Still trying to figure out the problem. How do my sparks look?

[rowan194]

Front (#1) is the leftmost spark, rear is the rightmost

I noticed that #2 has more gunk on its threads than the others, but the tip looks pretty much the same. #4 has a more pronounced burn mark on the white part below the tip. I replaced them all earlier this year, maybe about 8 months ago.

I still have the packet and was surprised to see they came with a 1.3mm gap; I re-set them today to just under 1.1mm.

I can hear a burbling chugging sort of noise coming from around the rear of the engine, it only became apparent a week or two ago, so I'm wondering if #4 has anything to do with it, or whether it's an exhaust leak. It's a bit hard to tell if it's the wind fanning rich exhaust towards me but I think the smell may actually be coming from inside the engine bay. I checked the exhaust manifold bolts and some of them did need mild tightening (about 1/8th turn) but nothing super loose. I haven't yet checked the flange where the manifold joins, or the split in the manifold.

I also pulled each spark plug in turn before starting the engine to see if it was a simple misfire, but deliberately running on 5 cylinders is definitely worse than my prob.

I had to top up the oil a bit today, can't remember ever having to do that, and it's only been 5000km since the last oil change, so maybe it's something internal this time. :(