CB400A Keihin Carbs

chris_sav

chris_sav

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Joined
Oct 31, 2025
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22
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Great Mongeham
Had my first experience of these in having to change O rings in the balance pipe on CB400A carbs that the previous owner had paid megabucks for rebuild on.

What a convoluted torture chamber experience to get at two knackered O rings! Uncovered a whole nest of vipers at the same time, wrong push cable, knackered inlet port O rings etc. so not totally wasted.

If I have to do it again, is it easier to take the air box out rather than skin knuckles trying to get the gaiters back on? Old rubber and that air box orifice certainly don't mix, one of my gaiters had been previously installed creased and refused to sit on the rim, I had to boil it for a while to soften it up a bit, previous owner had tried to use contact adhesive to hold the blasted thing in place.

Try it out today!
 
It's almost always easier to remove the airbox on any motorcycle to service the carb imo.

I have a motorcycle where you can't remove the airbox without taking out the swing arm and the boots are hard as a rock. To work on that carb I find it easier if not necessary to remove a few of the engine mounting hardware and loosen the remaining so I can drop the engine 1/4 of an inch, makes all the difference. Fighting those tight spaces on old bikes leads to more damage and headache than an extra few minutes to provide space.
 
chris_sav said:
Had my first experience with these in having to change O-rings in the balance pipe on CB400A carbs that the previous owner paid megabucks for rebuild on.

If I have to do it again, is it easier to take the air box out rather than skin knuckles trying to get the gaiters back on? Old rubber and that air box orifice certainly don't mix, one of my gaiters had been previously installed creased and refused to sit on the rim, I had to boil it for a while to soften it up a bit, previous owner had tried to use contact adhesive to hold the blasted thing in place.
Click to expand...
"Balance pipe" ?

Not only do I find it much easier to unbolt the airboxes on my '82s and move them rearward to service the carbs rack, but it's also done so not to risk damaging the No-Longer-Available boots between the airbox and carbs.
And, it may not have been your bike's previous owner who used the contact cement, because Honda did just that at the factory when new as a re-enforcment and/or supplementary seal when they put the grooved-rubber boots into the airboxes of my '82s
 
"Balance Pipe" - fuel connection tube between the carbs

Thanks guys, need to take the carbs off my other CB400A as that does not run smooth at low revs so will follow advice.
 
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