SOHC.co.uk Forums > CB500/550

Sooty plugs.

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McCabe-Thiele (Ted):
I'll fess up, I've not had the plugs out of my 400 since I finished the rebuild. It runs well, no hanging throttle, no flat spots, good idle. It's only done 400 miles, there is no black smoke out of the exhaust and the end of the tail pipe is not heavy with soot. I see no reason to remove them until the engine has done at least 600 miles unless it starts to misfire, have starting problems, excess smoke, oil consumption, or a cold cylinder.
 
Spark plugs are designed to effectively self clean, on a cold engine with choke the plugs will be pretty black, once the engine is hot then plugs will burn off the carbon reaching the right colour. As you know  ideally you might want to look at the plug colour when the engine is hot and has been running 4k revs or higher for a few miles. By the time you have reduced the revs,idled for a minute, then reached home the plug colour will not full represent the situation.

On my 500 due to the fuel pipe fiasco and having to take the carbs off again I checked the plugs they were pretty black. Now I have the 500 carbs back on, with it now running evenly, I hope I can put a few  hundred miles on her before I need to check the plug colour unless I develop an issue.

What I'm getting at Johnny is don't get too worried about plug colour alone, it's not a very precise diagnostic tool imho unless you have other running issues. It's not as if these old Honda's are modern lean burn designs with injection & O2 sensors.

Now you are happier with how it runs, I would be okay with that plug colour, I would worry much more if one was very lean.

Johnny4428:
I agree Ted that if everything is running fine no need to remove the plugs however I twice lost a cylinder because of excess “sooting”. This prompted me to return to the fuelling issues, only this time it seemed to be getting excess fuel. It dawned on me then that by increasing the size of the pilot jets to improve starting had an impact of course on running from idle upwards too. It seems now that I have achieved an acceptable runner by tweaking the idle mixture screws.

McCabe-Thiele (Ted):
That's a good outcome then Johnny are you back on the original 42s.👍👍👍

Johnny4428:
No Ted still on the 45’s.

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