Running rich, part 972

My Miata has been running rich. I think it has, anyway. It'll backfire if you poke the throttle after letting it coast at middle-high RPM for a second or two. I can do this on command, anytime after it's warmed up. Often on upshifts, too.  Also, the plugs looked pretty sooty last time I checked (and replaced) them. And this soot ends up on the back bumper, too. This all leads me to believe it's running rich.

So I've been trying to figure out why. I've done a lot of things (plugs, plug wires, coolant temp sensor, O2 sensor, tested the coil, cleaned up electrical ground wires, dumped magic cleaner juice in the gas tank many times). This is the latest, and probably the last thing I'll do before sending my injectors out to get rebuilt.

I replaced the air filter installed by the previous owner. It's very obviously an eBay affair, you can find them all over there for around $10 (with another $30 to ship or something stupid). Anyway, it's a chintzy model, and I thought maybe this piece of shit was clogged up. The service manual says a clogged filter is the first thing to check, so I'm glad I did it right away. And by "right away" I mean... well, read the last line in the previous paragraph, my attention-span-challenged readers.

Anyway, I never liked that filter because I could do this:

Also, I could spin that rubber piece in the filter. This is hardly confidence-inspiring in terms of providing a seal against shit. Thankfully, area parts stores sell this kind of chintzy shit. And since a 3" flange is common, I found a direct fit without having to use adapters. It's the Spectre brand they sell at Autozone, in the same place you'll find the DIY intake kits.

It looks a bit better than the dirty blue thing, though very ricey. It also did nothing to help my running rich condition.