Topic: CB750 Carburator reassembly question...

[sbparks]

Folks,

 I am in the process of rebuilding a set of carbs from my CB750.  I have the rebuild kits from K&L.

 First question, I am not exactly sure what the metal washer and the 'O' ring in the kit were intended to replace (the parts at the right of my attached picture).  I am guessing they are for the carb vacuum measing port, but I'm not sure.

 Next question... The jet needles... In what position should the clip be placed?  My original ones were set to hang at their highest position (the last clip position counting from the top of the needle).  Is this correct?  Where should I set these to start with?

 I do thank you for your time!

Scott

[BobbyR]

People would need to know the year, the carbs changed at one point.

[Johnie]

Yah, the year would help. And there is a number stamped on the casting of the carbs. Give us that number and we can tell you all sorts of info: float height, needle position, jet sizes and air screw info.

[sbparks]

 Gee, I think I made this mistake before... Sorry!

 A 1974 CB750K4.

 Carb numbers, 5-R74-1 and 5-L74-1... Under the carb bowl covers.

 Thanks for your patience.

Scott

[HondaMan]

Gee, I think I made this mistake before... Sorry!

 A 1974 CB750K4.

 Carb numbers, 5-R74-1 and 5-L74-1... Under the carb bowl covers.

 Thanks for your patience.

Scott

Scott:
The number is on an oval flat spot on the top of the carbs, at their front. They are probably 657B on a '74.

That would make the needle clip position 4th from the top if your main jet is a #105 size, or 3rd from the top if the mains are #110. Air screws should be at 1 turn out from full (gentle) stop.

[Magpie]

Metal washers may be for the screws that are removed when the carbs are being synch'ed. There's one on each carb near the front of it. On carb 1 & 2 it's on the top left, on 3 & 4 on the top right. "O" rings for the float bowl drain screws. I think.
Cliff.

[Magpie]

A picture may help, the circled parts.
Cliff.

[sbparks]

 Thanks again for your patience with me.

 I swear I can't find any other identification on the carb body other than the number I found under (inside) the loat cavity.  I'll check again when it isn't quite so early in the morning!

 The diagram that Magpie attached contains much more detail than the parts diagram I was looking at from the Honda parts Fishe... Thank you.  I need to get a larger version of that too see all of the detail.

 On the two washers I asked about... Yes, I was guessing the Synch ports, but I just wasn't sure.

 Here's my next hurdle... One of the rebuild kits was missing the 'needle jet'.  The replaceable piece that presses into the carb's body.  Bummer.  (No, I didn't lose anything on the floor, I was extremely careful when I opened everything up!).  Now, I gotta get with the company and see what they can do for me!

 Thanks for the help!

Scott

[Johnie]

Hondaman is right you probably have the 657B number carbs as that is on my K4. If you do:

Float height - 26mm
Main jets - #105
Slow jet - #40
Needle clip in #4 position
Air screw 1 turn out

I happen to have my K4 carbs off and below is a pic of where you can locate that number. You can clip the pic to enlarge it.

[sbparks]

Guys,

 Thanks for the information.

Johnie,

 Thanks for the picture.  That's what I needed!

 Also... The number four (4) position... Starting at the top or bottom of the needle?  There are 5 positions on the needle.

 So, help me out here.  So many parts...  The needle I am asking about is the 'Jet Needle', which moves in and out of the 'needle jet', correct?  Under the needle jet is the 'Main Jet Holder'?  The jet holder isn't included in the rebuild kit, so I've cleaned it up.  Then in the jet holder is the Main Jet?  That's what Johnie is saying is #105?  I'll need a magnifying glass to see these things to confirm.

 As I was saying before, one of my rebuild kits conveniently (actually, a big pain iin the ass) forgot to include the needle jet.  Now, I need to find another one of these parts!

 I am ready to start re-assembly, but I'll need to wait for that last part.

 Everyone... Enjoy your Christmas Day!

Scott

[ev0lve]

That would make the needle clip position 4th from the top if your main jet is a #105 size, or 3rd from the top if the mains are #110. Air screws should be at 1 turn out from full (gentle) stop.

