Topic: Need diagram/manual for KeiHin PD 42A carbs for '78 CB750F3 Super Sport

[dboblet]

My shop manual doesn't show the PD42A style carb used on the '78 CB750F.  I really need have the maintenance/repair/schematic on this carb similar to what the shop manual shows for the earlier carbs (or later depending on the manual). 

I also have a question about the choke spring.  It seems there is a problem with the spring that helps close the choke valves... simply put, the return spring works well but when you open 'close' the choke by pulling on the control, the butterfly valves stay put.... the spring isn't hooked up corectly or is broken... I'll split the cab set apart and take a look but I'm not sure how it was supposed to be in the first place. 

Also, on e-bay I can find lots of rebuild kits for 'other carbs' and 'earlier' F1 carbs but none for the F3 unless I pay through the nose for an individual carb rebild kit.  Seems like I ought to  be able to get a set of 4 for around $50 US but can't locate one.  All I really need (all parts are in great shape and clean up well) are the bowl o-rings.... but it appears I must buy the whole kit for these odly shaped beasties.

Thanks in advance.

[upperlake04]

Parts diagram at CMNSL - http://www.cmsnl.com/honda-cb750f-750-super-sport-78-us_model7240/partslist/E++2001.html
 
Carb bowl gasket at CSI - http://www.siriusconinc.com/carb/parts.asp#

[eurban]

I have found two links to help you. 
First is the Honda 750 k8/f3 manual in JPG form (even though the link says 550):
http://community.webshots.com/user/cb550f
Second is the Honda 750 k8/f3 manual in PDF format that you can download.  Big file:
http://www.mediafire.com/?5ttzdg9yyuz

I searched the forum using "K8 manual" as the keywords BTW.

Enjoy

[dboblet]

Thanks !!!  The photo version was pretty good and had all I needed.  I'm also downloading the pdf version, hope it's better than the pdf version I have already.  Wish I had the ability to print it double sided, but as grapics, the laserjet I have access to is almost a minute per side....  And I priced having it printed from the file at a print shop, $28us.. I'll live with the computer version and print out the sections I need as I need them.  Thanks again sooooo much.

[Wayne Meuir]

I have an additional related problem.  The diagram in the link above was just the carbs.  They are simple enough.  What I need is an exploded diagram of the linkage and fittings that connect all them together.

Does that info exist anywhere?

Thanks,

Wayne

[scondon]

I have an additional related problem.  The diagram in the link above was just the carbs.  They are simple enough.  What I need is an exploded diagram of the linkage and fittings that connect all them together.

Does that info exist anywhere?

Thanks,

Wayne

   Bookmark cmsnl.com  ,it has every diagram for your bike with part #'s and cross-reference listings. Indispensable     Here's your linkage page...

http://www.cmsnl.com/honda-cb750f-750-super-sport-78-us_model7240/partslist/E++19.html

[Wayne Meuir]

Scondon, thanks, those pages will help a lot.  I have one of the carbs soaking in carb cleaner now.  I am taking them apart one at a time because I am so unfamiliar with this particular carb.  I must say I don't like it a hell of a lot.  I am considering buying a set of earlier carbs to put on my bike that has  real pilot jets, real jet needles and no accel pump but would like a opinion of someone who knows whether thats a good idea or not.

I do have one more question, is the fuel valve seat replacable, or do you just change the needle,  I can't figure out how to get the seat out, or even if it comes out.

It would sure be nice to have a rebuild manual for these.  My Clymer does not even mention this particular carb even though it says it is for a 78 CB750.

Wayne

[jtb]

If the seats come out, I haven't been able to figure out how.  I don't think they do.

[scondon]

 These are the better carbs in my opinion. The accelerator pump makes up for the press in jets ten-fold and your needles can either be replaced by the 77 750F needles(same needles but with clip positions) or, as I do, shim the needles you have with well machined small washers. If your rebuilding with the stock engine and airbox then the stock jetting should work fine and your proper cleaning should be enough.

  To be honest, I don't know about the fuel valve seats. I've just replaced the float needles and that solved the problem with drippy carbs, although the aftermarket ones I've used can take a while before they seat properly.

  Take care to keep track of the small o-ring that sits in the mating surface of the #2 float bowl. Don't know how many times I forgot it was there and assembled without it just to have my accel pump squirt gas out the back of #2 bowl

[scondon]

If the seats come out, I haven't been able to figure out how.  I don't think they do.

   There ya go. If John couldn't do it then it can't be done unless eurban pops through here, he knows way more than I care to learn about these carbs

[Wayne Meuir]

Thanks, guys, I think I will just finish cleaning the stock carbs and see how well it will run.(That, of course, assumes I can figure out how to get all 400 pieces of the linkage back in the right place )

It looks like Parts-N-More has gasket/o-ring kits with the float needle for $10 each, so that's fair enough.  They also have an accelerator pump diaphram for $16. Should I just go ahead and replace that as well, or how do I tell if the one in it now is good or bad (what shoul I be looking for)?
Is there other parts that that I should replace while I am at it?

