Topic: Carbs on a 78

[jas750]

Hey everyone,
 Great site.
I've been working on a 1978 CB750k my father gave to me.
I need a set of carbs.

Do they have to be from a 78?
Or would anything from 69-79 work?
I've seen a few different styles on ebay.

Any suggestions?

Thanks

[TwoTired]

Direct replacement  - 77 and 78 K  PD style carbs only.
Same style carb but with different internal jetting are 77-78 F model PD carbs.
Older models are different style, can be made to work the proper adapters and internal jetting.

79  750s are DOHC and have CV carbs, I think.  You can adapt anything if you work hard enough at it.

Cheers,

[BobbyR]

Are the carbs missing, or do they need to be rebuilt? All the parts are available. 77 and 78 carbs have accelerator pumps so they are different.

[DrSummerOff]

Check the forumn above Keihin carbs..I was asking for help in identifying a set of carbs I have.
turns out they are from a 78k model..
Look at the pic and see if these are what u need..

[sandcastcb750]

You can put 69-76 carbs on 78s but you need the manifold rubber to the head from the same year carb

[DrSummerOff]

Are these them?

[BobbyR]

Those appear to be 78 pumper carbs.

[jas750]

Are these them?

Those are the ones!
The carbs are not missing, I do have them. When my dad first put this bike back together
He had alot of problems getting the bike to run well.
Seemed to me it was fuel starved.
He put in a 836 piston kit and a dyna? electric ignition.
I think he forgot to change the jets. Anyone know if a 110 main jet was the stock size for these?

[fuzzybutt]

i think the stock jet might have been a 105, 110 possibly, either way, way too small for an 836 motor.

[sandcastcb750]

I have had almost all the CB750s from 1969 to 1978. Basically, I found the 1977-78 carbs to be the worst and even my neighbor up the street has had little luck with his 1977. My K8 got so bad it only ran with the choke.

To me, the 1971 to 1976 carbs were the best. Easy to clean.

[jas750]

I have had almost all the CB750s from 1969 to 1978. Basically, I found the 1977-78 carbs to be the worst and even my neighbor up the street has had little luck with his 1977. My K8 got so bad it only ran with the choke.

To me, the 1971 to 1976 carbs were the best. Easy to clean.

Is it hard to fit the 71' - 76' ? Or are they pretty close?

[BobbyR]

Just carefully clean them and they are fine. Jet to 120 and with the accel pump you get good jump when you crack open the throttle.

[sandcastcb750]

1969-76 have no accell pumps. Air screw, slow and main jets. #40 slow and #120 main are the jets for 1969s.

You must obtain the rubber head/carb boots to connect. Should be cheap enough to buy. Used okay. Hard rubber yes. Use rubber mallet, heat and WD40 to pound them on.

No choke cable; lever on the carbs.

Duck soup for you.

[eurban]

I have had almost all the CB750s from 1969 to 1978. Basically, I found the 1977-78 carbs to be the worst and even my neighbor up the street has had little luck with his 1977. My K8 got so bad it only ran with the choke.

To me, the 1971 to 1976 carbs were the best. Easy to clean.

Did you ever pull the pressed in pilots and do a proper clean?  Probably not!  Not particularly hard either, but the manuals don't encourage this step..  My 836's performance is excellent with its clean and properly adjusted  77/78 carbs. Jas750, go with the late carbs.

[sandcastcb750]

Still easier to unscrew the K1-6s slow jets. Less parts in the carbs too.

[eurban]

Agreed, greater simplicity.  In my experience, however, lesser ridability and performance.

[TomC]

Hi
     If you want greater simplicity how about some Amal monoblocks?
quote author=eurban link=topic=37634.msg399536#msg399536 date=1218150720]
Agreed, greater simplicity.  In my experience, however, lesser ridability and performance.
[/quote]
          TomC in Ohio

[BobbyR]

Carefully clean your 78 carbs and all will be good. I have done it and it is not hard. If you have to run wit the choke on, you are running very lean.

[I Zombie]

I have had almost all the CB750s from 1969 to 1978. Basically, I found the 1977-78 carbs to be the worst and even my neighbor up the street has had little luck with his 1977. My K8 got so bad it only ran with the choke.

To me, the 1971 to 1976 carbs were the best. Easy to clean.