Is there a formula for the relationship between the jet size and clip position? Sorry to have to ask but I've never seen it just spelled out like that and I kind of like it  

At any rate I thought the stock clip position was smack in the middle. Is the info in the carb FAQ or did I just miss it in the manual?

Too early, too little coffee... How do you guys know?

Merry Xmas!

As I was saying before, one of my rebuild kits conveniently (actually, a big pain iin the ass) forgot to include the needle jet.  Now, I need to find another one of these parts!

Scott

One of my K&L kits was missing a float valve seat. Kind of a pisser for sure.

[steamnjn23]

as i understand it, the needle position is determined by the factory to work properly with the main jet, idle jet, and location of the country where the bike is sold. 

i think i have a chart somewhere of the needle positions.

[HondaMan]

Guys,

 Thanks for the information.

Johnie,

 Thanks for the picture.  That's what I needed!

 Also... The number four (4) position... Starting at the top or bottom of the needle?  There are 5 positions on the needle.

 So, help me out here.  So many parts...  The needle I am asking about is the 'Jet Needle', which moves in and out of the 'needle jet', correct?  Under the needle jet is the 'Main Jet Holder'?  The jet holder isn't included in the rebuild kit, so I've cleaned it up.  Then in the jet holder is the Main Jet?  That's what Johnie is saying is #105?  I'll need a magnifying glass to see these things to confirm.

 As I was saying before, one of my rebuild kits conveniently (actually, a big pain iin the ass) forgot to include the needle jet.  Now, I need to find another one of these parts!

 I am ready to start re-assembly, but I'll need to wait for that last part.

 Everyone... Enjoy your Christmas Day!

Scott

The "count" starts at the top: #5 is the groove furthest from the top, about 1/4" down the whole needle.

A word to the wise: if the needles do not have the number "27201" engraved on them, they are not Honda's. in nearly 100% of non-Honda kits we've seen installed at SOHC4, this causes enormous problems. I suggest re-using the old needle jets and jet needles: mine didn't wear out until 128,000 miles, and even then all I had to do to striaghten up the air/fuel mix was to lower the needles one notch. Usually the rest of the parts are OK.

Keyster makes most of the aftermarket needles, and they do not have the tapers correct. Lately, they have started selling their needles to other kits we've seen. If the engraving on the needle is not as described above, it's probably a Keyster: they can be a pain in YOUR keister to fight with...

Get some tiny drill bits, or at least stiff wire, and clean the little holes in the mainjet holders (these are also called "emulsifier tubes"). The holes calcify shut partially, and cannot be cleaned by any other method than scaping. You'll see tiny amount of white crust come out of them as you scrape: usually enlarging the upper 8 holes to 0.039" is recommended for smoother performance (and better MPG) with today's gasolines. You can buy this size drill bit and a pin vise at a hobby shop.

[sbparks]

Guys,

 I've been away from my project for a few days, but I am ready to dig back in.  I did confirm that my carb bodies are the Keihin 657B's as Johnnie indicated earlier.

 Here are the specifics of the other parts:

 Original parts: 115 Main Jet, 40 Slow jet, 27201 Jet Needle
 Rebuild (K&L) parts: 110 Main Jet, 40 Slow Jet, D17 Jet Needle

 My original Slow Jets were completely clogged even after a long chem dip.  I plan on replacing the slow jets with the new ones from the rebuild kit (both are number 40's).

 The Jet Needles and the Needle Jets... I read and re-read what HondaMan had to say.  I am leaning towards reusing these original parts after I make sure they are completely renewed and clean.  The original needles are labled '27201' just as he indicated.  The needle parts don't seem to be worn (22K miles on the bike), just dirty.  The K&L needles are labled D17.

 If I reuse the original Jet needles, should I reuse the original needle jets (keep them as a pair)?  Or isn't that a consideration?

 Oh yes, I'll set the needles at the #4 clip position.

 Does this all seem reasonable?  I do appreciate the help!

