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Motorcycle Repair/1975 CB400F running problems



So I have a pretty cherry cb400F that's basically never given me much trouble at all. It has 6700 miles on it, and runs like a dream.
I drove the bike home yesterday without any issue, but when I woke up today and tried to take the bike for a ride, about 3 blocks away from my house the bike started acting strange.
It sounds like its running out of gas a bit, and it will die if I dont keep the RPMs up.
It starts and runs, but runs rough, and only if I give it gas.
It even backfired once while I was revving it. The bikes never done anything like this before.

The most recent thing that happened was a magnet in the ignition came loose and I epoxied it back in and have been running it that way no problem. I also just recently replaced all the plugs, but I've been running on those for the past 80 miles with no issue.

I have been running the bike more on the highway, but I've never had a problem with it at speed.

Any ideas where I should start looking?

Christopher, I am a little puzzled about your "magnet in the ignition came loose" comment. The bikes came with points and condenser ignition, so have to guess that perhaps you have a Dyna electonic ignition instead.

I suspect that either your battery is failing or the charging system has a problem. Have the battery fully charged and then load tested to see if you have a weak cell. With these bikes, the headlights come ON automatically, so when there are issues with the battery or charging system, then the voltage draw-down usually affects the ignition system first, when the voltage drops down into the 10-11 volt range.

Once you have a good battery in place, check the voltage output, which should run up into the low 13v range beyond about 3k rpms. If you are not getting much more than the original battery voltage before you start the bike, then the charging system is having a problem. Check the stator connection wiring beneath the countershaft sprocket cover. These bikes have a mechanical voltage regulator which are not very reliable. There are aftermarket regulator/rectifier units out on the market that will replace the mechanical regulator and give better voltage regulation.

Other than a blocked gas cap vent system or petcock filter plugged up with rust/corrosion, the fuel system shouldn't give a sudden performance issue as you describe.

Another trouble spot on these bikes is the starter button on the right switch assembly. These are failure prone, often leading to either a lost of electric starter function or headlight failure, even when the bulb is good.

Otherwise, they are great bikes. I have owned about a dozen of them through the years and always have fun with them.

Bill Silver  

Motorcycle Repair

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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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