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Motorcycle Repair/CB77 lean condition on right cylinder


Hi Bill ! Yesterday,during a ride, the engine started to develop a misfiring at 1/8 throttle (steady cruising).It climbs to redline without a hiccup,except in fourth gear.I did a plug chop check.The left cylinder plug was the right brownish color.However, the right side plug is completely white ! I cannot figure out how come the right cylinder developped a lean condition all of a sudden.Air filters are stock and clean, timing is right on (with electronic ignition).I put new plugs, and did another test, and now,both sides are white !!Before lowering the carb needle one position to richen the mixture (good idea ?), maybe it's better to find the cause of this.Worn carb parts ? What do you think ? Thanks again, Miche from Canada.

Michel,  First of all, check your compression and valve settings to make sure nothing is getting damaged.

My setup for US bikes on alcohol-based fuels is #140 main jets, needles at #4 clip notch. Lowering the needle leans the mid-range out, not making it richer.

Be sure that your float levels are 22.5mm and carb slides are synched properly and not sticking.

Verify the ignition timing with a dynamic timing light on BOTH sides. Even with the e-ignition, the timing can vary from side to side and the right sides tend to run a few degrees more timing than the left, which makes them seize first!

Check your o-rings on the carb flanges and insulators. They flatten out and cause air leaks.
Use the 260-004 o-rings, not the PFO suggested by erroneous Honda parts listings. They only fit CB72s.

Also, bear in mind that the intake ports are not symmetrical, so one side has the intake charge pointing onto the spark plug tip and the other side points away, causing some variations in the way the plugs read.


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Bill Silver


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.


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.

VJMC (Vintage Japanese Motorcycle Club) of North America

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

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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