Scott

[Johnie]

I have also reused the long needles as Hondaman mentions just to keep the OEM in there. As he said they usually present very little wear. I just use MAAS metal polish to clean them up and it is a very easy clean. I then will install new OEM needle seats and float valves. All you need is a little wear or scratch on the needle seat or float valve to leak. You could always leave the originals in there to see if it works. If so you are good to go. If they leak then change them out. Or if you have the new ones already in the kit, change them out and forget about it.

[Ben Woodward]

Hey guys,

I am currently cleaning all 4 cabs on my 1976 CB750 K6 SOHC, and seem to have run into a bit of problem.
Let me first state, this is my first time working on a motorcycle. I have purchased a few manuals and am following them religiously.

On that note, I have made it through 3/4 carbs with no problems, then have been very clean and I have just been checking everything and giving them a nice wipedown.

This morning I pulled the flood bowl off the last carb, and noticed something missing.

Under the floats, there are two small holes that appear to let gas into the next chamber, then into the cylinder (i think). well, on all other carbs, each of these two holes have a very small circle piece of metal, resembling a small washer. However, on this last carb, only one hole has that gold piece, the other is missing.

(link to image below)
http://img31.imageshack.us/img31/127/dsc9500.jpg

Note, in the photo the left hole there is lacking that golden "washer"

I am curious if anyone know where I can find this piece, or even what this piece is. I cannot pinpoint it in the exploded view of the carbs in the manual.

A picture is attached and any help is much appreciated.

Thanks,

[Johnie]

That is the way it was designed. Carbs 2 & 3 have the gold restrictor you mentioned in both holes. Carbs 1 & 4 only have the restrictor in one hole. I just pulled apart a set to confirm.

[Ben Woodward]

Thanks so much for your fast reply, upon reading your post, I went back to the garage and confirmed this for myself and you are correct! Thank you very much.

If anyone is interested, I will be documenting the entire rebuild in photos in a flickr album that can be found here:

http://www.flickr.com/photos/46075014@N04/sets/72157622978240133/

Cheers

[Johnie]

Thanks so much for your fast reply, upon reading your post, I went back to the garage and confirmed this for myself and you are correct! Thank you very much.

If anyone is interested, I will be documenting the entire rebuild in photos in a flickr album that can be found here:

http://www.flickr.com/photos/46075014@N04/sets/72157622978240133/

Cheers

Cool...you even have your mug in there. Looks like a great project.

[sbparks]

Guys,

 I made a little more progress with this part of the project yesterday, but I may have caused a problem.

 I used the Wintergreen Oil, Water and some heat (I heated the mixture) to soften some of the carburator's rubber parts.  I soaked the carb insulators and also the top grommets that seal the top plate where the shaft goes into the carburator itself (please see the picture).

 I was happy with the results of the Wintergreen oil and I even pulled the small gromets out of the water early (after, say 15 minutes).  The grommets 'rested' on the side until this morning when I attempted to put them vack into the top plate. 

 Oh, oh.  Not gonna happen!  My top grommets won't go back into place!

 The gromets have swelled to the pint that they won't go back in.  I even applied a small amount of lubricant.  It didn't work and I didn't try to force things further.

 What next?  I am I out of luck with this set?  I don't see these listed as an available part on any of the parts suppliers (the fiche).  Anyone have a set to sell me?  EBay?

 Darn it.  I thought I had this licked and here I am in a jam!

Scott

 

[jsaab2748]

I'm not where my bike is right now, (Holiday) but if you have a few days, I have some spares. Send me a p.m.

[sbparks]

Jsaab2748,

 Wow.  That's very nice of you to offer that to me.

 I was also looing to see if there were any ways to 'reverse' the swelling that I caused.  I'm not sure if waiting a reasonable period of time would cuase them to lose a little of their swelling or not.

 PM on the way.

Scott

[newday777]

Scott
Are you on here still?
Did you get the grommets fixed/replaced?

Jsaab2748,

 Wow.  That's very nice of you to offer that to me.

 I was also looing to see if there were any ways to 'reverse' the swelling that I caused.  I'm not sure if waiting a reasonable period of time would cuase them to lose a little of their swelling or not.

 PM on the way.

Scott