MFI simplification thread (1st draft)

[David E. Clark]

Yup, it's a supplementary cold start nozzle fitted to the lower portion of the intake stacks. If the enrichment solenoid is operating correctly, this supplementary starting device is activated below 14°. Here is a picture:

[Flieger]

Thanks for the reply.

Since I do not have the heat exchanger hose connection to the enrichment circuit, do I need to block these supplementary injectors off?

[Peterfrans]

The cold start enrichtment nozzles in your stacks are actuated by a solenoid on top of your fuel filter. This in turn is controlled by a thermo-time switch that sits in the breather cover on your engine case. It has no relation to the (missing) heat exchanger hose. It is just to help the engine start, after that, during the warm up phase the hot air from the heat exchanger comes into play. This controls the mixture during warm up.

So to answer your question: there is no need to block them

[David E. Clark]

Peter's explanation is correct as is his conclusion that you do not need to block the nozzles. The cold start nozzle just gives an extra squirt of fuel to help start a very cold engine (under 14°). Even though the control rack is set to full enrichment when the engine is cold, the extra bit of fuel from these nozzles helps start a very cold engine. After you've started, the thermostat regulates the mixture up to about 113°. Since you live in Southern California I don't imagine these nozzles really ever come into play for you anyway!

Now if you remove them you might save ½ an ounce each. Think of the performance improvement.

[Flieger]

Thanks David and Peter for your technical explanations. I like learning more about the workings of mechanical fuel injection.

[kach22i]

This is an excellent MFI thread, is there another one for CIS?

[silverc4s]

David, great posting, thanks for the leadership here..
Small correction in your second post, third paragraph, you state: The air/fuel mixture ratio is maintained at 14.8 psi
Of course you meant to say the ratio is 14.8:1, and the psi slipped in there someway...

[gridgway]

A couple of questions on a fantastic thread...

- I assume the temps are in degrees farenheit, so the cold start temp of 14° is way below freezing, not the balmy temperature of our English summers?

- for a '73 MFI setup, as far as I can see there's nothing specific that happens when starting a warm engine, no "warm start" mechanism, no enrichment. Is that correct?

Thanks
Graham

[David E. Clark]

Originally Posted by gridgway

A couple of questions on a fantastic thread...

- I assume the temps are in degrees farenheit, so the cold start temp of 14° is way below freezing, not the balmy temperature of our English summers?

Correct. For you Brits that's about -10°.

- for a '73 MFI setup, as far as I can see there's nothing specific that happens when starting a warm engine, no "warm start" mechanism, no enrichment. Is that correct??

Correct again. On a properly running MFI system you don't need to enrich the mixture on a engine that is already warm (above approximately 113° Fahrenheit, 45° Celsius).

[gridgway]

Originally Posted by David E. Clark

Correct. For you Brits that's about -10°.



Correct again. On a properly running MFI system you don' need to enrich the mixture on a engine that is already warm (above approximately 113° Fahrenheit, 45° Celsius).

That'll be the problem then "properly running"! My current list of things to fix:

- injection pump (completely out of calibration) - now calibrated
- injectors (one completely duff and 5 v poor spray pattern) - new ones to come
- throttle bodies (shockingly poor) - to be replaced or refurbed
- fuel pressure only a few PSI - to be fixed

Hopefully getting that lot sorted will get me properly running and able to start a warm engine without pouring gas down the trumpets!

Graham

[Zeke]

Good job. In addition to the MFI resources thread by DEC, this is as good as it gets. I recommended it for the Wiki.

We now ordain you Dr. Injection.

[porschenut]

An awful lot of copyrighted information posted here. I hope this doesn't ruffle up any Pelican feathers.

[David E. Clark]

Originally Posted by porschenut

An awful lot of copyrighted information posted here. I hope this doesn't ruffle up any Pelican feathers.

It is absolutely not my intent to violate anyone's copyright. In fact, by noting the copyright and the source after each posted picture it is my hope that readers will seek additional information and go to the copyright source for additional reading. The purpose of this and my other MFI threads is solely to help educate the Porsche community about a very complicated system. By writing this stuff I learned quite a lot about MFI - not only from the sources cited but also from members of this board who have forgotten more MFI information than I'll ever know!

I do not claim to be an expert but I believe that limited use of copyrighted material - like in this and my other posts - is authorized for educational purposes:

The Copyright Act of 1976 specifically states:

Notwithstanding the provisions of sections 17 U.S.C. § 106 and 17 U.S.C. § 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright.

[Zeke]

Originally Posted by porschenut

An awful lot of copyrighted information posted here. I hope this doesn't ruffle up any Pelican feathers.

picky, picky.

[FenderBender]

+1 on the PelicanWiki Great thread!!!!

[porschenut]

Originally Posted by milt

picky, picky.

Not at all! I'm not complaining, just pointing out the fact that there is copyright material presented. Wayne has flogged others for doing this. I wouldn't care if the entire book were posted here.

[Eddyvdw]

Thanks, really outstanding MFI documentation!

All MFI owners have at least the tools to become true MFI experts (and we need to...)

Regards from Prague.

Ed

[j911brick]

I think I will raise this from the dead. I've looked at allot of information, but I don't see anything about adjusting the governor? The mixture screw sits on a round plate with four other screws; two silver and two black. Anybody know anything about adjusting these, or baseline settings?

BTW: PacificFI was the last ones into this pump and it looks like they assembled it incorrectly.

[Flieger]

Try this thread for the screws: MFI Pump - Open Heart Surgery

What makes you think Pacific put it back together incorrectly? No offense intended, but I trust Gus more than you.

[j911brick]

Originally Posted by Flieger

Try this thread for the screws: MFI Pump - Open Heart Surgery

What makes you think Pacific put it back together incorrectly? No offense intended, but I trust Gus more than you.

Yes, Gus does have a good reputation, but everybody makes mistakes. THe control lever from the barometric cell does not go into the lever. Its just sitting on top controlling nothing.