SOHC.co.uk Forums > CB350/400

Sooty plug

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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)

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