Topic: Questions about "A 1975 CB400F 102.000 km (65000 mi) complete engine rebuild"

[supersports400]

I started a treat with the name : "A 1975 CB400F (102.000 km) full engine rebuild" (http://forums.sohc4.net/index.php?topic=65233.0) in the projects shop thread but I have several questions. I do have experience with the older Honda twins, including rebuilding engines, porting heads, and extended machining.    

I'm new to the sohc4 engines, and while starting to rebuild my engine I saw some things that bothered me.

First of all, when dismantling the intake side of the engine, I saw that the tube before the air filter that let's the air into the air filter has a small diameter. I compared the diameter with the diameter on the CB72 an actually it has almost the same diameter. The CB72 is a 250 cc twin engine, revs to 10.000 rpm and has a square bore / stroke ratio. From these points of view I suggest that the area of the air filter, including the air intake hose on the CB400F is way to small. I'm I right ?

When the engine is rebuild and after breaking in I will do a dyno run with and without the air hose before the air filter, I like to see if it's making any difference. Did someone else this already, if yes, what was the outcome ? In the threat in the project shop I also included some dyno runs,the first one how I got it (very lazy), and the second one after a few runs on the dyno.

I saw that Honda glued the parts of the air filter case and intake rubbers together. But looking careful to it, I see that it's not air tight any more. Does someone have any sugestions what to use to glue these parts together again ?

Jensen

[supersports400]

no one ?

[camelman]

I haven't pulled the intake hose off yet, but anything you do to drastically effect the breathing of the engine will also have an effect on the fuel mixture, so big changes will require new jets.  I had the cap off my airbox, and it made the engine run VERY lean.  If you are going to the trouble of rejetting, then I'd suggest pulling the top off the airbox too.  The other areas of the intake definitely leak.  Sealing them up will help airflow, but it isn't going to create a new engine.  I am considering a custom intake with straight runs and a less restrictive intake plenum for my bike.  I plan to run a standard auto air filter in it... but that is just a dream for now.

One word of warning though.  If you drive the bike in the rain after removing the airbox top and the intake snorkel, then you will not have as much protection from the elements for the intake.

Also, I have a 2002 VW Jetta 1.9L Diesel.  The intake on my diesel isn't much larger than the air hose on the 400F (it can actually be less whenever the snow screen gets clogged), and it makes much more power than these engines.  In fact, the VW diesels from that era have an issue with clogging the intake, and it was down to about 2cm diameter of breathing room at the beginning of the intake manifold before I cleaned the intake.  It still drove fine like that, and made plenty of power, although output was probably down to 70HP from the original 90HP.

Camelman

[Deltarider]

First of all, when dismantling the intake side of the engine, I saw that the tube before the air filter that let's the air into the air filter has a small diameter. I compared the diameter with the diameter on the CB72 an actually it has almost the same diameter. The CB72 is a 250 cc twin engine, revs to 10.000 rpm and has a square bore / stroke ratio. From these points of view I suggest that the area of the air filter, including the air intake hose on the CB400F is way to small. I'm I right ?

I doubt it. I would start with identifying your specific model. There might have been models tuned down to 27HP for the German market. As far as I know those were never on the Dutch market, if they were there at all(!) I would start by bringing the bike back to its original state first.
Do not conclude hastily there's something wrong with your air intake. My first question - years ago - in this forum was: how can the CB500 Four models (meant for Germany, Holland, Switzerland, Italy and some others) that were equiped with a snorkel over the airfilterbox and #78 main jets perform equally good as the models for other markets that didn't have the snorkel and had #100 main jets? That question remains still to be answered.

I saw that Honda glued the parts of the air filter case and intake rubbers together. But looking careful to it, I see that it's not air tight any more. Does someone have any sugestions what to use to glue these parts together again ?

I don't know why Honda glued them together. Is it really glue or just silicons? Mine are not glued together (anymore) and seam airtight nevertheless. From the carbs to the head it should be airtight ofcourse. I don't think that for from the airbox to the carbs it's that critical, but others may chime in.
You may find my proposal for an experiment in http://forums.sohc4.net/index.php?topic=47771.0 interesting.

[supersports400]

Everythings is as should be, and it's an original dutch machine. Look at the pictures of the exhuast ports before cleaning, I thinks that and a few other reasons where the power restrictions.

Dutch and vacuum cleaners, what else is new ? I use a vacuum cleaner for checking the relative flow resistance.
I have a simple differential pressure sensor to measure the difference between two chosen points. But I also build in a electronic tachometer to measure the rotor rpm. It turns out that this is highly accurate, by measuring small, very small differences in air resistance.

