CB400F Engine Rebuild

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My manual is at the machine shop while they check my head and cylinder so I looked up the float height on SOHC Fours and it said 21 mm. This doesn't look right.




You are holding your scale wrong, turn it 90 degrees (across the floats). Hold it over the main jet and slide it that way. :) + turn the carb 180 degrees so the float is hanging down on an angle. The float should be just resting on the little needle inside the needle, not compressing it. :)

Hope this helps
 
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bmcdonau
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I finally have all the parts to put the bottom end back together.

The first thing I did was rebuild the starter clutch and replace the rubber cush drive. I used new springs, rollers and caps.



I also was able to locate a new needle bearing and decided as long as I had it apart I might as well replace that too.



I also found a NOS neutral switch, the original worked, but the body was chipped.



Here you can see the new main bearing shells with engine lube on them ready for the cranshaft. I put the transmission and primary into the upside down top half of the cases, then dropped the crankshaft on to the top half.



Then I installed the cam chain tensioner.



Then I did a dry run by setting the bottom case on the top half and inserted the primary shaft. Everything looked pretty good.

 
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bmcdonau
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My manual is at the machine shop while they check my head and cylinder so I looked up the float height on SOHC Fours and it said 21 mm. This doesn't look right.




You are holding your scale wrong, turn it 90 degrees (across the floats). Hold it over the main jet and slide it that way. :) + turn the carb 180 degrees so the float is hanging down on an angle. The float should be just resting on the little needle inside the needle, not compressing it. :)

Hope this helps
Don't really understand how you could measure anything by turnin the tool 90 degrees, then the measurements on the tool w?uldn't line up with the line of movement of the floats would it? If you look right above the word "Pit" you can see that the tang on the float is just touching the end of the float needle.
 
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Dennis, I see what you did there. Looks like the float is "off" by a ~50mm. That's a lot.

Good on for ya taking on a basically thankless task. You must love the doing because the having will never pay ya back (financially). :D

Brad Mobely in Dallas had a CB-400F, all original and pristine, setting in his living room. That was in 94, tho, don't know if he still has it.
 
Joined
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'07STA
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7235
Don't really understand how you could measure anything by turnin the tool 90 degrees, then the measurements on the tool w?uldn't line up with the line of movement of the floats would it? If you look right above the word "Pit" you can see that the tang on the float is just touching the end of the float needle.
:D
I modified your picture. In your picture the needle is at the bottom on the float. Turn the carb so the needle is at the top with the float hanging from the pin (my picture). Angle the carb back so the float rests on the pin of the needle, but not to far as to compress the spring in the pin. Lay your ruler cross the 2 floats directly over the main jet and then lower your ruler indicator.:D

I did some searching on the net. Here is demo

Enjoy, gotta love playing in gas
 

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bmcdonau
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Dennis, I see what you did there. Looks like the float is "off" by a ~50mm. That's a lot.

Good on for ya taking on a basically thankless task. You must love the doing because the having will never pay ya back (financially). :D

Brad Mobely in Dallas had a CB-400F, all original and pristine, setting in his living room. That was in 94, tho, don't know if he still has it.
George, they are at about 26mm which is still pretty far off. If you push the float up until the float needle plunger is fully seated it's about 21 mm. I think the PO adjusted the float for 21 mm when the needle is fully closed instead of with the float tab just touching the float needle.
 
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bmcdonau
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:D
I modified your picture. In your picture the needle is at the bottom on the float. Turn the carb so the needle is at the top with the float hanging from the pin (my picture). Angle the carb back so the float rests on the pin of the needle, but not to far as to compress the spring in the pin. Lay your ruler cross the 2 floats directly over the main jet and then lower your ruler indicator.:D

I did some searching on the net. Here is demo

Enjoy, gotta love playing in gas
Blair, I see what you're saying now, I was spinning the gauge around the wrong axis:doh1:. That's an intresting observation, it looks like it'd be close to 21 mm if I did what you suggest. My book is at the machine shop, I'll check it when I get it back. Thanks
 
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bmcdonau
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My float gauge wisn't wide enough to measure across the carb body so that's out.

