MFI running rich I think?

[the911Tprocess]

My 72 T had been running reasonably well over the last few years with the only issue being an slightly wondering idle that was on the high end ( 950 to 1200 rpm).

The engine did not have excessive smoking (black) and actually was running pretty good.

This season when i took it out of winter storage, things were about the same then over time, the idle started creeping on until last week it was idling at 1500 to 2000 on average. Cant drive like that.

After looking at all the threads regarding MFI and using the CMA guide, I was confident enough to try going thru the CMA.

I've done that.

Set the main pump arm to 114 mm. Adjusted each throat flap so all are correlated (used the sync tool to set air flow to each throttle body to be same (all seem to be running at 9 to 10). And of course set the air screws to the standard 3/2 turns. Adjusted and replaced on the throttle linkage bars and tightened all the throttle stops as well.

The idle came down to below 750 and seems to hover around 500 to 750 rpm. Too low. The interesting thing is that at the tail pump there is black smoke at idle coming out. Not alot but definitely black smoke.

Based on the info I've read, it looks like the engine at idle at least is running rich. Over 1,000 rpm there is much less smoke or none at all. it runs great except while driving and is definitely alot smoother. At idle, it is running slightly rough, I guess due to the low rpm and the rich mixture.

The next step is

1. Do I adjust the main pump mixture screw (one below the allen key bolt) to run leaner. Based on CMA I seem to understand I should adjust this first then go to 2.

or

2. Adjust the idle mixture screw only to a leaner setting.

MFI expert opinion needed and much appreciated.

[mobius911]

I'll assume your comment that "it runs great except while driving" is a typo and you meant "while idling"...

I'm going through some of the same issues, and the sequence can get confusing, at least to me!

In my view there is a fair amount of iteration required in the whole CMA process, so you end up doing some steps several times. For example, you have to adjust the ignition timing (step 5) before you do the correlation (step 9). Correlation is the first place that adjusting the air bleed screws is mentioned. But ignition timing can only be done at the proper idle speed, which, as far as I am concerned, requires adjusting the bleed screws...

So, to answer your specific question, I would recommend:

1. Adjust bleed screws to get idle speed correct. The suggestion I've read is to "turn them all the same amount" if you've already synchronized at part-load. I would suggest going back and checking again at the part-load to make sure they are still synched. As I understand, if you have to choose between being perfectly synched at part-load or idle, choose part-load. If you can't get close at both, you're throttle bodies aren't adjusted well.
2. Check timing at correct idle (this may requiring going back and adjusting the air screws again since adjusting timing can change the idle rpm...)
3. Do part-load mixture adjustment using CO meter or seat-O-the-pants meter (make it "popping" lean, then increasingly richen until transition response from 2500-3500 is good). Idle speed may also change based on this adjustment...
4. Adjust idle mixture (which again may change the idle speed and require you to reset the air bleed screws...)

The reason that part-load is done first is that it's a sledge hammer compared to the fine adjustment of the idle mixture, so part-load setting is really dominant. Anything you do first on the idle first will get overwhelmed by your part-load adjustments.

Good luck!

Jeff Jensen
SF
RS 1282

[Early_S_Man]

Mark,

You haven't mentioned anything about spark plugs, ignition system checks or adjustments, which, along with valve adjustment, must be working perfectly according to specs BEFORE any MFI adjustment is attempted. The 10° range of advance operation where the vacuum retard module works [5° ATDC - 5° BTDC] is a very critical range regarding idle quality and speed, and is one reason I advocate disabling the vacuum retard and plugging the vacuum ports. The centrifugal advance mechanism in the distributor must be working perfectly, also, as verfied by plotting the advance curve from idle to 6000 rpm. Specs for your engine and a discussion of distributor maintenance can be found in the following thread:

distributor lube...

BTW, one of the worst things you can do to an MFI engine is dive in and start altering MFI pump and linkage settings before attending to all of the prerequisite work on other engine systems in the order presented in C, M, A!

[svandamme]

Originally posted by Early_S_Man

BTW, one of the worst things you can do to an MFI engine is dive in and start altering MFI pump and linkage settings before attending to all of the prerequisite work on other engine systems in the order presented in C, M, A!

+1

ask me how i know

[mhackney]

I've been fighting a high idle problem too and have found and fixed a lot of things ala C, M, A. One thing in your original post that caught my attention is that at a 1500-2000rpm idle, your microswitch-rpm transducer-stop solenoid should be kicking in and the engine hunting between 1300-1500rpm at idle. This will not have any effect on getting the idle speed correct. Do you get a lot of popping and backfiring on removing your foot from the accelerator? These things are easy to check and are outlined in CMA. Don't forget to check the fuse to the rpm transducer too.

Don't overlook your thermostat. Mine had a dead mouse plugging the tube running up to it! That will contribute to a rich idle.

cheers,
Michael

[the911Tprocess]

All
thanks for all the replies and great suggestions,

to answer some of the questions, i have carefully and several times gone thru each step as outlined in the CMA. I gone back a few times to make sure that my settings are still correct.

The plugs, wires, points, timing, etc are all spot on. I checked these twice to make sure and when I did make a change to the correlation of the butterflies I went back and checked everything else as well. All is within the spec's as outlined in the CMA.

Driivng the car , it runs great with no signs of rich mixture (black smoke) above idle, the only black smoke I observe is at idle so thats why I was asking if I really need to touch the main mixture screw rather than just going to the idle mixture screw.

Based on the recommendations, it looks like I need to work first with the main mixture and then using a CO meter adjust the idle mixture screw. Is this right.

All in all, before I used CMA the car ran worse, at least the car is running much better now, even thou the idle is a little low. Once I get to the next step I think I will be there.

My other question is, is it worth getting a CO meter or can I adjust the mixture using " how it runs best" approach as outlined in Rice's write up.

Thanks again, much appreciated.

[mobius911]

You can't having the timing set right if you're idling at 500 rpm, and you can't start messing with the mixture settings until you konw you're timing is correct. So I still say get your idle to its proper point by increasing airflow with the bleed screws. Since you know it's already rich at the current idle it needs more air anyway.

[the911Tprocess]

Originally posted by mobius911
You can't having the timing set right if you're idling at 500 rpm, and you can't start messing with the mixture settings until you konw you're timing is correct. So I still say get your idle to its proper point by increasing airflow with the bleed screws. Since you know it's already rich at the current idle it needs more air anyway.

Jeff, you are right, I will do what you are recommending first and go from there. Will report back sometime today, if it stops raining. Unreliable Cdn summer weather!

Thanks again.

[mhackney]

I have exactly the same rain problem here in Boston Mark! I've been trying to check the 6000rpm timing on my 70 S for the last 5 days and everytime I have the time, it is pouring. Maybe today I'll get lucky.

cheers,
Michael