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]

Any diesel residues still in the fuel system ?

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]

If the leads are long enough try swapping the two cylinders to eliminate a HT coil lead issue.

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.

[exvalvesetdabbler]

When the rebore was done, were the valves checked and also clearances set properly upon re assembly.?

To soot up plugs it's either getting way too much fuel or burning something else to create the soot. If it's not firing it would be blowing out fuel from the exhaust, ie smell of petrol.

All choke flaps opening properly?
Float heights correct?
Compression on each cylinder?

Does the no.2 header pipe feel cooler than the others after a short fire up?

Just as a side note, your spark may look to  be a bit weaker when cranking on the starter because the battery voltage drops, pull the lead of 1 or 4 and test the same suspect plug on the other  coil.

 Other than that I would be looking to see if fuel is getting  past the o ring on the main jet.

Regards
Dave

[K2-K6]

Swap the #2 plug with #3 .... then run it to assess.

Both of those plugs run on the same coil .... if the problem remains where it started ... likely the carb is next suspect. That's assuming you already swapped  2&3 plug leads with nil effect ?

[Athame57]

When the rebore was done, were the valves checked and also clearances set properly upon re assembly.?

I had a different garage do those at the same time as I had them fit the header pipe properly. Bike was great for a day and now this. I expect I'll be heaving the carbs out. 

[McCabe-Thiele (Ted)]

That diagram is all wrong.

As you sit astride the bike Cylinder 1 is on the far outer left, then its cylinders ,2,3,4.

[Ken4004]

I had the same issue last week one cylinder not firing turned out to be the carb putting too much fuel into the piston so carbs out and changes the float fuel valve that wasn’t closing properly and flooding the carb.
After replacing valve and reassembly started up with no issues.

[Athame57]

That diagram is all wrong.

As you sit astride the bike Cylinder 1 is on the far outer left, then its cylinders ,2,3,4.

Right, I'll delete before it does anymore harm!

[Bryanj]

I only had the block so no idea about valves