1973 911E MFI adjustmet

[Jeff777]

Hello all

I have the above car and just had the engine completely taken apart and reseal done with tuning of the engine by a very qualified tech. It now runs great except for low rpm/low gear when it is difficult to accelerate slowly without it bucking and coughing/backfire. If I hit the accelerator it clears. Otherwise car runs great...solid idle at 1000, starts well and very strong pulling. The tech used a gas meter to set the CO at 2.7%

I have gone through the CMA and am now at part load test and adjustment. Just bought an Innovate LM2 and have it hooked up.

So, my question is how to do the part load test? I know it's 2nd gear, 2400 rpm but the rest is pretty grey. Is the test done on level road, downhill with brakes or uphill. Any advice much appreciated

Cheers
Jeff

[30westrob]

I have never had the luxury of an exhaust gas meter, but I have had good results with trial and error. It sounds like your part load is lean. I would start with two clicks rich, and drive the car. If the lean diagnose is correct you should see an improvement and go from there. Always record all adjustments so you have the confidence that you can return home, if needed. You can probably make a few adjustments to part load, without having to adjust the rack adjustment. Rob

[nickelplated5s]

There's a reason 'qualified techs' don't touch MFI's, mine won't. Read the Rice paper. Air screws and a snail air flow meter are easy. Listen to 30west if you want to mess with the rack. There are only 3 people, companies, I know of for rebuilds. Mines been in since April 2025 and I should get it back next week or so. Then it's off for dyno work. Remember, MFI motors run what is considered dirty these days. Perhaps 12 MPG if you're on the throttle.

[PeteKz]

Jeff: Do yourself a favor and install a AFR meter. Then you will know what's going on while you are driving it, and what needs to be adjusted.

[stownsen914]

Once you have the AFR meter hooked up, just observe the AFR during the conditions when the car isn't running well, and make adjustments to make the problem(s) go away. Low RPM / low load in your case, sounds like.

There are two external enrichment adjustments on the MFI pumps. The main circuit adjustment is the one under the allen head screw. The "idle" adjustment is the spring loaded hex head thingy on the rear facing end of the pump. The idle adjustment actually covers lower RPM and part throttle usage, analogous to the idle circuit on Webers. From what you describe, you may need to adjust your idle enrichment. Remember that for the idle circuit, to enrich, press it in to engage and turn clockwise (note that this is the opposite direction of the main circuit adjustment). You'll feel as the detent clicks.

Your AFR meter will tell you if that's the adjustment you need to make.

[nickelplated5s]

Following this for the obvious reasons above and just reread the Rice paper for the umpteenth time. It's available here and someone has a picture of all the internal pump bits too, yikes. The 'qualified' shop which I will not name touched mine a few years ago. Didn't even attach the fuel cut off. Yup, you get flames on a hard decel lift that way. Had the motor out this spring for trailing arm bushings (different shop) and thought, send the pump in too. Probably 93K on a 69 that I've owned around 43 years. Bite the bullet and get shop time lined up or go carbs. Probably a wash cost wise for what you have versus carbs.

[john walker's workshop]

Probably not set rich enough. They like rich.

[nickelplated5s]

Hence the flames John. I didn't like the 1400 RPM idle they sent it back with.

[Jeff Higgins]

While using a gas meter to check and adjust CO2 is what is called out in CMA, we have better methods today. The gas meters were all they had when these cars were new so they really had no choice.

With your LM2 wideband O2 meter you won't be performing the part load measurement as described in CMA. There is really no way to do that with that instrument. So, as others have said, just drive it around. I like to run through a variety of test scenarios, from steady state cruising at various RPM ranges to full throttle pulls through the gears. One of the most valuable that I used was finding a steep enough, long enough hill to pull from idle to redline in, say, third gear. This pretty much replicates what we do on a chassis dyno.

What you are looking for is what we call "lean worst running" under load. In other words, the leanest A/F reading displayed at full throttle or part throttle with as much of a load on it as you can provide. This is the "tent pole", or "bottom line" item on these systems, where it can get dangerous and damage the motor. I prefer to keep this at 13:1 at the absolute most lean condition.

With a stock motor and stock pump (unaltered space cam), this may be the only number you can actually use. From here everything else will just fall where it falls. You will see operating conditions under which it will go a good deal more rich, but there is nothing you can do about that. You will also see some conditions wherein it will go a good deal more lean, but that will be at trailing throttle, coasting down a hill, lifting momentarily while cruising, etc. Don't let lean numbers here concern you, they won't hurt anything. Too lean under load is what hurts things.

Prepare to be surprised at just how widely the A/F ratio varies with this system. A full two to three points is not uncommon. We just had no way of knowing prior to wideband O2 measurement systems.

Also be prepared to be disappointed in just how little that idle/low speed mixture screw affects anything. The main rack adjustment simply overwhelms it. Just concentrate on the main rack adjustment and pretty much leave that low speed screw alone. All it really does is change tension on the tiny little spring in the center of the spring capsule, which allows for an infinitely small change in how far the space cam moves fore and aft as the clamshell weights open and close. The stylus remains on the same general area of the space cam, where it has a very gentle contour change, so not much happens by way of altering the A/F mix. Adjusting the main rack has a far more profound affect.

So, drive it around, get some numbers, adjust the main rack. It helps to have a willing accomplice to read off numbers as you go. That goes a lot quicker than recording, going home and downloading to your computer, and pouring over the graphs. Good luck.