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Motorcycle Repair/Honda CB750FA 1980

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Question
Carbs have been cleaned and reassembled with many new bits.On tickover there seems to be a misfire on one cylinder but it revs freely.Vacuum balanced seems ok.Removing a plug in turn fails to identify affected cylinder/carb.How can I tell which one is affected/Help!!!!please.

Answer
Bill, did you replace the air-cut diaphragm?  

http://www.cmsnl.com/honda-cb750fa-canada_model14850/partslist/E18-1.html#result  #13

If the diaphragm tears, then you have an air leak at the #1 cylinder. If the caburetor idle mixture screw limiter function has been defeated, you turn in each mixture screw until you hear it die, which will indicate that it is functioning. If you turn one and there is no change in the rpms or misfiring, then you have isolated the cylinder/carburetor.

Before you go too far, check the compression readings to ensure you don't have a leaking valve or two.

Check the spark plug caps with an ohm meter. They should readout about 5k ohms each, when unscrewed from the plug wire.

Spray some WD40 or carb spray around the intake manifolds while the engine is running to detect an air leak at the manifold junctions.

The spark ignitors on these ignitions are know to have faults in them, but that will affect 2 cylinders at a time... 1-4 or 2-3.

Make sure the charging system is working properly. They have issues with bad rotors, brush wear, stator connector failures and/or regulator/rectifier problems. The ignition systems are not happy when the battery voltage gets down towards 10-11 volts.

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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