Topic: 1978 CB550k carbs

[JC_550]

Hey guys, can't find an answer for this anywhere on line and no local motorcycle stores have had an answer, so I thought maybe someone here would know. I just bought a 1978 CB550k which I thought was all stock, I took the carbs off to clean them and the main jets are 102's, I read online that there supposed to be 90's, so I looked at the casting number and it says PD44a, everywhere i look says my bike should have a PD46c? Does this make a difference? Is it possible this carb is stock? And if not does anyone know what it's comes from so I can order the right carb kits for it? Thanks guys!!

[JC_550]

Hey, no I live in Canada but I bought the bike in the states.. I didn't check the numbers on all of them. Does each carb have a number on the casting? I thought it was just on the outside one.. I have pics I will post when I get home, my phone won't let me.. Thanks man!!

[Bankerdanny]

It's possible that the prior owner (PO) changed the carbs because the 77-78 carbs on the K bikes are much harder to find parts for.

With questions like this pictures are a big help. The PD46 carbs look very different than the carbs on the '76 and earlier bikes.

[JC_550]

Hey guys so this is a pic of the carbs that came on my bike hope it helps to identify what they came off... I also checked and each carb has the casting number (PD44A NL) Thanks again!!!

[brewsky]

Per the FAQ those are from  a 75-77 CB750A...definitely pumper carbs.

[Bankerdanny]

Yep, definitely not the carbs that came on your bike.

[JC_550]

Ok Thanks, much appreciated!! I will look for carb kits for a 75-77 cb750a! This is probably a stupid question, but why did you call them pumper carbs? Is that because they have an excelerator pump? Is that Difference from the stock 550 carbs, do they not have that? will these carbs cause me any issues when i get the bike running being not stock? The bike has stock exhaust and intake but Im putting on pod filters and aftermarket exhaust..

[Bankerdanny]

The 550 used conventional carbs. I know the last couple years of the 750 had a-pumps, I don't know enough about the A's to say about them.

[brewsky]

Ok Thanks, much appreciated!! I will look for carb kits for a 75-77 cb750a! This is probably a stupid question, but why did you call them pumper carbs? Is that because they have an excelerator pump? Is that Difference from the stock 550 carbs, do they not have that? will these carbs cause me any issues when i get the bike running being not stock? The bike has stock exhaust and intake but Im putting on pod filters and aftermarket exhaust..

Was the bike running when you got it, and why do you think you need re-build kits? Looks like they have already been cleaned or rebuilt? I would only replace what is not working, ie bowl gaskets or float needles etc.

The accelerator pump is actually a good upgrade if the carbs fit the boots properly and are the same size. Were they completely installed to begin with?

If you are changing exhaust and intake, you will be starting from scratch on the jetting anyway, so no way to tell what jets you will need till you do that first. The 102's may end up being ok after all.  May also need to play with idle and needle jets. There is a lot of info on jetting in the FAQ section.

(BTW, the #1 and #4 bowls are switched in the pic.)

[JC_550]

Well the bike would start but wouldn't run for long and only with the choke on, but they were installed and seem to fit fine.. The carbs look so clean because I cleaned them and polished the bowls, they were dirty when I bought It.. Your right I just need the rubbers for it, o-rings and bowl gasket were in bad shape. But Ive been looking around and looks like the 78 550k carbs have the same rubbers as the 75-77 cb750a according to the cycle recycle website..
Oh i see what you mean the bowls are switched!! Thanks guys

[chickenman_26]

...But Ive been looking around and looks like the 78 550k carbs have the same rubbers as the 75-77 cb750a according to the cycle recycle website..

That's because the '78 550K carbs are the same bodies as the 750A. Your carbs should be PD46C - no accelerator pump. The carbs you have there are 750A carbs. The air jets, jet needles, fuel jets, and float level calibrations will be incorrect for your engine. Still, because you intend to modify the air intake and exhaust, all bets are off. You might use the 750A carbs, just to see what happens. But the reason the bike starts and runs only with the choke on is due to either/or clogged slow jets/passages and vacuum leaks at the manifold rubbers. Those pressed-in slow jets need to come out for cleaning. If it were me, I'd leave the airbox and exhaust as it is and look on Fleabay for a correct set of carbs and overhaul them. Then you can ride and enjoy instead of tearing your hair out chasing jetting.

Stu

[Bankerdanny]

I would look for '76 and earlier carbs. Because the '77-78 carbs were just the 2 years parts are more difficult to get and more expensive.

The earlier carbs should work fine (although you will need a T fitting for the fuel lines since the earlier carbs used 2 lines and the '77-78 a single line).