Quick mfi question
 

1973 MFI .... assume everything else is spot on ... what is the exact relationship between the idle rack and the main rack

For example, if I was to lean off the idle rack what knock on effect if any would that have on the main rack ?

From what I have read, the idle adjustment helps from idle up to 2200-2600 rpms. I write that because I have read a few people state its in between those numbers. And that was years ago. Now I'm sure someone has spent time analyzing data and will chime in with an exact rpm number. So once you get the main rack adjustment setup properly, you fiddle with the ide and most likely will come to the conclusion that it doesn't do much. But if and when you do get it right, it supposed to help with the transition from off idle and up to say 2600, or where ever. Then the main rack adjustment takes over fully. Lean the idle to your hearts content, just write down the clicks so you know where you are at. I found that it doesn't do much.

These may be the threads the above post is referring to ...

http://forums.pelicanparts.com/porsche-911-technical-forum/386414-mfi-pump-open-heart-surgery.html?highlight=open+heart

http://forums.pelicanparts.com/porsche-911-technical-forum/617039-mfi-open-heart-surgery-iii-reworking-space-cam.html?highlight=open+heart

(Installment II appears to have been for the electric fuel pump in case you're wondering)

Just a few things to help you, hopefully.

First off to make sure you are aware and everyone is on the same page, there is only one rack - it refers to the position of the rack (as, in, rack and pinion) that controls the actual volume of fuel discharged by the pump. If you make a change to it the fuel volume will change by the same amount over the entire range of the pump. Idle, mid, high speeds and all throttle positions.

The adjustment for the main rack is inside the pump, behind the 6mm allen head capscrew just to the upper right hand of the stop solenoid. (That's from memory I would have to double check that as I don't have a pump in front me.

There is no idle rack, only an idle adjustment screw. This is the one that is engaged when you press in the spring loaded plunger on the outside of the pump. DON"T touch it with the engine running, BTW!!

The idle adjustment screw changes the amount of preload on a spring in the governor capsule. So, more preload = less movement of the stylus on the space cam, resulting in more or less fuel being injected.

In theory, the force required for the governor to compress the idle spring will affect the overall movement of the stylus long after idle speed had been exceeded, but due to the exponential nature of centrifugal force, by the time the pump speed has gone from, say 450-500 RPM (half of crankshaft idle RPM) to say 2000RPM, the contribution of the idle spring with only be a 20th of the overall resistance to governor flyweight affect.

Short answer - no relationship. Fiddle with the idle all you want, but in small measures. From all indications when people start to fiddle with the pump it's because something else in the system has gone off.....

The other way to lean or richen is adjusting the main linkage to the MFI pump. this is really easy to do and undo but affects the entire rpm range. Before you start messing with it, do you have an ARF meter installed? If yes, watch the gage or connect to laptop and log the results.

Opening the idle adjustment screws on the stacks will lean it at idle also. Use a synchronizer to keep them balanced.

About a million adjustments can be made in different ways. Keep track of them so you can return to baseline if necessary.

Chris
73 E

Thanks all .. great info and just what I was after. Was wanting to get a better understanding of what each adjustment would do.

I have dual afr gauges fitted so can monitor any changes I make.

At the moment it’s running too rich at small throttle openings eg when cruising etc. Wide open throttle it’s all good.

Sometime this year I’m going to be checking the throttle bodies and re tuning them , then I might play with the mfi pump

"The other way to lean or richen is adjusting the main linkage to the MFI pump."

This is bad advice! Follow the CMA manual.