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Strange spark plug test result
Athame57:
I'm not firing on my CB400Fs #2 piston. A look at the plug showed a fairy sooty result that was dry, the plug is new. I did the crude test of placing the plug on the cooling fins and hitting the go button to check for a spark. There was a spark but it came off the top of the electriod, not within the gap itself. Any ideas?
exvalvesetdabbler:
Carbon is conductive so the whole plug is now an electrode. Need to clean it or swap for a spare and retry.
Easiest way to clean a sooty plug is to heat the end up with a blow lamp to red heat and burn off the soot. Does less damage than a steel wire brush.
Any diesel residues still in the fuel system ?
Regards
Dave
Athame57:
--- Quote from: exvalvesetdabbler on October 04, 2025, 01:03:41 PM ---Any diesel residues still in the fuel system ?
--- End quote ---
Spark a little better I think but piston didn't fire. This is the carb that the garage put new seals in, maybe I need to clean it again, but why would one carb be more affected than the others? Could it have been because this was the cyclinder that needed reboring more than the others? ( I just had a rebore courtesy Bran)
McCabe-Thiele (Ted):
Cant see it being anything to do with the rebore.
If the leads are long enough try swapping the two cylinders to eliminate a HT coil lead issue.
Athame57:
--- Quote from: McCabe-Thiele (Ted) on October 04, 2025, 02:28:41 PM ---If the leads are long enough try swapping the two cylinders to eliminate a HT coil lead issue.
--- End quote ---
Alas, it didn't work either. :( Maybe I'm going to have to face the hell of putting carbs back in ::) but not today anymore.
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