But I only use it as a relative measurement, so to see if all ports are having the same flow, but says not so much about the ideal flow. Remember, air is air and mixture is air with fluids and fumes, these two behave very different.

I was looking for an alternative for the glue Honda used, turns out that this "glue" is not petrol resistant over the years.

The snorkel is bothering me, the diameter (square area or flux)) is much smaller than the four intake manifolds together.
That means that the air speed is much higher at the snorkel intake then behind the air filter (if the volume in time is the same the air speed must be increased).

Jensen

[Bodi]

The snorkel is bothering me, the diameter (square area or flux) is much smaller than the four intake manifolds together.
That means that the air speed is much higher at the snorkel intake then behind the air filter (if the volume in time is the same the air speed must be increased)"

No.
Each cylinder's full cycle takes 2 revolutions, or 720 degrees. A cylinder's intake stoke takes close to 180 degrees, valve timing may make it actually a bit less or more.
The Honda SOHC4s use symmetrical firing of the four cylinders: only one cylinder of four is on intake stroke at any time.
The snorkel area must be compared to the area of only one carb throat or intake runner.

[supersports400]

Thanks Bodi,

You're right, I didn't count that in. A CB72 has per cylinder (2) the same intake area as the CB400F in total.
When you compare the diameter of the snorkel to other engines I know, it seems just so small.

One down, one to go, and that's the question about the glue that holds all the parts together. The glue that Honda uses is brown and somewhat clear. I can try glue from a heat gun, anybody experiences with that ? If it doesn't work, heat it again and remove it (that's my thought).

Jensen

[supersports400]

Hi,

Found out that black silicone kit from wurth is resistant to petrol, and can "glue" rubber to plastic, so I'll try this to seal the intake rubbers to the air filter box. There are also a few other possibility's, but I'll try that after I tried wurth. Will keep the readers informed.

Jensen

[supersports400]

@ Bodi and others,

I did same research on the physics around air filter boxes, snorkels and inlet manifolds diameters.
As long the flow through the snorkel (air inlet of the air filter box) is laminar, it makes sense what you saying.

Honda did engineer the snorkel as IF the flow is laminar, but that is not the case. Later study and measurements where pointing out that the air flow is in fact in most cases turbulent instead of laminar.

In the case of the Honda CB400F snorkel, it's best to remove the gauze from the inlet intake, these small holes produce even more turbulence. It's not harming the engine when something falls into it, it's before the air filter.

The angle in the snorkel is not ideal either, but is necessary because of it's place (rear fender). It's not wise to remove the snorkel completely (without extensive re-jetting).

In more modern engines, the air intake area is much bigger in relation to the manifold, and the square area of the air filters are much bigger too.

Jensen

[Deltarider]

I'm interested in your working order when you install it all. I myself never bothered to glue couplers and plenum anymore. It's much easier now to remove and install these parts, since I can push the couplers partly inside the plenum, creating more room to remove the assembly. I have never noticed air was sucked in where the couplers fit in the plenum. I tested this first with cigar smoke and later by spraying brakecleaner. No change in RPM. My impression is: it's pretty airtight. But... this was at idle. Maybe other members have more experience.

In the case of the Honda CB400F snorkel, it's best to remove the gauze from the inlet intake, these small holes produce even more turbulence.

Ahaaa, very interesting!!! This could explain why the CB500 Four models (meant for Germany, Holland, Switzerland, Italy and some others) that were equiped with a snorkel over the airfilterbox (without a gauze) and #78 main jets perform equally good as the models for other markets that didn't have the snorkel but did have that gauze. As remarked years ago: obviously there is more than just bigger orifices and opener pods. I value your scientific approach very much, Jensen. Keep us informed.

[supersports400]

Hi Deltarider,

The scientific approach has something to do with my job in daily life,

Jensen

[Deltarider]

The scientific approach has something to do with my job in daily life

I wonder what that is.
I had to study basics about airflow when I was flying parapentes over the Alps. My other hobby is sailing. Isn't it funny that up to this day in magazines they're still discussing what exactly takes place between jib and mainsail.

[supersports400]

Deltarider,

Don't start laughing, I did study physics, so I'm a physician. Before that I did the LIS (School for precision technology - De Leidse Instrumentmakers School), short-cut :http://www.lis-mbo.nl/.

So I'm a physician with a practical attitude and I love small components, maybe that's why I like the CB400F.
For the last 11 years I designed turn-key vision systems for the bear and beverage industry, as I'm a specialist in optics and mechanical (precision) technology.

