Topic: Using carbs from a different year?

[Tretnine]

I have a 78 CB550K which is currently using a set of 46C carbs. They're pretty knackered, with one being somehow very corroded. I don't have another set of passable 46C carbs, though I do have a set of 46A that is mostly complete (needs jets and tubes, etc.)

Is this a 1:1 swap, or will I need to alter jet sizes and needle position? I realize there will be fine tuning - particularly as I'm at 5,000 feet - but I'm mostly looking for well it works to interchange carb sets between bikes and years.

[bryanj]

Cant answer that one as i hate PD's with a vengance and would fit 500 or 550 to K2 or F manifolds and carbs

[Tretnine]

Oh yeah? Why do you hate on the PDs so much? I've got a set from a 500 around here somewhere. Don't remember which set they are or if all the parts are around anymore...

[bryanj]

Because they block for a passtime, are a swine to reassemble and never seem to run right if left for more than 3 weeks. Unless its a 500K3, which is a rare bike no 500 had pd's also the 550 did not have an accelerator pump like the 750

[Deltarider]

I think you can swap easily from the -C to the -A carbs. Differences are minor. You can try to determine them by close reading the parts list: http://www.honda4fun.com/pdf_parts_list/pdf_spac500-550/CB500-550_K3-K4.pdf and/or the addendum in the Shop Manual that deals with the K3: http://manuals.sohc4.net/cb500/service_manual/HSM500550_16.pdf
Honda was forced by US legislators to lean out what had been a very succesful bike. They had to redesign the exhausts and carbs.
These carbs will show no problems, if you ride your bike regularly. Todays gas is not the same we had back then and that's where the problem is, especially if a K3/4 has not been used for a long time. If I owned a K3, I'd add a small socalled 'maintenance dose' of some fuel system cleaner to every tankfill. 15ml would  probably be enough.

[Tretnine]

Great links, Delta, thanks. There's all kinds of good info in there. I think I might print these out and keep them in the shop. Also, interesting thought on the fuel system cleaner - what do you use normally?

[Deltarider]

Ever since lead was phased out, my impression is that gas behaves somehow less 'lubricous', which - I admit - is a strange property when we talk gas. My carbs are the oldstyle type Keihins which are less susceptible than PD carbs. However I ride less than before, so I will experiment a bit myself. If you don't ride regularly, you can apply the regular products like Seafoam etc, or just add a little lamp oil (kerosene) maybe with even less naphtha (2:1). I suppose 15ml (in total) every tankfill will do. Anyway, that's what I'd use as a socalled 'maintenance' dose.

[Tretnine]

That works, and makes sense. Lead was added to prevent engine knock... which I understand it did, but also had all of those environmental and health effects. I know modern gas is a lot harder on old rubber as well, particularly if you can't find the ethanol-free stuff. I know there's also a "lead" additive. It's not actually lead (or I'd hope it's not) but it's meant to help old engines that were designed around the slippery, leaded gas. I wonder if that would have any effect. I doubt it would have any effect on the deteriorating rubber or on gas's tendency to turn into varnish when a bike isn't ridden, though.

[jonda500]

Lead replacement additive can cause some tank liners to fail. Hondas are designed to be able to run on lead free petrol so I wouldn't use the lead additive, better to use the most premium (priced) petrol as I think it has the best additives available in it.
John 

[Deltarider]

Maybe I have not expressed my experience with gas precisely. I don't know if the changed quality is related to the presence or absence of lead. All I know, is that in the days of leaded gas, the gas was much liked by my bike. Other things in gas may have changed also.
Our bikes don't need lead and as a consequence neither will you need substitutes that IIRC are based on kalium (potassium?).
You carbs need to be clean. Once they are, it could be beneficial to apply small 'maintenance doses' of a fuel system cleaner when you fill up. 5000 ft could be an advantage for a K3/4, so I wouldn't start 'fine tuning' rightaway.