Did you ever pull the pressed in pilots and do a proper clean?  Probably not!  Not particularly hard either, but the manuals don't encourage this step..  My 836's performance is excellent with its clean and properly adjusted  77/78 carbs. Jas750, go with the late carbs.

anyone have a pic of these soundz like it could be the issue with my 78, which is doing the same thing

[eurban]

I don't have a pic but you might like this link to down load the factory manual.  There is a page in the carb section of the manual that shows the location of the slow jet etc.  Of course, ignore the part in the manual where it says the slow jets should not be removed. 30 years of deposits pretty much make it a necessity. . .http://www.mediafire.com/?5ttzdg9yyuz

[I Zombie]

thanks, i've torn into em once but didn't have anything to go off of.
so i just sprayed what i could. didn't wanna break or lose anything my first time into it

[eurban]

Yes caution is wise when you don't know what you are doing.  Manual should help with that as will this forum.  Unfortunately, the routine you describe will not usually be sufficient with these carbs.  Pilot circuit will need a good bit of attention, including pulling and soaking / clearing the slow jets. Before you remove your carbs, check that accelerator pump is working (filter off, whack open throttle and look for fuel squirting from small brass nozzles located at the bottom of the carb bore next to the choke plates).  If not, give that extra attention while the carbs are off.  Set float level and once assembled check fuel heights in bowl with some clear tubing attached to the overflow spiggots (hold tubing up along carb body and open up drain screw; the level of fuel in the tube is the level of fuel in the bowls; all should be the same and should be at or just below the bowl to carb body seam.)  Bench synch the carbs while you have them off.  Have fun!

[I Zombie]

could you explain the "Bench synching" & setting the float height?

[eurban]

You can search for the subjects (and any other you desire) in from the "Tech Forums" page and find lotsa info. . . .Bench synchronization involves using some convenient object like a 1/16 drill bit as a gauge to adjust all the slides so that at a given throttle speed screw setting, they are all open the exact same amount.  #2 carb's slide is not adjustable so you turn the throttle speed screw until the gauge object just fits between the bottom of the slide and the carb bore on the engine side of the carb.  Now using the synchronization screws at the top of the slides, and the gauging tool, adjust all the other slides to be the same as as #2.  This procedure will typically get synchronization close enough that (assuming the engine is in a good state) the precision gauge synch will be quick and painless. . . .Adjusting the float height involves bending the float needle tang up or down to acheive the proper height spec for the carb. . . .  The procedures for float height measurement and gauge synchronization are pretty well covered in the factory manual.

[I Zombie]

sweet thankz for the info

[BobbyR]

Soak everything in Simple Green for a few hours and get some strands of copper wire from an old lamp cord. Take one carb apart at a time and you have the others as a guide.

[rbmgf7]

be careful not to leave then in the simple green for too long or it'll begin to accumulate some kinda white residue that can gunk up the orifices. did it to my keihins and it was a blast to clean that mess up. follow bobby's instructions and you're good to go.

hope no one is talking smack about the PD carbs. gosh, somehow i managed to gain 4 sets thinking it would be better to use these instead of the older K keihins. figure the accel. pump was a plus to have.

[eldar]

The later carbs are awesome. Much better for tuning, especially with the 77 needles, than the older carbs.
They are also much more flexible. People have run pods with them and have only had to tune the idle mix screw. Cant usually do that with the early carbs.

As for simpler, in what way? Only real additional part is the accel pump. And that is easy to work with.

[dchautin]

Can someone send me a link to the 78 carbs manual?  I'm desperately trying to get mine in working order.  Leaks at the alum fuel tubes between carbs.  Thanks


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[lucky]

I put the 1969 carbs on my 1978.
A lot more power and much easier to work on.

[dchautin]

Really?  Was it hard?  Need different carb boots?


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[dchautin]

Got them to stop leaking but now won't idle without the choke on. I guess they need to be synced.


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[TwoTired]

Either vacuum sync, or maybe some pilot jets are clogged. 
Checked for even head pipe heat?

By the way, the PD carbs have the very same venturi size as earlier carbs, so they make exactly the same power for those who are able to tune them.  They do have some extra complexity that is beyond some individual's ability to comprehend.

[dchautin]

Any advice or link to how to sync? Thanks guys


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[TwoTired]

Get a vacuum gauge set.  Adjust 1,3,&4 to whatever #2 shows for vacuum.

The Honda shop manual is a pretty reliable reference.

[Crack On]

Link appears to be invalid as of 9/14/16