There's no easier way to explain,

jensen

[Deltarider]

Well, you choose right, the 400 is the cleverest of the Fours. Soichiro Honda has stated that the CB350F was 'the finest and smoothiest Honda had made'. I suppose he said this before the 400 was launched. 400's are still highly praised in the UK.

[supersports400]

Hi Deltarider

Out of interest, what are you doing for a living ?

Jensen

[Deltarider]

Studied Dutch language and literature. After finishing university I had all kinds of jobs... as long as they offered me lots of free time to travel the world: teacher at high school, technician at a theatre. Nowadays I write articles, do textediting and teach others how to write.

[supersports400]

Now, hat's something completely different. You wrote earlier you lived around or in Naarden ?
Do you ride a lot on your CB ?, I see from time to time people driving there sohc's, but not many anymore.

Jensen

[Deltarider]

Actually I was born in the Jan Steenlaan, but our family moved when I was six months old. My father worked at the Naarden municipality (gemeentehuis). I live in Asd now. I ride when the weather is nice and on holidays. Have to confess I like bicycle riding too. Simple, no helmet, no extra clothes, costs nothing, in town much quicker and out direct contact with nature. Good for me, good for the planet. From a mechanical point of view I admire a bicycle much more than any product coming from the automotive industry and the Dutch make the best (see avatar). So little force needed to create such great effect. In Zimbabwe I once did 200 kms in a day. Compare any simple pedalbike with all these furnaces on wheels that cannot even convert more than 25% of the fuel's energy into motion. In fact, it's even worse: it takes 1,4 liter of fuel to produce 1 liter at the gazstation. God help us when the Chinese abandon their bicycles and start to adopt the american way of life...

[supersports400]

Hi,

Yes bicycling is nice, I do it everyday from and to work.
But often I doo have to go to customers, and then I take the motorcycle.
I like to be on time, and with these traffic jams today yhis is only possible with a motorcycle.

Funny to talk about Naarden, on a international forum, in English.

Jensen

[supersports400]

Hi,

I dismantled my front fork, bearings etc. Pictured is the lower bearing race (yes, it came out like this), any ideas what happened ? There's absolutely no damage on other parts (exept the balls)  like the steering stem, everything is straight and good.  Did Honda mount them this way ? 

The steering head bearings should be replaced, now I know why the bike did steer a bit funny.

Tapered bearings will be mounted (of course),

Jensen

[camelman]

Looks like a stress crack started either from a manufacturing defect, or at some point in the life of the bike.  Any nick in the race, maybe a steering tube that was slightly out of round, etc, could have caused this to happen.  Heck, a failing ball could have put a nick in the inside of the race.  As soon as you have a nick in a hardened material like that, the vibrations of 65000 miles will cause a crack to propagate through the rest of the material.  It looks like a pretty clean break, so that is my guess.
Also, if the bearings ever came loose, then one of them could have loaded up independent of the others, and overcome the yield strength of the bearing race causing the crack.  It looks like there has been some independent wear from the individual ball bearings, so that could be a likely culprit.

I doubt you have to worry much about an impact causing that kind of damage.  If the bearings are set anywhere near correctly, then the rest of the bike would show significant damage from whatever accident would have been severe enough to break that bearing race.

In other words, consider it expected wear and tear, and enjoy the new tapered bearings... they make one helluva difference.

Camelman

[supersports400]

Hi camelman,

Your explanation seems plausible, because I'm almost sure (99.9%) that the bike never was involved in an accident. I bought this bike from the first owner, and I believe him saying that the bike (and he) never had an accident. Until now, everything what he told me about the bike and history was true.
I am interested in the fact of this happened more often, where these races prone to braking because of the heat treatment (too hard ?).

Jensen

[supersports400]

Hi,

In my project shop thread (http://forums.sohc4.net/index.php?topic=65233.0) I documented a CB400F oil pump rebuild. Since the oil pump has 1 part number (as a complete unit) I wonder if Honda has new rotor sets for it ? Or after market?. I did a google scan, but found nothing on the www. This is why a I started the idea of rebuilding a pump, but maybe there's a short-cut ?

Any thoughts ?, any ideas ?

Jensen

[supersports400]

No one had any trouble with getting parts for their oil pump ?

[camelman]

Try modifying the 750 oil pump rotor to fit.  You'll get higher pressure and better flow.  Here's the link from TTR400's web page, but I think the link is broken.  http://www.ttr400.com.istemp.com/oil%20pump%20article.pdf.  E-mail him if it doesn't work for you. He'll get back to you with the info.

Camelman