Topic: '78 carbs on a '75?

[Dr. Noisewater]

Hey guys, I've been trying to resurrect my '75 cb550f and finding carbs has been quite an issue. I have found some carbs from a '78 that are perfect. My question is what all would need to be done, if anything, to get these running properly on my '75? Thanks!

[Bankerdanny]

The '78's use a different throttle and choke linkage. They are also set up a bit leaner than the '76 and earlier bikes. They are also a single gas line feed instead of a dual line.

It's not an ideal switch, but I'm sure you can use them. Just make sure to get the whole rack assembly, not just the carbs.

[lucky]

There are many problems to overcome.
why would you want to do that?

[TwoTired]

This has been covered before.
78 CB550Ks also have different headers, rubber couplers fore and aft, require a choke cable, and require at least one different throttle cable just to fit onto a 75 CB550F bike.  An extra fuel "T" will alleviate the single fuel inlet on the newer carbs.

Once they are on...
All the fuel metering adjustments are different as well.  Unless you are also fitting the 77-78 exhaust system and air induction components, you will need to change the main jet, find the correct or acceptable slide needle position, and a new pilot screw setting which gives the proper acceleration upon throttle twist.

It not a bad project undertake from the standpoint of learning your machine.  However, if you are looking for a quick and painless fix, this isn't it.

[Dr. Noisewater]

Cool thanks for all the input guys. I guess I'll keep looking. Just to make sure, only 74-76 are completely interchangeable?

[Bankerdanny]

The '77 550F also uses the same carbs as your bike.

The '77 550K uses the same carbs as the '78, but the F retained the old style carbs.

So you can look for carbs from any F bike, and the '74-'76 550k's.

[TwoTired]

If your bike is an F model CB550, and retains the stock air induction and exhaust, then you definitely want carbs from another F model, 75-77.
If the exhaust and/or induction has been modified with aftermarket components, then avoid the F model carbs in favor of the 74 -76 CB550K carbs, for reasons of internal jetting suitability.

069a carbs wer for the F model
022a carbs for the K (except 76 which was 087a)

These are set up numbers stamped on the carb body mount flange, that describe what internal parts and setting they had from the factory.

Be aware that the internal parts and settings could have been altered when serviced since the 70s and the stamped number ignored.

[Bankerdanny]

Shows you what I know. I thought the only difference between the F and the 74-76 K carbs was the jetting.

[Dr. Noisewater]

Awesome thanks a lot guys!

[TwoTired]

Shows you what I know. I thought the only difference between the F and the 74-76 K carbs was the jetting.

In essence, that's true.  But in the details, are some internal part differences and settings to make that effect.