Lowering Idle On Mfi (1970 S)

[denson]

The idle on my 1970 S is at 1300 rpm's. My mechanic advised that he cannot get it lower. I am not sure he understands MFI that well.

Anyone know what my problem could be? The car is also running rich and I am dumping a bit of gas into the oil.

Under the current conditions, is this a real problem if I continue to drive it like this?

Thanks,

John:
confused:

[boxster03]

check with Grady over on Pelican he gives excellent advice and knows MFI> It appears that high idle on the mfi is often a question and there is more than one reason for it. Not just a matter of turning an idle adj. screw.

[mobius911]

I second the recommendation to look at the Pelican site. there is also a link to a compilation of all MFI related stuff posted here at this site on the links page.

There is a book from Porsche called MFI Check, Measure, Adjust that is a critical reference, and the steps need to be followed in order. Most insiders call it CMA.

Here's a quick rundown:

The first step is to get the basic setup correct. This includes compression, plugs, ignition timing, dwell, and MFI pump belt timing. Then the MFI has to be adjusted. The throttle bodies need to be set so all the butterflies are properly closed at rest. You need to get the linkages set properly (there is required length that must be exact for the link from the pump to the large crosspiece). The other linkages need to be set to come off there stops at the same time. There is also a beginning spec for how much the air bleed screws should be opened. These are used to adjust the airflow at around 3,000 rpm. It's important to get the airflow the same to avoid excessive popping and spitting (but in my experience you will never eliminate these "endearing" characteristics completely). Once this is done you can check and adjust the part-load mixture adjustment for proper mixture (there are CO specs for this, but I've found I can dial it in without the meter).

I've told you all this because it all happens *before* adjusting the idle mixture and idle speed, and you really don't want to do idle without having all the prior stuff set correctly. There is an idle mixture screw (your setup may be different from my 73) to set the fuel mixture. The final step is to adjust the idle speed by using the air bleed screws. If you adjust the screws equally in/out they won't upset your part-load calibration. So for example, if your idle is high and you are running rich at idle, you would turn the screw to lean it out, and then dial in the bleed screws to reduce airflow. The process takes a few iterations to get the speed and mixture right.

So in conclusion I recommend getting the CMA book, and the basic tools (MFI adjustment set about $40, air flow meter, timing/dwell meter, etc.), and going through the CMA process step by step. I've decided to learn it myself because I agree with you that most mechanics are not expert at this. It seems complicated, and my overly simplistic description is probably confusing, but it's not that tough once you get into it, especially with help from board experts (not me, though I hope to achieve the level of "master of my own MFI system" someday...).

Jeff

[MJHanna]

"book from Porsche called MFI Check, Measure, Adjust"
Is this book still available from Porsche?

[304065]

http://forums.pelicanparts.com/showt...=mfi+high+idle

[LeafGreen]

The individual throttle levers must sit against the stop screws on the throttle body, and if not, check the hand throttle (lever closest to the drivers seat) isn't pulled up or mis-adjusted. Also check the linkage for interference and that the micro switch plunger moves freely.

[raspy2point2]

also if your stacks are worn out, the butterflies will not close off the air properly at idle so a little more air is getting by at idle than should and therefore your engine is wanting to go when your are at rest. Regards Chris Early S reg .#205

[Grady Clay]

Guys,

I’m flattered and Jeff is basically correct – download CMA and Search Pelican (my name and MFI, CMA and CMA2 will yield much) I have been tinkering with MFI since new in late ‘68. I am a proponent of YOU becoming your own best expert. It adds to the enjoyment and success of ownership of these very special cars. Us ‘old farts’ won’t be around forever.

Best,
Grady