What Does This Plug Tell Us?

My 97 Thunderbird 900 has been running g rough for a while now. Compression test was good. Carbs have been out and cleaned. New filter. New plugs. One of the coil packs has been replaced.

The right plug keeps oiling up. After only 3 or 4 runs of about a half hour each it looks like this.



Does this suggest anything to you?

I'm going to switch the coil packs around to see if a different plug is affected.
I wonder if it's a valve issue?
Maybe the carbs need balancing?

Thanks.

 

Spark plug test.
Does the spark look right?
https://youtube.com/shorts/-B-4U5ptj1A?si=T79OsFhD7baMN1-q

 

I have not.compared.it with the other. A good suggestion. I have Swapped the coil packs round though. If a different plug fouls we know its electrical. I've been in touch with Paul Messenger and may well get the stick coils if I can pin down this issue.

 

Bike starts well with a clean plug but gets progressively grumpier over a few days. Fouled plug. Riding to work and back today she pulls well but is a bit iffy idling and occasionally stalls. I wonder if she is running rich, particularly on that right cylinder. When I cleaned the carbs i didn't touch the fuel mixture or balancing. Something new to learn. I'll pull that plug when I get a moment to see if it the same one fouling even though I've switched the coil packs. My (latest) guess is it is not electrical.

 

Yep. To me too. Not wet. Google seems to think that is a symptom of it running rich.

 

Ok. Had a look at all three plugs today.
Interestingly, the most sooty one is now on the left cylinder and the dirtiest is on the right. Previouly the right side was fouling the most. I had swopped the coils round last week. One coil pack is new. The others do look pretty old. I did an ohm test. the new coil reads 10.88 k ohm
The others were 8.81 and 8.78 k ohm which seems a bit low.

Just to recap, the bike runs steadily worse over a few rides, becoming hard to start. Carbs have been cleaned and the bike has a new airbox filter. When I check the plugs one tends to be sootier than the other two. When I clean the plugs she starts fine again. After a couple of runs she gets lumpy.

I was all geared up to mess with the air fuel mixture, but this suggests the issue might electrical. Do you agree?

I could buy two more replacement coil packs as £25 each. Or I could stump for the £200 stick coil upgrade.?