Topic: 069A carb rebuild

[mustanger67]

My tale of woe.  I have a 72 CB500k that has low miles.  The carbs had bad metal corrosion inside and had to replace them.  I found used 069A carbs that looked good.  The local Honda dealer cannot tell me which rebuild kit these carbs take.  He wants the year of the carbs and I don't know the year.  Any help would be greatly appreciated.  I have allready ordered and paid for the wrong gasket and o-rings for these once.  (They don't match the old ones).  Thanks, Jerry 

[TwoTired]

069A Carbs were installed on 1975 through 1977 CB550 F model bikes.

What confuses me is that the 72 Cb500k should have 627B carbs and all the gaskets and o rings are interchangeable between these two castings.  The jets, emulsion tubes, slide needles, and air bleed screws are different between models.

I use Honda part number  16010-323-305  for both carb models.

Cheers,

[mustanger67]

So to make the 069A carbs work I'll need to use this o-ring and gasket set (16010-323-305), and jets, slide needles and such for a CB550F model.  Correct?  Also another question.  This bike, a 72 CB500 had four into four exhaust.  Now it has four into two exhaust.  The CB550F that my present carbs came from had a four into one exhaust.  What different parts to the 069A carbs will I need to purchase to make it run right?  I know this is confusing, (at least to me).  Thanks for your help.  Jerry

[csendker]

The 550F came stock with the 4-1's, the 500 came stock with 4-4's.  The 4-1's will breathe easier than the 4-4's, leaning out the mix.  But that assumes all else is stock.  Whatever your intake situation is -stock filter/foam replacement/pods- will also affect the mix.  Pods will almosy definetely lean things out, foam replacement less so and stock is stock.  This will all generally impact the jets you need to compensate for the leaner mix; bigger jets, more fuel, even out the air/fuel ratio.  But.......the 069A's were built for a 550, not a 500.  They should breathe slightly more as a result of the extra 50 cc's.  That will also lean things out if I'm thinking clearly - still on my first cup of joe here. 

So, I would suggest that you start with the stock settings -->  http://www.sohc4.us/forums/index.php?topic=5410.0  and then compensate for whatever your specific set-up is.  Unless someone chimes in with some specific suggestions, get a couple different sets of main jets and experiment.  Take your best guess, install then and run some plug chops, then check your plugs --> http://www.sohc4.us/forums/index.php?topic=9817.msg91737#msg91737.

[TwoTired]

The carb body castings were the same for the two model carbs.  Internal parts varied to match requirements of the engine's breathing characteristics.  The castings were then stamped 627B, 069A, 022A, etc. to identify what parts were installed inside them and where the needle clip was set (this is a set up number).

Both carbs have the same breathing capacity as they both have 22mm bores.  It is the mixture at any given throttle setting that is different between them.  If the only change you have made to the bike is a 4 into 2, I would be inclined to transfer all the 627B internal hard parts (not gaskets and orings), over to the 069A body stamped carbs.  This would give them the same setup as 627B carbs native to the CB500.  And, to be kind to future owners or mechanics of the bike, the setup stamping on the carb body should be altered accordingly, so there is a visual indication externally that the carbs are not what they were when they left the factory.

The only exception to the parts transfer might be the Idle Air bleed screw, I suppose.   These have a different tapered tip and are solid vs hollow in tip design.  I'm uncertain if the carb bodies had seats machined into them to match the tapered tip, or whether there would be some sort of performance issue with the mismatch.  But, from this aspect, the 069A carbs are "the most different" from those found on the CB500.  For a conversion subject, I'd rather see a 627B conversion from a 022A set than a 069A set.  However, you should move the following parts in either case:
Slow jet
Main jet
Main emulsion tubes
Slide needle (use clip setting for CB500)

optional:
The idle Air Bleed Screw.  (I don't know what, if any effect this may have on final correct setting.)

About the 4 into 2:
My 74 CB550 with 022A carbs ran fine for 15 years with the stock carb setings and 4 into 2 exhaust.  I changed nothing in the carbs when I changed back to 4 into 4 and it still runs great.

But.......the 069A's were built for a 550, not a 500.  They should breathe slightly more as a result of the extra 50 cc's.  That will also lean things out if I'm thinking clearly - still on my first cup of joe here. 

Then venturi effect is reliant on airflow mass.  Increased displacement increases the mass.   In short, higher displacement sucks harder on the jets for a given RPM, causing them to flow more, making the mixture richer.  Air filtration changes likley have a greater effect on jet selection.  Note, however, that the CB500 has #40 slows, and the CB550 has #38 slows in stock form.  Seems reasonable to assume this change was a reaction to the increased displacement.

Cheers,

[csendker]

Must...get...more...coffee.

See, I knew someone who knew what the hell he was talking about would clarify things.  Excellent post TT.  Interesting info on the different carbs and their associated guts.  I need to change my perspective, I keep thinking volume, not mass.  And while I knew there was an issue there, I had it completely bass-ackwards.  I must now spend some time pondering the dynamics of mass vs. volume vs. suction vs. flow, and then apply that to the various pieces-parts...

[mustanger67]

Thanks for all the help guys.  I start on the carbs again tomorrow.  All the information has helped illuminate the dark.
Jerry

[csendker]

Still pondering...higher displacement=more mass flow through the venturi.  And the venturi is kinds like an aircraft wing rolled into a tube.  At this point, the light bulb goes on and Bernoulli takes over: the greater the velocity, the lower the pressure.  So by jamming more air through a fixed bore {as a result of the greater displacement}, you actually get a higher velocity, which leads to a lower pressure in the venturi.  And it's this lower pressure that sucks more fluid out of the jets.  So by increasing the mass flow you actually may richen the mix, not lean it out.  I suspect there's a limit somewhere, but I'm restricting my pondering to a 500 vs 550 cc differential, and not like trying to stick these carbs on a 1200cc monster.

However, by adding pods to an existing engine you don't increase the mass flow rate because the amount of air pumped is a function the engine cc's and not the filtration device.  Piston engines's are positive displacement pumps, but the fluid is compressible, hmmmmm.  Back to more pondering...