Spark plug reading question(Miata)

I’ve developed a hesitation when hot on my 90 Miata. My hunch is I may have a small crack on my $70 eBay header, but I’m in the process of ruling things out, so I pulled the spark plugs to check them.

The plugs & wires have just over 700-miles & 12 autox runs on them. Here are plugs 1-4 in order. 

You’ll notice plugs 1 & 4 are darker than the rest, while #2 is almost completely white still. Any idea why? Is it something I should be concerned about?

Ok, slight update: I took it for a drive & got it up to temp. I’m not sure “miss” best describes the problem, but it’s definitely down on power under acceleration. Ambient temps are lower than they’ve been in weeks, so that doesn’t seem to be a factor. 

I also don’t think I have an exhaust leak. What I’d heard as a “putt-putt” sound actually seems to be coming from the p/s pump...and I seem to have p/s fluid disappearing, so maybe the pump is going out? I don’t think that would cause random power loss under acceleration though?

So I’m not sure what would be causing my power loss? Nor do I have any idea where my p/s fluid is going either?

Cylinders 2&3 appear to be lean. 1&4 look correct for a proper mixture. I would investigate why 2&3 are lean.

In reply to Don49 :

Thanks Don, that’s definitely a clue. 

Oh, one other thing that might be a clue: I get a slight “pop” out the exhaust on every up-shift. 

I'll second that 2 & 3 are lean. 1 & 4 are as they should be. 

I would guess dirty injectors and start there. Dump a bottle of Seafoam through a half tank of fuel and check it again.

Another worse possibility is a head gasket. Could be dumping a little water in those cylinders and steam cleaning everything. 

In reply to Toyman01 :

I unplugged injectors 2 & 3 and sprayed them with contact cleaner. Number-2 had a bit of crud in it too. 

I just realized cylinders 2 & 3 share a coil, but if it was misbehaving intermittently the plugs should be wet, correct?

In reply to Pete Gossett :

Maybe not, the coil could just be weak and breaking down at high loads. A crappy ignition will fire at idle and low loads but puke when the cylinder pressures increase at WOT or under heavy accel. That could also wash the cylinder and plug.

Agreed, running lean.   If I remember correctly, wasn't there a minor issue running a plug to the coils?  Might be a connection issue.  Maybe back track to that, and go forward from there.

Even under the best conditions (properly done) reading plugs with modern fuels is a waste of time, at least for fueling.

In reply to Toyman01 :

Autozone over in Gulfport had a coil assembly, so I picked it up & will install it later once it cools off. It sure feels like an ignition problem to me, but the spotless spark plugs were throwing me off. 

My vote goes for coils, since you're seeing the same thing in both shared cylinders.  If it were injectors, odds are against you that two started failing the same amount at the same time, and those two happen to be your shared coil cylinders. 

Swap the coils, go for a full (up to temp) drive and see if the color follows it.

Edit: looks like you were posting at the same time as me :)

In reply to Dirtydog :

You are correct, but it was my own ignorance in not realizing that part of the connector had broken off & was still stuck inside the plug. 

Todays gas does not put color on the plugs like the old days.

A clean plug with a shadow of grey ash is normal for todays fuel.

The 2 end plugs got a lot hotter than the others, look how far the heat comes up the threads on #4.

Detonation on the 2 end plugs?

Too many runs on those plugs to say more.

I replaced the coils & took it for a drive. When I was pulling off the old coils, on of the potted connectors cracked underneath, so it was probably a good thing I replaced them anyway. Unfortunately it didn’t fix the problem...

Then I started thinking about when I first noticed the symptoms - about 20-miles after filling up on the way home from the autox a couple weeks ago. At a gas station that was a little sketchy(I wasn’t sure they were even open when I pulled up). 

Since I was down to almost 1/2-tank I went ahead and filled up a Shell today, and on the way home it seemed to start running better. So maybe it was just bad gas?