Topic: Mismatched carbs on 1979 CB750 Ltd edition

[gunner]

Hi,

I'm rebuilding my carbs for the first time and following (to the letter) the well written guide by MacGregor Carb Cleaning Services.  So far things are going well, but now I'm convinced that the carbs are partly from a CB900.

The carbs that were on the bike when I bought it, and that are now being rebuilt, have the following casting numbers 1: VB42DASC, 2-4: VB43ABSK.  Carbs 2-4 have 4 holes under the butterflies, plus the pilot hole.  Carb 1 only has 2 holes under the butterflies.  According to this post (step 34) and this table carbs 2-4 belong to a CB900 and carb 1 belongs to a CB750.

When I bought the bike the PO also gave me another bank of carbs and these appear to have the correct casting numbers: 1, 2, 4 are all VB42AAQG, 3 is VB42APRA, which matches what's in the table for my bike.

So, do I care?  Should I break down, clean, rebuild and swap in the 'spare' bank (meant for the 750), or just go ahead and put in the bank from the CB900?  If I use the CB900 bank, what do I do about carb 1, with the different number of holes?  Will this be a problem, or can I account for it during syncing?

Also, if I stay with the CB900 carbs, I noticed that carb 2 has a port/tube downstream of the butterfly.  Looking through drawings I'm guessing this is for a vacuum assisted petcock...?  Do I just put a good plug on there like the PO?

Thanks,

Gunner

[Xnavylfr]

You could run the CB900 carbs if you had a matching set for all 4 but since you don't I'd rebuild the 750 carbs. The 42/43 denotes what diameter throat is on the carbs 42mm or 43mm, you could get them to work together by tuning but it could become a frustration.
Yes. the line is for a VAC. petcock, if you are still using stock petcock it has to be hooked up or the petcock won't work properly.


Xnavylfr(CHUCK)

[gunner]

Thanks Chuck.

I decided to proceed with the full set of 750 carbs as they looked to be in good shape and only needed new air cuts and passage plugs.  Got em installed on Friday night and they worked first try.  Idles pretty smooth around 1000 rpm which it had trouble with before.  Next up is valve shims, new cables, timing, and finally carb sync.  And maybe the HEI ignition mod I've been reading about

Thanks for your help.

Gunner