Bike is stock as far as I know except some kind of 4/1 header.  Stock air box with what appears to be a new EMGO filter.  Should I change it to a K&N, and if I do, will I have to go up a size on the mains?

Wayne

[78CB750CAFE]

WARNING! In the K8/F3 manual the exploded drawing of the carb linkage assembly is wrong/misleading. I thought it looked different when I tore mine down, but when it came time to put them back together I tried to do it as the diagram showed. It didn't work.... the little divots where the the linkage gets the pivots screwed in did NOT come anywhere near lining up for carb 2. Look at the pictures and compare them to the line drawing and you will see what I mean.

Later this week when I get mine back together I will take some pics for your reference if you want.

Drew

[scondon]

Bike is stock as far as I know except some kind of 4/1 header.  Stock air box with what appears to be a new EMGO filter.  Should I change it to a K&N, and if I do, will I have to go up a size on the mains?

  I would think a small shim under the needle would be the only change, if any, for using a K&N filter. The '78 750F ran a tad on the lean side which was OK for some nice acceleration when the throttle is twisted and the accel pump dumps some gas in the mix. The K&N might allow a bit more air into the mix in the mid-range at steady throttle.

   I would stick with the Emgo until the bike is up and running and you can see how well everything is tuned. If you can run with the Emgo for a while without getting white plugs then try a K&N, they last forever

It looks like Parts-N-More has gasket/o-ring kits with the float needle for $10 each, so that's fair enough.  They also have an accelerator pump diaphram for $16. Should I just go ahead and replace that as well, or how do I tell if the one in it now is good or bad (what shoul I be looking for)?
Is there other parts that that I should replace while I am at it

   That Partsnmore kit looks pretty decent and is definitely priced right. Make sure it's the 77-78 pack(I'm sure you did, just stating the obvious). They appear to be out of stock at the moment so maybe email them to make sure they still are offering it.

    To be honest, I don't go much further than removing jets, idle screws and slides and dumping the rack in cleaner. Separating the carbs from each other can cause the old o-rings on the hard fuel lines between carbs to start leaking cause they're old and cranky. This is the only part you haven't covered yet and you may not need new o-rings, but test by adding fuel to the carbs before putting them on the bike. If fuel doesn't come seeping out somewhere after five minutes then your good.

    Usually I end up swapping the float needles between carbs until I get the right combination that doesn't overflow, and I hate to find this out after putting a freshly rebuilt set on the bike

[Wayne Meuir]

Keep the info coming guys and I might have an outside chance of getting these things back together.  I do have another question tho:  There are two screwed in fittings in the float bowl.  The one that is pointing straight up I assume is the Idle Mixture Screw and it has a tiny needle end and there is a spring on it and no o-ring.  The other is sorta shaped like a slide stop screw on a Round slide Mikuni, sort of a round nosed pin. It has a rubber O-ring on it and was just screwed down tight in its fitting in the float bowl.  What is the function of that screw?
I think I will try to order the Parts-N-More Kits if I can get them.  I destroyed two of the float bowl o-rings getting them out, they were in pretty bad shape already and then they were stuck to the bowl really bad.  I had to scrap the residue out of the groove in the bowl after I got the pieces of the o-ring out.

Any other advice, I am listening!

Wayne

[scondon]

   The one that is pointing straight up I assume is the Idle Mixture Screw and it has a tiny needle end and there is a spring on it and no o-ring. 

 It is the IMS and is actually outside the float bowl on the carb body. The needle and spring come out easy but there should also be a washer and o-ring down in there. With the needle pointy side up the spring goes on first, then washer, and finally o-ring. All of these parts should be removed along with the press-in idle jets in order to properly clean the idle circuit which is the most problematic area of these carbs.

The other is sorta shaped like a slide stop screw on a Round slide Mikuni, sort of a round nosed pin. It has a rubber O-ring on it and was just screwed down tight in its fitting in the float bowl.  What is the function of that screw?

   If this is the one on the bottom of each bowl, facing sideways, then it's the float bowl drain screw and allows gas out the drain nipples at the bottom of the bowl. Nothing special to note about it.

[Wayne Meuir]

OOPS, I didn't know about the washer and o-ring on the IMS, and I soaked the carb body in Carb cleaner which probably ate the 0-ring, I don't know where the washer is!

Are the two press in idle jets the two little brass tubes in the throat of the carb that point toward the rear of the bike?  If so, how big a PITA is it to get them out and how well do they stay put when you replace them?

Wayne

[scondon]

 No worries about the o-ring if you get those partsnmore kits. Looks like they come with it. Just keep an eye for those washers when you take your carbs out of the bath. We're talking about #1 and #7 in this diagram, for confirmation

http://www.cmsnl.com/honda-cb750f-750-super-sport-78-us_model7240/partslist/E++2001.html

Are the two press in idle jets the two little brass tubes in the throat of the carb that point toward the rear of the bike?  If so, how big a PITA is it to get them out and how well do they stay put when you replace them?

    Those are the air jets for your Main and Idle circuits. They allow air to pre mix with the gas before it enters the carb throats. No need to pull those, just make sure they are cleaned out.

