Topic: CB550 difference in intake manifold?

[Teddyhoeg1982]

I have a european CB500K3 from 1977 which is similar to a CB550K3 except for the displacement.

The bike used to have 022A carbs, and as I am working on bringing the bike back to 100% stock, I purchased a set of PD46 carbs.

I thought these PD carbs would mount to the same manifold, but it seems like the intake spacing is slightly different, making the intake rubber stretch too far and not providing an air tight mount? I could perhaps make it work but seems I would stretch the intake rubbers too much. 

What also tells me about a difference is that my intake manifolds have a vacuum screw, while the PD carbs also provide this, which 022A carbs did not.
Anyone have experience with this, do I need to find the newer aluminium intake manifolds from 1977 to make it work, or is something different wrong?

[bryanj]

The intake rubbers are different, you need to look up the correct part number, and you will get to hate the pd carbs.
The carb to airbox rubbers are different to and from memory not available
Also the alloy manifolds are different, the pd ones do not have syncing vac port tackoff holes as they are on the carbs

[Little_Phil]

But apart from the addition of the screw for vac balancing on the earlier bikes the intake manifolds are identical AFAIK. I ran 627B carbs on my K3 with the pd manifolds for a while.
And yes rubbers at both ends of carbs are different and at the intake difficult to find. There used to be someone on the site who made and supplied them.
The carb rubbers at the intake are longer for the PD46.
The rubbers for carb to manifold for the PD46 are actually shorter.
As the dims about cancel out, I don't know if anyone has tried using the PD 46s with the wrong rubbers. Or if there are other dimension differences.

Just looked and the carb to manifold rubber for the PD46 seems a fraction larger on the i/d at one end. So probably won't work.

[bryanj]

Never having had both types of manifold in my hands at same time i cannot comment with certanty

[jonda500]

I don't currently have easy access to my parts but I managed to find a photo - but maybe I have it mixed up which rubber goes with which manifold? John

Edit- actually I checked and the picture is correct - the PD manifolds and rubbers are both shorter than the earlier ones.
However the airbox side, I have no information as I got these parts on a 550 I bought in the eighties that came fitted with earlier model 750 carbs with pods. (I don't know how PO would have be able to sync the carbs with no sync ports!)

[bryanj]

The manifolds definately look different heights to me, plus one has two grooves the other one so no they are not the same except for vac points

[TwoTired]

The 550’s PD carbs are physically longer than the older Keihin types.  So, the rubber couplers are different at both ends of the carbs to properly place the carbs in the ducting.  As already noted, the manifolds were different too, with the balance ports the most obvious to notice.

[Deltarider]

Personally I'd opt for the oldstyle 022A, 627B or 649A carbs. More K3 owners have chosen for them as the PD carbs are known troublemakers. The oldstyle carbs are easy to service/maintain whereas the PD carbs... they can be a nightmare unless you use the bike almost daily. And yes, the intake manifolds do differ. The K3 is less sporty and more economic as far as fuel consumption.  If you want the PD carbs nonetheless, pay attention to the orientation of the rubber boots (see pic).

[Teddyhoeg1982]

I don't currently have easy access to my parts but I managed to find a photo - but maybe I have it mixed up which rubber goes with which manifold? John

Edit- actually I checked and the picture is correct - the PD manifolds and rubbers are both shorter than the earlier ones.
However the airbox side, I have no information as I got these parts on a 550 I bought in the eighties that came fitted with earlier model 750 carbs with pods. (I don't know how PO would have be able to sync the carbs with no sync ports!)

Could you please measure if the intake hole spacing on the carb side of the manifold is similar?

[jonda500]

I am quite sure the intake hole spacing is exactly the same but I'm afraid I have no loose parts accessible to accurately measure to confirm, I recall a thread in here where the owner had no sync ports which can only happen with early model carbs mounted to later model (PD) inlet manifolds?
John

[Teddyhoeg1982]

I am quite sure the intake hole spacing is exactly the same but I'm afraid I have no loose parts accessible to accurately measure to confirm, I recall a thread in here where the owner had no sync ports which can only happen with early model carbs mounted to later model (PD) inlet manifolds?
John

Meanwhile i found a pair of the newer style ma
Manifolds, and they are actually Spaces a bit wider.

[jonda500]

Oh, so they are! and slightly larger diameter too. I always assumed they'd be the same but never actually checked. I wonder which if either exactly matches the 750 spacing or 650... John

[Little_Phil]

Had the opportunity to measure some bits whilst I had the carbs off my K3. All measurements approx.
The manifold where the rubber fits (as shown in Teddyhoegs pic above):
Old 500/550 o/d 33mm dia.
Pd carbs type o/d 35mm dia.

Also the positioning is slightly different as PD carbs are not spaced evenly.
Old 500/550 spacing between all carbs is about 76mm
Pd carbs: spacing 1-2 is 76mm, spacing 2-3 is 82mm, spacing 3-4 is 76mm.

Length of carbs, excluding spigot parts that are covered by rubber ducts:
Old 500/550, 77mm
Pd carbs, 81mm

As I said I have run early type carbs (627Bs) with the later manifold as there is obviously enough wiggle in the rubber to accommodate this. But don’t think it would work the other way round.