Topic: CB550K PD46 carburetor questions

[JBMorse]

Hi everyone,
I recently picked up a spare 78 550K engine and carbs, which I plan to slap in my CB500/4 after a tuneup and cleanup. I was hoping you all could help me with the carburetors. I opened them up to clean and inspect them, and I don't know how to get the slide and needle out. It was a different procedure on my 627B carbs, and I can't find any help in the manual about this.
I just want to take the needles out to look them over and verify clip position. These carbs are in great shape and so far have all original settings.
Also, while searching on this subject, I found mention of the PD46 carbs not fitting the airbox rubbers from the earlier models. Is this true? If so I need to start shopping for a 77-78 CB550K airbox?
Thanks

[rocs]

Hey, Wnyoutlaw I believe has a stock airbox for sale.

http://forums.sohc4.net/index.php?topic=134222.0

[mickwinf]

Air box is the same on all models, but the rubbers from airbox to carb are longer on the later carbs.Hard to find but someone does a repro set cant remember his name but think he may be a forum member.

[TwoTired]

The PD carbs are longer than the earlier style, so both the fore and aft couplers are shorter to allow placement in the same space between air plenum and manifolds.   The manifolds are different too, in that the vacuum taps were removed from PD centric installations.
The rear couplers are also different diameters to fit the carburetor inlets properly.

The slide needle is removed by opening the top covers, raising the slides to access the cross head screws on the attachment plates.  then remove the screw for the lifter arm through and at the actuating rod.

The original settings on the PD carbs will not work properly on a CB500, even with stock exhaust and air filter.  So, plan on a rejet/adjust process.

You also need a choke cable and different throttle cables for the retrofit from 627b to PD46.

[JBMorse]

Thanks for the tips folks. I should have been more clear in my first post about what I'm trying to do.
I have a whole '78 CB550K engine with carbs that I would like to put in my CB500K. The 500 engine is apart for rebuild and I'd like to get the bike on the road in the meantime since I have no idea when I'll be done with the rebuild.


So after some reading and searching on swapping carbs and engines, I guess I have a decision to make. I have a good set of 627B carbs from my 500k and the PD46 carbs that came with my 550 engine. I'm trying to decide whether to find and buy the appropriate parts to fit the whole 78 setup, engine and carbs, to my bike or try to jet and tune my 627B carbs to the 550 engine.
To use the PD46 carburetors I need:
78K airbox rubbers
78K manifold rubbers
78K throttle cables

I already have everything I need to use the 627B carburetors but would need to re-jet and tune them for the CB550K engine. I'm not sure which is easier. My 500K has the stock airbox with a UNI foam filter and a 4-2 exhaust. Not sure who made it. When I had the orignal 500 engine in there the carbs were stock and the bike ran great.

What do you all think? Would it be difficult to make the 550K run well with 627B carbs?

[TwoTired]

What do you all think? Would it be difficult to make the 550K run well with 627B carbs?

Unless you are going to fit the stock high pressure 4to4 exhaust from a 77-78 K model, the PD carbs will need rejetting regardless.

You've got all the bits for 500.  Use the manifolds back.  Assuming you have something close to stock pressure exhaust, the slide needles should be positioned in the 4th groove from the top, and you may need to open the pilot screw another 1/2 turn to keep for sooting plugs at idle.   The stock 100 mains will be fine for such a set up.

[JBMorse]

Thanks TT. I think that sounds like a plan. I do need to clean up the 627B carbs since they've been sitting in a box for a couple of years. I might also replace the gaskets again.
I do think this old 4-2 exhaust must be pretty close to a stock system in terms of pressure. Thanks for the tips, can't wait to get this thing going!

Also, for future reference, if anybody else is looking into this, it looks like the rubbers for the '78K are available. David Silver has the set for engine to carb:
http://www.davidsilverspares.com/CB550K-1978-USA/part_145065/

and rubbers for the airbox side can be purchased here:
http://classicmotorcyclesolution.com/

Throttle cables are available from Motion Pro. For the choke cable, I would have just made a lever on the carbs, but that's also available from Motion Pro.
Anyway, for me the 627B carbs make more sense.

[TwoTired]

I do think this old 4-2 exhaust must be pretty close to a stock system in terms of pressure.

The 77-78 muffler has smaller outlets and no balance tube compared to what the 500s and 550Ks had prior.  This made them unique in their pressure peaks and averages.

Generally speaking, manufacturers of aftermarket systems, if they did any testing or performance tune planning at all, tried to make one product for all 500-550s mimicking the stock characteristics of the early systems of the very early 70s.  Mechanical fit was the primary concern.  By the time the 77-78 models came to be, they simply changed the applicability label to include those years and let the buyer sort out run issues caused by EPA meddling.

If an aftermarket system was close to any pressure characteristics it was that of the 74 CB550K (for example) which makes it quite different than the 77-78 exhaust pressure profile.

Anyway, for me the 627B carbs make more sense.

I believe you are correct.  It should be much easier to use these with fewest mods.