I finished assembling carb 3 and 4, nothing is torqued or crimped because I still have to disassemble them and plate the parts that were rusty, but they look great.



So then I started on number 1 and 2 and found this:





Another busted float pivot. I don't know how you can break these off when you are working on these carbs. This is fixed pretty well unlike number 4 which was loose. This is the same black plastic stuff that was stuffed in between the fins of the cylinder. I'll leave it as is unless I can find a replacement.

The number 2 carb body has a lot of metal erosion in evidence.



This piece was missing between number 1 and 2. I think I can use brass rod to replace it.



I finished 1 and 2 this afternoon and now I just wait for my plating chemicals to show up and I can finish the job.
 
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After a few taps, this fell out of the cylinder.





It's some kind of plastic putty that was put into the slot in the front of the cylinder where the studs pass through the fins. You can even see the the person's finger prints who put it there. Nothing about it in the manual. A few taps with a small screwdriver and it was out and the cylinder was free.

This was where it was, four of them.

Why would they have blocked those off? Isn't that additional air flow for cooling? The cb400t has similar "ports", so does the 750. Should flow right through the head past the inside cylinders.
 
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bmcdonau
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I finally heard back from the machine shop.

They did an air test and two valves are leaking. Then they disassembled the head and neither valve looks bad so they are going to see if they can lap in the valves and get a seal. He said the valves looked too thin to grind so I'm hoping they don't have to recut seats or replace valves.

They checked the cylinder and verified my measurements--the bores are good, the pistons are in spec and the ring end gap is good. He suggested just a light honing and no new rings. He said they should seat just fine.

Still waiting on the cam chain to put it back together.
 

Bones

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Sounds like good news...looking forward to your continued progress.
 

8-BALL

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F.Y.I.


Is this a CB400F? It shows up at our local bike nites.

4JUN07 (39).JPG



Dave C. :cool:
 
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bmcdonau
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I sure hope I don't have to take this back apart this week.

The first thing I did was turn the top half upside down and transfer the transmission from the old case to the new cases. That was simple and pretty foolproof. Its sorty fiddely because you have engage the shift forks with the shift dogs on both sets of gears. I also replaced the crank, countershaft and transmission seals at that point.



Then I installed the new main bearings into both halves making sure that they went into the right places because they are backwards at that point (since the case is upside down). I coated both sides with assembly lube per the manual.

After that I wrapped the primary chain and cam chain around the crank and dropped the crank into top half by holding on to the primary chain.

Then I installed the cam tensioner rod, springs, and adjustment bolt and nut.



I decided to replace both main bearings (which are in the bottom half) so the first thing I had to was replace the left side bearing. It sits in a machined cavity in the case and is held in place by a large circlip.



I drove the bearing in with a very large socket so I was only bearing on the race. The bearing had been in the freezer for a week so I was hoping it would slide in fairly easily without heating the cases and it did.





Then it's a simple job to insert the circlip.



You have to finish installing the main bearings and primary drive after the cases are assembled so I prepped the cases by cleaning the gasket surfaces with simple green and solvent to degrease them.



Next I put thin coatings of Yamabond on both halves. I put a thin bead on and spread it with a popcicle stick. here's a pic in the middle of the process. The coating ends up really thin and I removed as much as I left. I wiped up any that ended up inside the cases before assembling the halves.



Then I installed the bolts from the bottom and torqued them, then flipped the cases and dropped the top bolts in and torqued them to specs.



Once the top and bottom were back together I could install the primary drive parts inside the primary chain and install the remaining main bearing and primary shaft.

A spacer goes in here, but it has to go through the cavity for the main bearing before you install the bearing.



Like this



Then you install the other main bearing.





And finally insert the primary shaft.



I sure hope I don't have to take this apart and install a dowel in the cases I might have forgotten.
 
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