Topic: CB400F Engine swap due to poor compression?

[Alaxy Galaxy]

I'm wondering if I should swap the motor in my CB400F.

The engine I've been running for 3 years was sold to me as a good runner. Although it's been reliable and has good power, it does leak a bit of oil and, more importantly, consumes oil due to poor compression.

I have a spare motor on the bench that I know nothing about other than it's not stuck, the cam-chain shoe is free and it has great compression (135 psi on all 4). My current motor barely has 90 psi with oil in the bores.

What would you do? I can swap everything over if need be. (magneto coil, starter, points, clutch, hell, even the cam and rockers if I need to)

[Kevin]

I would swap it.
It's not that difficult to do.
Then you can do a rebuild of your original motor.

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[Alaxy Galaxy]

Well, it looks like the engine with his compression has at least two leaky valve stem seals on the exhaust side, so I'd be trading one set of problems for another.

[Don R]

Consider swapping the seals for new if the guides are usable.

[Alaxy Galaxy]

Consider swapping the seals for new if the guides are usable.

That's true. And the head doesn't need to come off fur that, right?

Would you change them all while you're in there?

[Tim2005]

Yes, you can, you just need to make up some sort of lever arrangement to compress the valve spring. You can actually get away with bringing the piston to tdc and resting the top of the valve on it too. I've done it on a 400f with the motor in the frame.   

[Alaxy Galaxy]

Yes, you can, you just need to make up some sort of lever arrangement to compress the valve spring. You can actually get away with bringing the piston to tdc and resting the top of the valve on it too. I've done it on a 400f with the motor in the frame.

Thanks. I already have a jig like that that I made to remove the springs on this motor. The fact that it's not in frame makes it easier.

Again though, would you replace them all or just the obviously leaky ones?

[Tim2005]

From what I remember it was pretty easy and quick to do, so I'd do all 4 while I have got the opportunity

[Alaxy Galaxy]

From what I remember it was pretty easy and quick to do, so I'd do all 4 while I have got the opportunity

Can they be removed without special tools and without removing the head?

[Bodi]

There are no seals on 400F exhaust guides in my experience, although the fiche shows them.

[bryanj]

350bsome didnt have exh seals 400 did

[Alaxy Galaxy]

There are no seals on 400F exhaust guides in my experience, although the fiche shows them.

This motor does not have seals on the exhaust side. I will order some and see if I can fit them nonetheless.

[Kevin]

I don't think they will fit, the valve guides are not machined to accept them on the 400F.

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[Alaxy Galaxy]

I don't think they will fit, the valve guides are not machined to accept them on the 400F.

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You're right. The parts fiche must be wrong in this case. In any event, I'll leave it as is and if the intake seals seem to be problematic down the road, it's pretty simple to change them. Might even be able to do so with motor in frame.

Now I just have to decide whether I want to remove an engine that I know works in order to replace it with one that I know nothing about...

[The Lone Builder]

Hi Alaxy,

FWIW, the parts guide I have doesn’t show seals on the exhaust side. Happy to send you the whole doc if you’d like, but it’s 2MB so may be a bit big to attach.

Are you going to part out the seized engine? I’m struggling to find some parts for my resurrection, so I’d be interested if you are. Main missing bits are the cam chain bar, the clutch arm and its spring.

Cheers.

[Alaxy Galaxy]

Hi Alaxy,

FWIW, the parts guide I have doesn’t show seals on the exhaust side. Happy to send you the whole doc if you’d like, but it’s 2MB so may be a bit big to attach.

Are you going to part out the seized engine? I’m struggling to find some parts for my resurrection, so I’d be interested if you are. Main missing bits are the cam chain bar, the clutch arm and its spring.

Cheers.

Of that is interesting. What market is that parts fiche from?

As for the engines, neither is stuck. I've got a running engine with sub par compression and oil consumption, and a spare motor with good compression but otherwise untested. I am not planning to part out either at this point.

[The Lone Builder]

I’m not sure, but I have another document that shows, presumably, the models it applies to.

It also shows no exhaust-side seals.

The first one came from here:
http://www.manualedereparatie.info/categorii/honda.html