MFI Mixture Control
 

I have a 1973 RS replica running a 2.7 engine on MFI. It has all the right bits and runs very well but it is running too rich. It gives about 7% CO and the plugs get a bit sooty. I have adjusted the usual mixture control with no success and have a feeling that someone has been there before me and adjusted the main rack. My question is, which way ( clockwise or anticlockwise ) do you turn the adjusting screw to alter the setting of the main rack. How much effect would, say, half a turn make. Or should I leave well alone.

Many thanks Nick White ( based in the UK )

Nick:

Before you do anything else, download a document called "check measure adjust." This is the BIBLE when it comes to making any MFI adjustments and must be followed to the letter.

Anyway, the answer is, turn to the right for the main rack to lean, to the left to enrichen. Remember the phrase "I Lean Left" for "idle screw lean to the left-- and then remember that the main rack is the opposite of the idle rack, so the main rack is lean right, rich left.

I suggest also that you do a search here using the term "MFI" which will reveal a few dozen threads and everything you could ever want to know about MFI. Armed with that technical basis, there are some genuine experts here to help you.

Good luck!

Dear John
Many thanks for your reply and sorry for my ignorance but where do I find the document you suggested. Is is part of the pelican parts site or elsewhere?

Thanks Nick White

To add a bit to John's good advice: 2-3 clicks in any direction will make a noticable difference in a car that is already running pretty well. Start with that and keep good track of what you did so you can get back to where you started if need be.

Also, 7% is not that bad for an MFI car, especially if you got that number at idle. If it runs well through the rev range, you may try adjusting the idle first. A really fat idle could dirty up some plugs quick in around town driving. On the idle adjustment, it takes many more clicks to make a difference than on the main adjustment.

Last, what heat range plugs are you using?

Paul

5%-7% makes for a happy MFI car!

Here are some links below.

Everyone you talk to will stress the LAST thing you do is adjust the mixture. Yes, you should adjust it and keep track of the changes during the process. It is very common for there to be other issues and someone tries to “adjust the MFI.” All this does is to get you farther from a solution and many times causes more problems.

Read and study these threads. You need to become your own best expert. It isn’t difficult. Start yourself a printed notebook with CMA and add & organize more. The simple process of organizing your own MFI manual will help get you started maintaining your system.



Check Measure Adjust (CMA)
http://www.scatliff.mb.ca/pelican/MFI_Check_Measure_Adjust.pdf

“MFI Size Chart”
http://forums.pelicanparts.com/porsche-911-technical-forum/117171-mfi-induction-chart.html

“MFI thermostat spacers”
p. 1 end has CMA extension, p. 3 has nozzle, fuel flow, and fuel pressure.
http://forums.pelicanparts.com/porsche-911-technical-forum/169445-mfi-thermostat-spacers.html

“More Fun with MFI and LM1”
How to use protractors for correlation.
http://forums.pelicanparts.com/porsche-911-technical-forum/179707-more-fun-mfi-lm-1-a.html
“More MFI”
http://forums.pelicanparts.com/porsche-911-technical-forum/98441-more-mfi.html
“MFI Problems”
http://forums.pelicanparts.com/porsche-911-technical-forum/173700-mfi-problems.html
“MFI Speed Switch”
http://forums.pelicanparts.com/porsche-911-technical-forum/160057-mfi-fuel-shut-off-solenoid-questions.html
MFI tools
http://forums.pelicanparts.com/porsche-911-technical-forum/170242-mfi-tools-diyer.html
“MFI electrical”
http://forums.pelicanparts.com/porsche-911-technical-forum/197806-different-mfi-thread-electrical.html
“'72 Hose from engine tin to MFI WUR - what material?”
http://forums.pelicanparts.com/porsche-911-technical-forum/201734-72-hose-engine-tin-mfi-wur-what-material.html#post1715584
trouble shooting “Grady/ Sherman(72 Guys) Ok now what?”
http://forums.pelicanparts.com/porsche-911-technical-forum/203301-grady-sherman-72-guys-ok-now-what-post1729700.html#post1729700

“Cleaning MFI Throttle Bodies”
http://forums.pelicanparts.com/porsche-911-technical-forum/152206-cleaning-mfi-throttle-bodies.html

Motor break in with MFI
http://forums.pelicanparts.com/porsche-911-technical-forum/246015-motor-break-mfi.html

“Mechanical Fuel Injection” site with OK stuff:
http://www.my930.com/MFI/TipMFI.htm


If any of these links don’t work or you find more/better, please post them for everyone’s benefit.

Best,
Grady

Many thanks everybody for all the advice. Will look at all the advised stuff and take it from there.

ATB Nick White

try denso irridium plugs. thy don't foul easily. i usually go somewhat hotter than the "book" heat range, due to the crap gas these days. i doubt that any of the plug charts have been updated for today's gas. remember when it used to leave the plugs clean and the tailpipe light gray?

Nick,

By the way, welcome to the Forum. You will find a lot of MFI and other information here. Perhaps the best in the world.

Please feel free to question everything. This Forum is at its best with everyone questioning a previous answer that you don’t fully understand.

Tell us more about your 911. (Some images would be nice.) What are all the engine details? What do you know about who set the MFI to run 2.7RS? What are the heads, cams, ignition & advance curve, throttle bodies, intake stacks, etc.? What pistons and cylinders? What compression ratio? Stock ’73 exhaust system? All these details are important to get your MFI and engine correct.

Where in the UK do you live?

Best,
Grady

Dear All

Thanks for all the advice re the MFI injection system on my car. Will not do anything for a while as she is running well and seems to have a fine engine. As for the engine it is virtually brand new. It has the correct 2.7 barrels / pistons which came from the USA, the heads are reconditioned and the cams are genuine S cams ( I believe from a 2.4S ). It runs the standard plastic throttle bodies and a full SSI exhaust system. The car itself was a 1971 non sunroof 2.2T model but now looks very close to 2.7RS with a lightweight interior and genuine Recarro seats and correct bodywork. The shell is perfect and has never been welded! It has 7 and 8 inch Fuchs wheels. All rather nice. Will post a pic as soon as possible.

Best regards Nick White

Nick,

Your car sounds great. One modification you might consider if you are focusing on the correct air/fuel ratio is the LM-1 data logger. Here are a couple interesting threads on the subject: with the LM-1 you can log AFR in real-time as you drive!

http://forums.pelicanparts.com/showthread.php?s=&threadid=239329&highlight=lm1+mf i
http://forums.pelicanparts.com/showthread.php?s=&threadid=193331&highlight=lm1+mf i

Phew : It's all coming thick and fast and at the moment it's winter here and the roads where I live are covered in mud and the car only gets a good run out once every 2 or 3 weeks. However, all advice is noted and will be assessed in due time.

Ta Nick White

Please take a look at the Ultimate MFI resources thread and add your links and resources. It will make a great resource for all of us. SmileWavy