height, damaged/leaking o-rings for the float valve or a damaged carburetor body. Carburetor intake manifolds and o-ring seals to the cylinder head will affect the mixture settings if worn/damaged.

Oil fouling comes from damaged/worn valve stem seals, work guides/valve stems, worn cylinder bores, worn/damaged pistons and rings.

On top of that, the carburetor metering is affected by ignition timing errors. Either base/idle timing or full advance timing affects the vacuum signals to the carburetor metering circuits. Faulty spark plug caps can cause misfiring along with a weak/defective condenser or coil.

Make sure that both carburetors are from the same series. They are stamped with a calibration mark just above the inlet.

Watch out for the choke plates which have a spring-loaded flapper door which can break off and be ingested into the cylinder with damaging results. The throttle shafts can become worn and cause air leaks, too.

Bill Silver  

Motorcycle Repair

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Bill Silver

Expertise

Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.

Experience

I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

Organizations
VJMC (Vintage Japanese Motorcycle Club) of North America

Publications
VJMC newsletter, as editor for two years and as contributing editor currently.

Education/Credentials
3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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