    The press in idle jets are the #38 jets next to your main jet(#105) in the upper float chamber. The main jet unscrews along with its emulsion tube. The idle jet has no threads or screwdriver slot and is press fit into the body. A gentle pull and slight twist with pliers is usually enough to remove them for cleaning. Gently tap them in straight until seated to replace.

[Wayne Meuir]

OK, now I'm really confused.  I think what you are calling the #38 idle jet is what I normally refer to as the pilot jet, only on this carb, it is pressed in and you can just pull it out then clean and re-install it.  Is that correct or am I really screwed up? 

I already pulled the main jet and the emulsion tube and cleaned them, so I'm OK there.

I also understand about the o-ring and washer on the IMS but you are referring to two washers in your description (#1 & #7)  I looked at the diagram and there are #1's and #7's all over that page, so I don't know what you are referring to.  Where is the second washer that I need to locate.Please straighten me out. 

I expect the washers are gone because I did not know they were there and I first soaked the carb bodies in carb cleaner dip, then sprayed carb cleaner and compressed air through all the orifices, so they are probably in never never land somewhere.

Sorry for being a little thick here, but right now I'm really appreciating the Mikuni carbs on my other bikes.


Edit:  Good news is that I found the washers.  Bad news is that the cheapest carb kits I can find are original Honda parts at $70 and all they include is the o-rings and bowl gasket.  The needles for the fuel valves, also original Honda parts, are another $70.  Siruis Consolidated in Canada gets $46 per carb for their Keyster Kits. Part-N-More said it is expecting more of their kits in about a month.  I really did not plan on having the bike down that long but if I were sure they would have them by then, I would wait.  Anyone else know of a source.  All I need is the bowl gaskets, o-rings, and fuel needle.

Also, I pulled the slow jet(Pilot Jet, idle jet, or what ever they are called) and they have the number 35 on them instead of 38.  Were all of the 38's from the factory?  Am I likely to be OK with 35"s?

Wayne

[scondon]

    Sorry if I got ya worried Wayne. It's been a while and some of the #'s have changed in my head

       Idle jet=pilot jet and it is #35 not #38 as I remembered. I get used to calling it the idle jet when talking to people who call it that. The correct term is pilot jet as you know.

    There are not two washers even though I gave two numbers. The two numbers were just to show the general area where the washer is because it is not listed separately.

    It was late and I was generalizing


    The Sirius kits are good and include everything for a full carb rebuild if I'm not mistaken. Jet needles and needle jets among other things. A bit of coin, but if your carbs are worn out it's a good deal. I tend to go with the Parts Unlimited type kits unless there is something obviously wrong when running the bike. They are cheaper and I like that

[Wayne Meuir]

Scondon, thanks, I think I finally got it.  I am trying to find some cheap kits without waiting a month.  All I need is the bowl gasket and o-rings.  I may also need fuel needles, but I think I will take your suggestion first and see if they leak.  Since I never had gas in the carbs yet, I don't know if they are working or not, however, by looking at the inside of the carbs, I would have to say that it has not been very long since someone cleaned them up.  No2 had a lot of gummy old gas residue and the slide was stuck, and the pump was gummed up,  but the other three look pretty clean and all the jets were fairly clean.

I'm still worried a little about getting all the linkage back together correctly, but I put everything in baggies and sorta labeled how it was supposed to go.

Wayne

[scondon]

I'm still worried a little about getting all the linkage back together correctly, but I put everything in baggies and sorta labeled how it was supposed to go.

   Exactly the reason I haven't attempted a full disassembly. My memory lasts about a week

   There are lots of people on this site with experience on your carbs. Someone even posted a nice pictorial of the process and posted it here. If I find it I'll link to it. Otherwise, just keep posting if you get stuck and someone will have an answer or a pic to help

[Wayne Meuir]

I ordered the bowl gaskets and o-rings from the local dealer for about $15 per carb.  They will be here Friday.  I ordered the Accel pump and fuel needles from Parts-n-More for $24, so I should be in business pretty quickly.  I am going to put everything back together as far as possible tomorrow, maybe I can remember how it all goes if I don't wait too long.

Wayne

[jtb]

Wayne the linkage is pretty simple, except for the spring that attaches to both choke shafts.  I'll probably have to pull my carbs again after this weekend, (can't stop the accel pump from leaking).  I'll take some pics of that spring when I do.

John

[Wayne Meuir]

John, that would help a lot.  I looked that over pretty good before I took it apart, but it seems to me to be more complicated than it really needed to be.  Pics would be good.

Wayne

[Wayne Meuir]

John,  I really need to see the pic of the choke spring set up if you can get a good close up.  I screwed with that for an hour today and I can't figure out how it goes back.

Another question:  The throttel slide return spring that fits on the #3 carb and fastens to the slide actuator rod: How much tension is it supposed to have?  I hooked it up by just twisting the spring the minimum amount to get it hooked (least amount of tension possible) and it sure feels stout to me, I'm thinking how tough that is going to be on the wrist.

Also, where the felt packing fits around the choke shafts, I can see daylight around those shafts.  I took the packing out and looked at it, it looks OK but I guess it is just worn out.  Is the air leakage there critical or does it not even matter.  If its critical, what is the fix?

Wayne