Topic: Synchronizing Keihin Carburetors but running into manifold issues

[aguerra]

I bought a 78 cb550 a few years ago and it seemed like the previous owner(s) had fiddled with it, but not using best practices. Anyway, I recently bought some Keihin CR29s and I have been working on getting them synced and running properly. However, it seems as though one of the vacuum ports on the manifolds that are on the bike were cross threaded at some point and the brass fitting that goes in there does not seat well (i.e. pressure being lost).

After doing some research I discovered that the '78 manifolds did not come with ports because their original PD carbs had integrated ports. This fact would explain why it has been a struggle to get the Keihin carbs mounted onto the manifolds even though they sit on an adjustable rack. After some searching, I found that the CR29s do not have integrated vacuum ports.

So the dilemma is as follows. I want to replace the manifolds with the correct/original ones so that the carbs mount properly (avoid leaks from the rubber boots) but, if I do I won't have a way to synch the carbs. Older manifolds, like the one on the bike right now have ports and seem to fit the engine, but do not necessarily line up well with the carb rack. Has anyone faced this issue before? Is drilling and tapping into a new set of manifolds a poor decision?

Thank you in advance for your help.

[harisuluv]

I don't think there is any difference in the manifolds save for the vacuum sync port incorporated into one.

So you're using the stock rubbers with CR29 carbs?  They don't have a matching rubbers that at least fits the carb side? 

[aguerra]

Yeah, I'm using the stock rubbers between the engine inlet and the carbs. The rubbers fit the carbs nicely as well as the manifolds. However, there is a slight misalignment issue between the carbs and the manifold. In other words, if you take a rubber, it will fit a carb and the manifold but the rack of carbs as a whole does not fit on smoothly. You kind of have to force them on, which is not ideal.

When you say, "I don't think there is any difference in the manifolds save for the vacuum sync port incorporated into one" what do you mean? Do you mean that the manifolds are dimensionally the same across all of the CB550 year models, with or without the vacuum ports?

Thank you for your response and I apologize for the additional question.

[harisuluv]

Yes that is what I mean.

So if the rubbers fit ok, then just respace the CR carbs seeing as how they are adjustable, no?

[aguerra]

That is one detail that I might have neglected to mention. They are adjustable but I can no longer adjust them in the direction that I need to get all four of them to fit nicely. It seems odd, but one of the carbs (#2) is fixed on the rack. The other 3 are adjustable and I can get #1 and #2 to fit nicely, but I cant get #3 and #4 to come in tight enough so that they can fit into the boots without essentially deforming the boots. No matter how I adjust, I just can't get the spacing close enough.

It was this wall that leads me to believe that the manifolds might not work well with the CRs. I noticed that you rebuild carbs. Based on your experience, do you know if the Keihin CR 29s have vaccum ports on them that I can sync from? Dime city cycles says they do, but I don't know where exactly and the booklet they came with is not very helpful.

Since the PO cross threaded the old vacuum port, I will definitely need to get a new one. You believe that one from '76 or older (with the vacuum ports) will fit or align properly with the both the engine and the carbs?

Thanks again.

[harisuluv]

The manifolds are the same part except for one has vacuum ports, that's it.

Your issue is you have an aftermarket part.  If it doesn't fit your stock part, it doesn't fit.  If the rubber by itself can go on the carb fine, then the manifold fits, but due to the spacing it does not fit.

I don't know where the CR29 vacuum ports are located.

The crux of your issue is with the adjustment of the spacing on your carbs.  Isolate the reason why the range of adjustment is limited, then you will have to modify it.

If you need manifolds I have the early style and I'm sure a lot of other members do too.

Hope that helps.

[aguerra]

Thank you guys for the help. I'll try out some earlier model manifolds with the vacuum ports. and continue working on orienting the carbs correctly. Any way I can get a look at some of your older manifolds harisuluv?