What's your take? Approximately 2500 miles on these plugs. Car is running great, good power, good economy, doesn't seem like it's getting too hot. CO is set to 2.5% at part load. From what I can see, they look good but on the lean side. Top row is cyl 1, 2, bottom row is 3, 4. Ignition is a Crane XR3000 with a Crane PS60 coil.

I'm inclined to leave it as it is.

Click to view attachment

By this matrix, I'm too lean. Hmm, maybe I'll try bumping up the CO by 0.5%

I think you answered your own question, but my aircooled experience is limited. I would normally like to see a little color on the insulator. I'm interested to see what more experienced people say.

Yeah, I agree, the more I look at that matrix, it's too lean. No wonder I get such good gas mileage! Runs fine, but it also explains why I have recently found that I need to adjust the idle mixture richer past the factory mark on the ECU knob to get the best idle characteristics, because the main mixture is too lean. I will experiment with 0.5 and 1.0% richer (which would be 3.0 to 3.5%) for the part-load mixture.

One thing to keep in mind when deciding to check plugs: They will change rapidly depending on driving conditions.

They guys at the track that are serious about checking them will do a hard run, then shut the engine down as they coast into the hot pits.
Then they either check them there or get towed back to the paddock.

Just a few seconds of idle or casual driving can skew the results.

Our plugs are such a pain to get to frequently, I rarely check them.
I have an AFR & EGTs to help me instead.

Stu, agree, plugs can be misleading. I pulled these from a cold engine in the morning, after I'd done a 20 minute drive the night before, I think they're reasonably representative. Plus, it makes sense in light of what I noticed about my idle mixture. I'll bump it up a bit and see what they look like after a similar run. I'm just glad I haven't melted the engine into a pile of slag .

I've heard plug reading became obsolete with unleaded fuels. That said, yours look perfect Brad.

Probably right, plus all the additives and even ethanol. It sounds like A/F meter is the only way to know for sure.

I think you are near right on the money.

Don.t know when those charts were made.

So, I have a MPS in the car that I'd removed the epoxy from the full-load stop so that I can adjust it. I'd adjusted it down to a part-load CO of 2.5% some time back. I pulled out my EFI Associates bench D-Jet tester, and took two NOS 0 280 100 043 MPS's I have and profiled them. The simulation conditions were engine speed 2500 rpm, throttle switch open, and temp sensors at operating temperature. I measured the injection pulse width at 0, 5, 10, and 15 inHg of vacuum.

The MPS in the car profiled lean in comparison, with 0.1 ms less duration at 15" and 10", and about 0.4 ms at 5". I adjusted it to match the NOS units as closely as possible, then re-installed it and clicked the ECU idle mixture knob 2 clicks leaner. From some tests I did a couple of months ago, that should correspond to a decrease in CO of about 0.5 to 1.0%. I also verified with a my bench setup that 2 clicks leaner is 0.1 ms lower injection pulse width, which was the delta I adjusted the MPS to in a similar manifold vacuum range.

Did a 30 minute test run, I only had to make minor adjustments in the air bleed screw to get a good idle. Power and driveability were good. Will try it out on a longer run to see if that got it closer to the correct mixture, and will check out the plugs when I get a chance.

I suspect the comments regarding cleaner fuels are true, plus the higher temperatures of an air-cooled motor, probably shift the plug readings to a bit leaner/hotter, so I think with this adjustment I should be OK.

Oh, I almost forgot. A shout out of "thanks!" to Chris Foley at Tangerine Racing, for the great "MPS Tuning Kit" and stop screw (full load stop). I'd bought a full kit from him some time ago, including the diaphragm, but as I've had good MPS's since then, haven't done a rebuild. But tonight, when I was doing the adjustments, I used the "tuning kit" which was really awesome!

The problem the kit solves is that you first adjust the inner main mixture screw at 15" and 10" of vacuum, then from 5" to 2", you adjust the outer screw to get the transition to full load correct. Problem is that when you turn the outer screw, the inner screw turns, too, and you mess up your initial main mixture adjustment. Enter Chris's kit, which has a tubular wrench to adjust the outer screw, which allows you to run a screwdriver inside of it to hold the inner screw stable. Works like a charm!

I also appreciated the drilled-out full-load stop, which has been milled for an 8 mm hex key for adjustment. This setup allows you do all three adjustments without leaving the diaphragm unconstrained by the full-load stop. Plus, it has a nice vinyl cap to keep out the dust. Way better than what Bosch ever envisioned! Thanks again!

http://www.tangerineracing.com/mpsdiaphragm.htm