Sooty plug

[watty51]

Just been out on a 20 mile ride, the Cb350f was running fine up till 6000 rpm but had a long hesitation to finally get to the red line.
Did a plug test and noticed that number 2 plug was very dry sooty and the others were a bit lean.
Could this be causing the hesitation.
The carbs have been striped and new viton seals installed, all thoroughly cleaned and the float heights checked and checked again.....new air filter etc.
What are the major causes for a sooty plug.
The carbs were benched sinked only as I have ordered a carb balancer which has yet to be delivered.

[Nurse Julie]

I would say your No 2 is running rich (I'm stating the obvious here 🙄) I had the same issue on one of my bikes, the struggling to get beyond 60 mph and oh, so slow getting from 60-70. I just adjusted all the air screws out, depending on resulting plug colour, making it leaner and all was sorted, instant pulling power and no sooty plugs. Hopefully it's something as simple with yours.

[watty51]

Thanks for the quick reply Julie.
I will try adjusting all the air screws out as suggested, I thought they would have to be set all the same across the carbs
Would you suggest about 2 turns out and try from there

[Nurse Julie]

I always start with them a 2 turns out and take it from there. I rarely find that the need much adjustment to make things better but also find it unusual to have all 4 set exactly the same also. The manual gives a tolerance of 2 ± ½ turns, so anywhere between 1½ and 2½ is ok depending on plug colour,on cleaned plugs and after a good hard ride and keep adjusting and re checking until you get what you are looking for, or the performance improves. Screw in for more fuel (richer) screw out for more air, leaner.Running slightly rich is preferable to running too lean though.

[watty51]

Will give that a go tomorrow, and report back, hopefully a simple fix (heard that one before)

[Nurse Julie]

👍👍👍

[K2-K6]

Thanks for the quick reply Julie.
I will try adjusting all the air screws out as suggested, I thought they would have to be set all the same across the carbs
Would you suggest about 2 turns out and try from there

It's described quite well in many of the Honda manuals,  how to set up the idle circuits to get operating parity of combustion and not physical equivalent screws.

In effect they are acknowledging that there can be small differences in flow from carburettor casting and jetting variables that the screws are used to normalise,  effectively calibration of each circuit. 

In some of their later manuals it warns against changing the factory offsets,  or recording the position prior to removal if worked on.  Further it describes how the factory set them to allow you a method if they've been completely stripped.

It surprises me that this method that they were using in the 1960s is essentially what modern fuel injection systems do to control mixture,  tickover etc.  They were so advanced in so many subtle areas that are not readily seen.

[watty51]

Thanks for all the advice, and after loads of reading up on the subject took bike out for a 10 mile ride.
Before I went out I adjusted number 2 air screw out to 2 1/4 turns to try and stop the plug from sooting up and thankfully it’s a lot better (see photo).
Is this about the condition you are looking for.

The bike ran loads better, still slight hesitation at 6000 rpm but only slight.
Will be checking all of the spark plugs now, to see if I can get it bang on

[Nurse Julie]

It does look a lot better. If it were my plug, I would turn the air screw out by just a tad more, somewhere between then 2¼ and 2½ and hopefully that should improve performance even nearer to perfect. I found that once I had adjusted all 4, I had to lover the idle revs down by adjusting the big idle screw. You may or may not need to do this though.

[watty51]

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.

[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]

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.

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 

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.