Topic: 1975 Honda CB400f Project - Beginner

[puddle pirate]

Well man, glad you got the carbs sorted.  Best of luck rebuilding the motor.  I'm currently suck at waiting to get the cases soda blasted so i can paint it before re assemble.

[Powderman]

Have you considered vapor blasting instead of paint, much more maintenance free. I know there is some one in the Carolina's doing it.

[gjunkie]

Here's the best picture I could get of the damage. It's directly where the bottom front bolt goes on the frame.



Here's a picture of the case I purchased and where the cracks are in relation to the new case...



Two other issues I'm sure I'll be able to tackle while moving everything over: Low compression, infamous leaks.

Question: while the mechanic was trying to fix the carbs he found it strange that the air screws didn't have o-rings in them. I used brand new rebuild kits for the carbs which came with the air screws, but not rings... should this bike have o-rings to go with the air screw?

[gjunkie]

The Resurrection:

Over the weekend we finally (U)hauled the bike back to my buddy's dad's shop to get started on the organ transplant. The first stage was to remove the motor and remove the head completely. I was amazed at our speed. Granted we had done this once before in January, but it had taken us almost 2 full days to get all these pieces removed. This weekend it took us just an hour and a half. I'm planning on keeping a close look at the clock and report back the overall man-hours required to do a complete engine rebuild... at least for this bike. Hopefully that helps other newbies out there.

So we decided to pause at this point so that I can gather any necessary tools I'll need to diagnose every piece I can, and determine what original parts I'll be keeping and what I'll have to replace.

Now... I know that compression is low on all cylinders. Visually I'm not sure how to tell if the rings are bad. Regardless, I was thinking of replacing them as I'm already inside. However, I called a Honda shop to order OEM parts, but they informed me that Honda no longer makes these rings... How safe is it to go with after market parts for something like this? I haven't done too much research yet, but I also may be able to find OEM rings on eBay, etc.

Some photos:

[gjunkie]

We continued the effort today. First thing we did was remove the pistons and measure them. I bought a $20 digital caliper last week on Amazon, seems to work great. The manual says that the pistons should measure at 51.1mm at the skirt. All four pistons measured between 50.95mm and 50.98mm. The book doesn't mention a range, so I'm wondering at which point the pistons are considered too worn out. To me this seems like a big drop in width, but I really have no idea.

Before we tried to start opening the the case to begin the transfer we decided to clean the new one as well as we could. I think it's looking as good as it can without having it done professionally.

We then began trying to open the case. We got all the bolts from the top and bottom removed. Took off all covers, etc. We got the two halves loose, but were not able to take off the top half. The manual is a bit cryptic as to which things you absolutely have to remove before attempting to open it, but from what we gathered we have to at least remove the clutch and transmission assembly. Is this correct? I did a little research here and found that the right way to do it is to flip the motor over and remove the bottom side first due to the cam chain tensioner and other chains, etc. But still, it wasn't budging. Must the clutch assembly come off completely first? Like I said, we're trying to follow the manual as close as possible, but the drawings aren't exact and it is a bit hard to follow. Any advice is appreciated! Thanks!

[straat-toe]

Hey GJ. Take it all off man.  It's a #$%* if you dont. Just be careful of the Shift stopper on the clutch. I Broke mine, or it was already broken by the PO when I split my cases, not sure but it is .  Best of luck to you.

[kajtek]

Hey, when splitting the cases, make sure u pull out the primary shaft, cant remember exactly how it comes out (big circlip i think??) but unless u take the shaft out the primary chain holds the cases together. Also make sure u undo the 2 m6 bolts that are normally hidden by the sump. Easy to miss. mine tool a few whacks with a rubber mallet to get seperated. Good luck with the rebuild! Ive read that piston skirts can wear a lot on these bikes, and can knock as they move around in the barrels.

Ashley

[gjunkie]

Straat-toe: thanks for the tip and graphic. I'll keep an eye out for it.

Ashley: This makes perfect sense. I suspected this, but like I said I was having a hard time deciphering the manual. I definitely have all the bolts out and the case already split. It's just held together by the primary chain. Thanks for your response!! This helps a lot!!

[gjunkie]

We've hit a bit of a brick wall. We have been unable to remove the alternator rotor. The book simply says to "Remove alternator rotor with a puller." Makes no mention of size, etc. I found another post on this forum mentioning 18mm. We called a local shop to see if they had them, they don't, but mentioned we might be able to use the rear axle. It's unfortunately 17mm and just shy of fitting perfectly, confirming that we need an 18mm.

We do have a puller that is far too small, but big enough to push on the crank inside the rotor. However, we have tightened it very very tightly and the rotor does not budge. It's also hard to keep it perfectly straight as it is too small. Is my only option to special order an 18mm puller?

Thanks!!!

[puddle pirate]

You can pull it off with a cheep puller from auto zone after you remove the crank.  Mine was really on there but I used PB blaster and a propane torch to finally break it free with the flywheel puller I rented.

[gjunkie]

You can pull it off with a cheep puller from auto zone after you remove the crank.  Mine was really on there but I used PB blaster and a propane torch to finally break it free with the flywheel puller I rented.

AFTER I remove the crank? Don't I have to remove this to remove the crank?

