SOHC.co.uk Forums > CB350/400

Sooty plug

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Nurse Julie:
Great, do the same process with the other 3 and your machine will be flying. I was really pleased when I sorted mine out, not only with the result but doing the diagnostics and adjustments too. We can now say we have tuned our bikes I suppose 😁😁😁😁

watty51:
Also, for all the people like myself who thought there must be something wrong with the actual carbs, and took them on off on off on off (you get the picture) checking float heights, changing this, changing that, maybe this could be the answer they will be looking for.

Learnt a lot about carbs though !

Thanks Julie

K2-K6:

--- Quote from: watty51 on July 16, 2019, 04:48:23 PM ---I will turn the air screw out by a tad more as suggested. I did notice the idle tick over rose so had to turn that down a bit, but tick over was a lot smoother/ less erratic.

--- End quote ---

If everything else is set correctly,  then this is the "icing on the cake" of smooth tickover for these engines. Keep it quite though,  most think it's the carb balancing with guages that gets chased relentlessly to not much effect  ;D

The tickover rising is exactly as Honda describe it,  as you get more accurate burn cycle the efficiency of combustion rises and causes that cylinder to run faster. It's this that makes the engine rpm rise.

billywingnut:
Forgive my ignorance, but I was under the false impression that the airscrew settings on these carbs was purely for adjusting the idle and slow running side of  things. At 6000 rpm with wider throttle openings I wouldn't expect it to have much of an effect on the overall mixture.
I have set all my airscrews at 2 1/2 as per the book. It runs fine though lean. I will now re-examine the settings.
Could somebody put me right on this.

K2-K6:
Both view's are technically right that you've stated,  hence some obvious confusion.

If you were to run under full load and pulling hard at 6000rpm,  then you'd definitely be up onto needle and main jet area as carb slides would be well open.
So you'd be looking at those jetting volumes,  this is a classic plug check under full load method.

Conversely if you're riding gently,  then wafting along at 6000rpm on a flat or slightly downhill road in fith gear could be well into idle circuit response as it extends a fair way up but dependent on carb slide position not on absolute revs. 
In this scenario you'd have plug colour dominated by light use and lower slide position.

The idle circuit overlaps considerably with the carb getting into its needle profile,  the better the match here as it hands over to higher range then the more smooth and linear response the rider experiences. Get it too lean or rich and it'll probably make a flat spot about 4/5000rpm at which point (on light throttle)  it's getting into the mid range flow of the carb and arriving at the point where the above idle jetting can clean up the running,  if that too is accurately set.

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