Topic: Carb Tuning

[steve]

I just finished a complete rebuild for a set of carbs on a 1976 CB550.  While vacuum syncing them I will grab some throttle a few times and the idle stays up around 3K rpm's.  It comes down in about 15 seconds.

Any thoughts on causes?

Thanks

[BomberMann650]

Provided you have the proper jetting.

I’d say it’s time to tighten the springs on your timing advancer.

[Deltarider]

Typical for an idle set (far) too lean or airleaks between head and manifold intakes, best checked by spraying brakecleaner and see if rpm changes. The O-rings engineside over the years become hard, brittle and then leak. Could also be an advancer that doesn't work as it should. Ideally checked with a timing light. But if the phenomenon started right after you did the carbs, I'd start there.  Remember: don't try to achieve a highest rpm at idle by leaning out the airscrews. This may be common practice with other motors, your model needs an idle a bit on the rich side to facilitate good driveability on acceleration.

1976 CB550

The Year tells us nothing. Model F? K? Could you inform us what stamped number is on your carbs? You can find the number... see pic below.

[BomberMann650]

I don’t know if lean conditions really happen that easy on these cb550.

There is a difference between a motor slowly returning to idle and one that wants to hang up and rage because it’s too lean.

Once you’ve seen it, it’s obvious.  Hot starting a bike with an over-lean condition, seen it catch and just go straight for 3k without anyone touchin nothin.  Then it stayed there.  Pissed off as all hell until someone hit the kill.

Where as, a slow return to idle suggests that the bike wants to mind its manners.  And if you use a timing strobe to watch the advancer mechanism.  It’s plain to see that when it sticks at full advance, the engine rpms stay there too.  Eventually the crank slows enough on its own that the advancer can reset and the revs drop.

That’s my $0.02

[TwoTired]

I just finished a complete rebuild for a set of carbs on a 1976 CB550.  While vacuum syncing them I will grab some throttle a few times and the idle stays up around 3K rpm's.  It comes down in about 15 seconds.

Any thoughts on causes?

Thanks

Have you checked head pipe temps?

[steve]

New Intake gaskets and boots, and it is a K.  I think I got it.  I kept syncing them as I lowered all the slides and keeping the idle adjusted to 1K.  Air mixture screws at 1 1/2 turns, and it seems good to go.  I have the needles shimmed, #45 pilot and #110 main.  Air pods and a 4 into 2.  Sounds good.  I also went to the petcock the has 1 outlet and that eliminated all delivery issues. 

Thanks for the input.

[BomberMann650]

45/110 should be fine jetting for that setup.

[Deltarider]

I have the needles shimmed

Just for the record, Steve, shimmed in what position? I'd like to experiment with that too.

[steve]

C Clip is in the lowest position so that the needle is at the highest and 1 shim under the c clip.

[DaveBarbier]

Needles seem pretty rich just for pods and a 4-2.

[steve]

It make start up so much easier. I have it on my  1975 with pods and stock pipes also.

[DaveBarbier]

Interesting, I just raised my needles by one notch on my 550 and it ran great. You ever do a 3/4 throttle plug chop?

It just seems to rich to me, but what do I know.


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