Topic: Anybody Know about 1978 cb550 carbs?

[Geeto67]

Anybody know about how to set up to factory spec a set of 1978 cb550 carbs?

A buddy of mine is having a heck of a time with his (read about the problem and see pics here:)

http://www.caferacer.net/forum/topic.asp?TOPIC_ID=1480

[MetalHead550]

Yeah he has a mess there.  As you said those carbs arent covered in the Clymers.  Maybe he can get ahold of an actual Honda shop manual for the 78.  The screws he messed with under the top cover are for syncing the carbs.  He seems to be under the impression that there are two screws for air/fuel mixture.  As you know there is one screw per carb and in his case they meter fuel.  They are the screws under the carbs, in front and center of the float bowls(toward the engine) and they are mounted vertically.  If he bottoms them out clockwise then turns them out 1.5 to 2 turns, that should be a good base point.  He'll need to pull the carbs again and try to bench sync them(sorry I dont know the specs for the slides) then sync them while its running.  He should be able to measure the clearance on the cut away side of the #2 carb and match the others to it.  The #2 carb is the "preset" carb and is the one that you match the others to durring a sync.  Match carb 1 to 2, then 4 to the others and finally 3.  He'll probably end up having to increase the vacuum on 1 and 4 a bit once he does a running sync since their inlets are longer.  I think the screws he mentions having the yellow torque paint on them are for adjusting choke but Im not certain.  All said, he should probably find a set of carbs that havnt been pulled off the bank or messed with, and use those as a reference.  Nothing wrong with having extra carbs and parts right?

[TwoTired]

Yes, he screwed up the carb sync.  This sets all the slide so they are open the same. You can get them close with mechanical sync.  But, you need 4 manometers to do the job right.  But, only after every single tuneup item is within specification.
His 4000 RPM problem is likely the fast idle cam.  I think this is the yellow paint he is talking about.  When the choke cable is pulled, the cam forces all the slides open more so the idle increases.  I can't tell from his pics if he even has the cable connected.

The only documentation I am aware of for these carbs is in the Honda Shop Manual supplements toward the end of the book. It can be found in the FAQs on this site.

The idle screws on these carbs meter fuel when the throttle is in the 1/8 or less position (choke off, see above).  It has very limited range even then.

The slow jets on these carbs are pressed into the carb bodies. They easily block with crud being only about .016 inch in diameter.  They can be damaged irrepairably with metal cleaning tools.  The circuits behind the jets often plug up too.  And yanking the jets for cleaning is often the only way to get these carbs to idle.  Even then, they are very sensitive to air filtration restriction, as well as exhaust flow different from stock.

[Kelvin8]

at 4000RPM the engine is running on the main jets, not the slow speed jets. I'll bet that at least one of those circuits is clogged. You can blow carb cleaner backwards through the circuit to clean it (maybe). You should be getting a strong steady stream of solvent coming out the slow jet intake aperature (or whatever it's called). I had a similar problem, engine died below about 3500RPM. Cleaned out the #4 slow jet and I was able to get the engine running closer to normal (carbs still need balancing). I actually have it now where it idles around 500 RPM once it's warmed up (but there's other problems, so ...)

Kelvin

[free85]

Holy Thread Resuscitation, Batman!

I have a 1978 CB550K that I'm working on putting on the road for some fun before I make any changes from stock. That said I'd like to change the exhaust, and airbox setup. THAT SAID I have heard about the sensitivity of these carbs to these sorts of changes. I was wondering if it's possible for the bike to run happily with pods and exhaust changes (I'm leaning in the direction of 4 into 1 at the moment) or not so much? If not so much is a carb swap possible to make things a little more solid?

Thanks in advance. I figured this would be a good discussion to have before I do anything outside of stock.

[Bankerdanny]

Possible? As they say, all things are possible. Probable? Well that's a whole different question.

Plenty of 550 owners run pods and aftermarket 4-1's and manage to find a tune they can live with. The 77-78 PD's are perhaps more difficult to get to a liveable tune because jets aren't as easy to come by and change. Some changes are easier to make by drilling the jets than replacing them, but of course this is a permanent change.

Swapping to a set of the older carbs should not be that difficult. Besides the carbs you will need the manifolds that attach to the heads and the throttle assembly.

[TwoTired]

The carb changes (internal as well as entire unit), can only do so much to adapt to cheaply made pods, and there are different styles and operational specs among the various brands of pods.  Simply shortening the intake tract length has a dramatic effect on all the fuel metering orifice sizes needed, then there is the membrane type and the turbulence they create so close to the carb inlets.  These aren't low RPM tractor engines.  These are high rpm engines with pretty high inlet air flow rates.  Duct aerodynamics are far more of a concern, and don't much care about the styling preferences of the rider.
Unless there is a dyno in your garage or you pay to use one, dialing in the changes to ANY carb you chose will require a recursive test and try process.  A test track that allows full power runs of 30 seconds or more is pretty much a requirement to obtain the correct main jet size (along with several sets of clean spark plugs to read the combustion deposits).  Even the earlier carbs will need the same tuning skills and attention to the same metering orifices as the PD types.

Can it be done?  Yes, and easier if optimum street performance isn't a requirement.  Is it just a matter of installing different physical assemblies?  No.  Is learning how it all works, and the hours of adjustments, worth your time and effort?  Your call.

[free85]

I didn't expect this to be "bolt on new carbs, throw away airbox (which was engineered, at great cost and time, by Honda for a reason), add pods, ride away happy" kind of process. I have heard (and read) that it "simply can't be done" and was wondering if that were true on these carbs or if it were more of a "can be done but takes time, learning, and patience" situation.

If I were to install pods, and yes, I can say that it's a looks thing, I would demand of myself that I spend the time to get them running correctly, for various reasons, not the least of which would be learning something as well as making the bike operate in a way that pleases me (Read: well). I understand the nature of this isn't "bolt on simple"... and I kind of like that, honestly. If I didn't want to learn about motorcycle mechanics I'd have just bought a bike, learned to change the oil, and called it a day. Would have, ultimately, saved me a lot of time, and money (even if you throw out the idea that my time is worth money).

I intend to start with the exhaust.