[puddle pirate]

Yes you can take the rotor off after you split the cases and pull the crank.  The metal shroud on the left side of the engine that covers most of the rotor is no longer in the way.  You have to take the crank out in order to plastiguage the bearings anyways ( assuming you are going to do that which i would if you have it split)

this was taken about 5 mins before i pulled the main drive c clip and pulled the drive out and split the cases.  You can still see my incredibly rusty rotor in the top of the picture.  I had a B**** of a time getting those two parts separated.  I PB blasted the back side of the rotor and then hit it with a propane torch several times and then backed the bolt out about 1/8th of an inch and put the 3 arm crank puller i borrowed/rented from Autozone, tightened it down and finally it separated.  I don't have any pictures of the crank but the part that goes into the rotor is tapered.

JT

[puddle pirate]

actually if all your doing is swapping cases there is no reason to pull it off that i can think if your not replacing all the rubber lower bushing parts, which i would do.  When you put that crank into the new cases make sure you plastiguage and double check all the bearing journals and if you put antiseize on the bolts make sure you adjust your torque specs accordingly.
JT

[gjunkie]

Yes you can take the rotor off after you split the cases and pull the crank. 

Wow ok, thanks for clarifying and for the photo... Yeah that rotor looks pretty beat up...

Well, I have bit the bullet and decided to put this in the hands of a professional mechanic. We went to Steve Bignaski near LAX. He's a friend of a friend. I was feeling a little overwhelmed by the fact that once we actually got it open and started the transfer, impurities could be left in the new case, I could miss something and pay the price later, etc. We had also spent three weekends trying to open it to no avail... our confidence took a definite hit.

He's deep cleaned not only the new case, but also the cylinder head and everything else. It's looking brand new. As we already had the motor in pieces, it's saving him time and me money.

This should be done this week, so I could be potentially riding by the weekend. Took these shots today.

[puddle pirate]

Those cases look so much better, I wish i had a parts washer.  If you have not I would advise you to go ahead and buy a CB400f Engine Oil Seal Kit to replace all the rubber pieces while you have it torn apart like that.  If you dont have one i'll try and look up the part numbers later and message you with them.  All in all looking really good.  I got stuck on the rings with my rebuild because i broke the last one putting it back on the piston after verifying that it was within specs, so new rings for me i guess i guess.

JT

[gjunkie]

All good Pirate. I had new sets of everything. It's almost fully assembled. He's gonna be taking a look at the float levels as well, make sure everything is right. He cleaned up the pistons a bit, as well as the plugs. I bought myself a set of new bolts to have them all match and toss the worn out ones, you can see them on the clutch cover. Side note, the cam chain tensioner was snapped, add it to the list of things that prevented this bike from running properly. That's all taken care of now. Photos of the progress:

[gjunkie]

Last night we put it back together and started it up. It sounds great! It took a few tries to start it up, but I'm sure that's just because A: it was a dry motor with new oil. B: Now that compression is at about 145psi on all of cylinders (up from 100-110), and everything else is set right, the carbs I'm sure are WAY off. I'll be taking it for a test ride around the block tomorrow morning, and if all sounds good I'm gonna take it back to the mechanic for a full tune up now that it's all back together.

The only scary moment we had was that I again saw a small leak coming from the bottom. It was coming from the bolt where the original case had cracked. However, when we checked it, this bolt was a little loose. We're not completely sure what the purpose of the bolt is... almost looks like a drain plug. We tightened it, will test again tomorrow to see if the leak has stopped. But no other leaks from the cylinder head, etc, which makes me VERY happy.

[neevo]

Awesome build bud! Thanks for the message via Twitter, will be watching your progress with interest.

[gjunkie]

Today we finished tightening what we had left and took it for a couple of short spins around the parking lot. This thing is running beautifully! It's obvious that its in need of some carb sync love and air screw adjustment, but that's fine.

Also.... NO LEAKS!!!! Well, one tiny one really. After running it for about 20 min we saw 2 small drops. I'll take it!!

A few things left to adjust: rear break pedal is positioned correctly but has very little resistance. Obviously something changed when we loosened the axel to move the wheel and take the chain off. Also, it seems a bit hard to shift. And lastly there's a rattle around the chain/axel area. Not sure what that is. Won't be riding today obviously, I'll be taking it to my mechanic to tweak it.

[neevo]

And what was the speed reached???

[gjunkie]

And what was the speed reached???

Ha nothing over 35mph. It's a small parking lot with speed bumps. Plus I didn't wanna push it knowing its not totally safe.

[gjunkie]

Success!! I just picked up the bike from the mechanic. He tuned everything up. Rode it home, about a 45 min ride, and zero leaks from what I could tell. Also got it up to 60mph no prob!

The idle started off a little high, around 1800-2000, but after riding it for a bit it settled down to about 1400.

I've been told that these older air-cooled bikes should be left to cool off after an hour or two of riding due to the metals expanding, etc... is this true? My mechanic tells me no. A perfectly tuned bike with no leaks, etc, should be able to keep on going. I was getting a little freaked at red lights towards the end of the ride.

Anyway, here she is, FINALLY, in my garage (that's not my Prius). I can go downstairs at any time and just look at her... crazy right?

[SOHC4 Cafe Racer Fan]

As Hondaman can attest, you an go cross-country on these old bikes. You are not limited to an hour or two of riding. Who told you that???

[gjunkie]

As Hondaman can attest, you an go cross-country on these old bikes. You are not limited to an hour or two of riding. Who told you that???

I honestly don't remember. But thank you for confirming that. Sitting at long red lights was freaking me out!

[Lyle]

I'm late to the party on this, but I have just finished reading this entire thread.   As a guy who is capable of doing brakes, plugs and oil filters...I'm very impressed.  Great work and hope you enjoy the fruits of your labor.