MFI Protractors (Or Lack Thereof)

[Jeff Higgins]

I have looked everywhere for a set of protractors for my MFI. I have the three piece tool set for mixture adjustment on the pump, but just can't find the protractors. Does anyone know where to get a set?

Since I don't have the protractors, and my car was idling quite rough, here is what I tried as a stop-gap measure. I set the rod lengths specified in CMA (the three 114mm's from the cross shaft to the pump and #2 and #5 throttle bodies). I then ran the car at 3000 rpm with the air bleed screws all 1 1/2 turns out and measured air flow through the #2 and #5 stacks. Since CMA allows plus or minus 5mm on the rod lengths to these stacks, and says to adjust the left (#2) rod length to relieve any tension or pre-load in the system (pump lever was on the idle stop), I used the #5 air flow as a baseline. Rather than using the air bleed screw, I adjusted the left rod length to #2 for both no pre-load at idle and to match the airflow through #5 at 3000rpm, at least as close as I could get it with rod length. I then adjusted the four short rods, to #'s 1, 3, 4, and 6 to match that baseline air flow through #5. I got them all as close as possible with rod length, adhering to 114mm rack to pump and rack to #5. I did the fine adjustment with the air bleed screws. I then adjusted the six individual throttle stops to just make contact with the arms on the butterflies. In other words, I used air flow to establish the shorter rod lengths. All six butterflies go to full open at full throttle, and appear to be at least visually very close to one another through the whole operating range. All of the bushings on the shafts are slop-free, and I cleaned out the air bleed orifices. There are no vacuum leaks anywhere that I can find.

The cars runs and idles better than ever. This eliminated the rough idle and occasional popping through the stacks I was putting up with before. Overal air/fuel checked under load through the full rpm range on a chassis dyno runs about 14:1 (kind of lean?) up to 4000 rpm, and then slowly richens up to about 13.5:1 @ 6000 rpm and finally 13:1 at 6700 rpm. This seems reasonable to me, but I realize it represents an average of all cylinders. The questions of the day are: Could I have a lean cylinder in the mix? Is this even close to a reasonable approach, or am I all wet?

[Tyson Schmidt]

Sounds just fine. You did a great job! Critical thinking always helps. I think you were playing with fire messing with the stop mscrews on the individual butterlies though. They shouldn't be messed with. But it sounds like you got lucky.\

I'd fatten the mixture up one click on the main rack, and call it good. Put it on a machine that can check HC. That will tell you if it's too lean on one cylinder. You'll get a lean misfire, which will show up as high HC content. But if all the airflows are very close, then no need to worry, since the MFI delivers the same amount of fuel to all cylinders, provided the injectors are all clean and operating properly.

[Grady Clay]

Jeff,

It looks like you have a good handle on things. Now is where you could use EGT or individual cylinder exhaust measurements to farther fine tune the system.

What are the most recent CMA numbers?

What are the relative air flow measurements at idle and just off idle (1500 RPM with the hand throttle)?

The first thing would be to install some cool plugs and do some road testing at part throttle. If you use hot plugs, they won’t show the mixture as well. Have several clean sets. A WOT run will clean them if necessary.

Use the plugs to identify a lean cylinder and the exhaust analyzer to keep everything in the ball park.

Best,
Grady

[Jeff Higgins]

Thanks for the reasurance, guys. There are a couple of things I didn't mention in my first post, as I thought it might have gotten confusing enough as it was. I'm glad you two were able to follow along and make some sense out of what I was trying to say. Anyway, the couple of things I left out: I don't have an exhaust gas analyzer; I know I eventually need to get one to properly set the two mixture screws in the pump. I figure it must be close, as the car has always run well off idle and under load. It's just never idled that good and has popped through one or two stacks at idle. With the rod lengths as they were before I started, the air bleed screws did not have enough effect to bring all cylinders into synchronization. I noticed I could get a couple of points of airflow variation with the screws, and I did use them for the final fine-tuning, so they are open and effective. A couple of different air/fuel ratio checks on a dyno have been in the 13-14:1 range, so any mixture adjustment at the pump that would affect those readings and overall drivability must be close. I have done a couple of spark plug checks, but never with cold plugs, just whatever is specified for the car. They always look like it might be on the rich side, but the air/fuel ratio says otherwise. For now, then, I don't have any numbers from the exhaust gas check portion of CMA. That sounds like the next thing I will need to do just to be sure of what I've done. I'll have to turn up an analyzer. The air flow numbers on my synchrometer were somewhere around 14 at 3000 rpm. I can't remember the number at idle, but I did check that they were the same there as well, and they were. I didn't think to check at 1500, so I will have to do that. Right now, though, the ground outside is frozen and snow is on the way - unusual for Seattle. I may have to wait awhile before the weather allows driving it enough to warm it up for these final checks. I was just hoping for a little peace of mind for now, and some reasurance that I probably didn't totally mess it up. You guys provided that and some more stuff to look at when I can. Thanks.