Author Topic: CB400F losing power and revving  (Read 892 times)

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Offline chrisp

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CB400F losing power and revving
« on: December 26, 2014, 08:46:32 AM »
Hi all.
I have a 75 CB400 that's been basically flawless for years. It was a barn find that I got cleaned up by a vintage honda specialist locally & then I upgraded the electricals a bit (Dyna, etc).

Recently I've been finding it losing power...which almost feels like something is slipping when I'm revving quickly or in 7k / 8k. What happens is the bike goes from a predictable increase in revs to jumping up much more quickly without any power increase.

I've noticed a bit of an oil deposit on the left side of the engine which seems to me like there's a gasket that will need to be replaced sometime soon. The right side is perfectly clean.

I was wondering if it might be the carbs that need a cleaning.

I'm pretty familiar with electronics, but mechanics are still new to me...so I'm just trying to learn how to diagnose mechanical issues.

Any help / suggestions in figuring this out would be really helpful.

Thanks!

Offline seanbarney41

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Re: CB400F losing power and revving
« Reply #1 on: December 26, 2014, 09:40:05 AM »


Recently I've been finding it losing power...which almost feels like something is slipping when I'm revving quickly or in 7k / 8k. What happens is the bike goes from a predictable increase in revs to jumping up much more quickly without any power increase.


Thanks!
Would it be more accurate wording to say ..."bike goes from a predictable increase in revs to jumping up much more quickly without any" speed increase?  If so, it sounds like a clutch issue.  May just need an adjustment.  Adjustment of the cable and clutch is covered in the periodic adjustments section of the manual.  Follow the directions to a tee, as it can be somewhat tricky on the 400, and see what happens.  Also, if you have been using motor oil that is not approved for wet clutches, you will need to stop doing that.
If it works good, it looks good...

Offline chrisp

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Re: CB400F losing power and revving
« Reply #2 on: December 26, 2014, 10:07:10 AM »
Yes it goes from a steady increase in revs to jumping 1000 revs or so and not increasing speed. I also expected it could potentially be the clutch. There's no problems otherwise...and it seems to get better as the bike warms up.

I'm using non-synthetic Motul 20w50 which is what a vintage Honda mechanic here in Northern CA told me to use. If there's a better option I'm all ears.

I'll take a look at the manual / clutch adjustment and see what I can find.

Offline Bodi

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Re: CB400F losing power and revving
« Reply #3 on: December 26, 2014, 11:26:16 AM »
Slipping clutch, I think if it slipped out of gear you would know.
The natural wear on your clutch plates will eventually cause this unless you occasionally do the adjustments. There are two, the throwout mechanism and the cable slack. If the clutch cable is tight with the clutch released, it may be holding the clutch partially disengaged - there is a weak spring in the clutch cover to take up slack but you should feel a definite increase in pull as the clutch lever moves in 1/2" or so. If it still slips with proper cable slack, it may be the throwout cam adjustment. If it still slips with all adjustments good, the clutch springs may be weak (not unusual if you have the original springs - they've been compressed for about 40 years). It could be worn friction plates or damage to them from anti-friction oil additives. I don't know if contaminated friction pates can be salvaged.
There's also a possibility the basket fingers have notches worn into their edges, with the plate edges locking into them and thus not able to move in to engage the clutch fully.
Once you have the adjustments good, it starts to cost money to look further. Removing the cover almost always means getting a new gasket. The springs are fairly cheap. The plates are not so cheap, and a special tool is helpful to get the clutch nut off. Notched basket fingers should be fixable with a careful bit of filing but you need to take the clutch apart to get at them.

Offline chrisp

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Re: CB400F losing power and revving
« Reply #4 on: December 26, 2014, 11:35:46 AM »
Thanks Bodi.
Do you know which manual would explain adjustment to the throwout cam?
I know how to adjust the cable.

Ya I agree it's definitely not slipping out of gear.

Offline bjbuchanan

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Re: CB400F losing power and revving
« Reply #5 on: December 26, 2014, 01:06:49 PM »
Loosen lock nut, turn flat head adjuster in until resistance is felt. Turn back 1/4-1/2" turn. Lock down again. Now adjust cable.

The cable has to be totally slacked to adjust it correctly
The dirty girl-1976 cb750k, Ebay 836, Tracy bodykit
Round top carbs w/ 38 pilots, middle needle position, airscrew 7/8ths out, 122 main jet
Stock airbox w/ drop in K&N, Hooker 4-1

Don't trust me alone with a claw hammer and some pliers