they're not rare...they're just rarely seen in MC shops as no one there knows how to work on them.
Dont go back.
What he said ^^^...
I get a LOT of the CB400F bikes here for these sorts of problems, and I would seriously estimate that 80% of them have been simply SCREWED UP by a local shop, even a Honda shop (if one would even work on them).
The #1 problem: the tiny emulsifier holes in the mainjet holders (which are called the "emulsifier tubes") are partially (or even fully) blocked with a calcium that comes from ethanol gas when the carbs are allowed to dry out. If gas preservative was ever used, this green stuff will completely plug those holes.
In addition, ethanol-laced fuels (and other EPA tweakings to our 'modern' gasolines) require much more air to make the fuel burn. This is their attempt to make it "cleaner' fuel. Trouble is: older carbs are not calibrated to that air supply. So, if the emulsifier holes are even slightly clogged, the fuel will not 'bubble' and then it won't burn well, making dark sparkplugs and poor performance.
Moral: pull out the emulsifier tube and clean out those holes. Start by chucking the tube into a drill, spin it and gently polish the surface CLEAN with 200 grit emery paper, lightly. Clean well, then push the largest wire or pointed object (or a tiny drill) possible into every hole to clean them out. You will find crud, guaranteed, if the bike has been sitting for a year or more in recent history.
Then take out the idle jets: you will see that they are a miniature version of the emulsifer tube, so repeat the process with them.
