What's the "Right" Part-Load Mixture?, Looking for inputs and experiences - OPEN DISCUSSION Options

[pbanders]

I'd like to get a discussion going on what the "right" part-load mixture (for a fully-warmed up motor) is for different engine configurations. Why? Because for D-Jet and other systems, this is the most "fundamental" mixture adjustment, one that all of the other conditions (e.g. warm-up, acceleration, full-load, overrun, etc.) are all affected by. It's also important from a fuel economy and emissions perspective, as part-load is where the engine spends the majority of its time in normal operation.

To define what I mean by "part-load mixture", I'm talking about the CO level as measured by a shop-quality analyzer on a fully warmed-up car either on a dyno or running on flat road, at a small, constant throttle angle, with engine speed between 2000 and 3000 rpm (pick 2500 rpm if you want an exact value). Air temperature around 70 deg. F, near sea level.

I recently (last 2K miles) had my 2.0L rebuilt. Only mod is Euro pistons, otherwise, it's mechanically bone stock, including the cam, outfitted with D-Jetronic and a Crane XR3000 ignition set to standard timing specs. I initially set it up with an NOS, unmolested Bosch MPS. What I found was that the part-load mixture under the conditions I described above was about 4.5%, quite rich.

I have an MPS that's in good condition that I have removed the epoxy protection from the full-load stop, so that I can remove it and adjust the main mixture screw. I did some experimentation and found from road testing (with my CO meter attached), that I could lower the part-load CO to 2.5% while maintaining good driveability. If I went any lower than this, I began to have lean mixture issues, such as poor on/off throttle response. I did a number of abrupt throttle openings from part-load, and didn't hear any evidence of pre-ignition (knocking), either.

The most obvious change I've noticed since this mixture adjustment is that my fuel economy has jumped to about 26 mpg in normal urban driving. I don't have oil or head temp gauges. I intend to do some oil temp measurements (thermocouple down the dipstick tube), but the car doesn't seem to be running hot at all. I also will yank the plugs the next time I get a chance to see what they look like after a run under part-load conditions (tips on the "right" way to do this would be appreciated).

My question to the group: is this a reasonable part-load mixture level? Is there a better method for determining the "right" part-load mixture? How do the guys out there with carbs, L-Jet, and aftermarket FI setups determine their correct part-load mixture? Is Jake posting here any more, I'd really like to hear how he does it.