SOHC.co.uk Forums > CB500/550

Black smoke and hanging idle after a full rebuild

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K2-K6:
Certainly the original airbox is preferred and particularly good on these Honda, but I believe the general view out on the big wide interwebs starts with false assumption (well aren't many assumptions false?  ;D ) in how they approach running alternative intake.

That assumption seems to be grounded in "well it's now getting more air so it'll need more fuel" and partly promoted by filter manufacturers claim in the improvement their offerings deliver.

For a std engine that's not true, but more that the change alters the carefully designed vacuum gradient that honda made and jetted the carbs for.
Essentially,  and for the same engine speed, the carburettor now enacts less vacuum and so draws less fuel from float chamber at the same engine demand. It's that which makes it run lean.
Inevitably you'll have to respond with some tailoring of fuelling to bring it back to a normal level, you'll have to add more fuel to less vacuum to bring that change.  Making a jet larger just allows that vacuum to pull more fuel through that jet as it meets less resistance.

Knowing what to add at which point is the part that's difficult to determine.

For a start point it's worth leaving the main jet alone and standard, but lifting the needle one notch.
Going up one size on idle jet,  but starting with airscrew out 1/4 turn from specification. 
Worthwhile using at least NGK 8 grade plug to help avoid overheating the plug tip.
Note;- increasing BOTH idle jet size and main jet will usually make it too rich at large throttle opening as the two changes are accumulated, the idle jet adds volume right across all carb opening.

To the OP,  which size idle jet have you used?

thomaspigeon:
Thanks for taking the time to explain this @K2-K6

I think restarting to square  one is probably a good thing. Now I’d really like to buy an OEM needle as the aftermarket ones at troublesome apparently.

My current jetting is :
Main 110
Idle 42
Clip on the 3rd position from top

I still have the original ones so I can do this.

I’ve got D7 grades plugs so that should be ok on that side.

One last question, I didn’t apply any oil on the K&N pods. I believe applying so K&N pod oil might increase the resistance so I’ll probably do this too.

fogrider:
After all the tests and trials I've done to get my 550 fuelling right, with foam filter , paper filter and no filter at all, it looks to me like all you're settings are way too rich.
 I think you have the 069a's like mine, if so, start again with 38 idle, needle in 2nd from top clip and 98 or 100 mains with air screw 1.1/2 out, and take it from there.

The idle jet seems  especially important, I've just discovered my idle settings were  badly effecting the whole system, after the knack***d float needles and valves were changed, float heights set, richness from the idle system had me chasing ghosts. Idle jet checks out as  41 ( marked 38 though) and seems to be way too big. I'm going to try some 35's just to prove a point.

All part of the fun.

K2-K6:
"After all the tests and trials I've done to get my 550 fuelling right, with foam filter , paper filter and no filter at all, it looks to me like all you're settings are way too rich."

As you find, there's delicate balance and shift around the lower end carburation, and more complex with the K&N type pods.

They effectively add another layer of pain with both the bell mouths and still air box removed.

I'd still start with just the idle sized up along with the setting changes as a first look as it'll give us a picture of response and something to go on.

robvangulik:
Does the black smoke persist with the k&n's removed?

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