MFI throttle deflection

carreraplanes · 06-27-2006, 08:54 AM

MFI newbie queries:

Whereabouts do you measure the deflection of the throttle for the part load CO measurement on the MFI? Is it at the end of the large x bar at number 1 cylinder throttle stack or at the butterfly arm on the same stack or does it make no difference (due to nil gearing between the 2)? I do not have the factory protractor but I think for this relatively simple task I could fabricate something with a pointer and some marks if I knew where to take the measurements.

I also want to check that the warm up thermostat is fully disengaged. I understand that there is a plate you can remove but I am not sure which one it is. Could be the one on the end of the unit but if not I might end up with loads of discs all over the garage!!!!

Any pics appreciated.

One final point. CMA says that measurement of intake temp (and subsequent compensation on the graphs) can be ignored on the 2.4 MFI which I will do. However I wonder why this is? I thought that the 2.2 pump was similar except for the space cam.

Thanks in anticipation.....

MBEngineering · 06-27-2006, 08:58 AM

HI i have the info you require PM me your E-mail add and i will try to send it to you

regards mike

304065 · 06-27-2006, 10:58 AM

I have never, nor do I know any who has, made that measurement. CMA requires you to set the throttle opening using a special tool and then go drive the car at part-load, ur use a roller dyno, to set the CO.

The trouble is that 30 years down the road, the factory's CO measurements are both hard to achieve due to wear in the system, and represent a mixture so lean that throttle response and transition suffer.

Why are you doing this test, is it something to do with MOT?

carreraplanes · 06-27-2006, 02:46 PM

To be honest another chap in the UK wiped out his top end with a car that was running overly rich. This was due to bore washing under load so I hear. I do realise that the factory figures are seldom achieved but neither do I want an engine that is running way to rich. I expect that the lean factory figures have more to do with emissions than outright power anyway. Most cars give better power and economy running a tad on the rich side. I doubt these early P cars are much different. It's all a question of magnitude and I'm a newbie here feeling my way around so to speak.....

Mr Beau · 06-28-2006, 04:27 AM

carreraplanes,

Top end or bottom end? I could see a bottom end being injured from being too rich, but the top end (valves etc) seems a bit more or a stretch.

304065 · 06-28-2006, 04:55 AM

Carreraplanes,

My experience, and the related experience of a couple of excellent race mechanics, is actually the opposite: even the "S" pumps result in a lean condition at high RPM under load.

You are correct to focus on attaining the proper mixture, however. As has been written here extensively, most recently by famous race driver, shop owner and all-round great guy Grady Clay, as the MFI system ages the tendency is to go very rich.

My recommendation to you is to get all the systems in perfect order and then set the mixture. This would entail a thorough run-through of "Check Measure Adjust" to verify that everything is up to snuff.

In particular, you will want to remove and clean the thermostat and verify that you have, in fact, 25 pairs of discs in the correct orientation. WHEN THE THERMOSTAT HEATS UP THE ROD PUSHES OUT (sorry for shouting but I wanted to remind myself because I once got that wrong) so if you remove the thermostat you will actually put the rack into the full rich position. Your mixture settings MUST be made with the engine at operating temperature with the thermostat in the "lean" position which means that the paper tubes must be intact from the heat exchanger.

MFI thermostat spacers

Finally, I recommend the use of an LM-1 data logger for setting the mixture under all conditions. Using this relatively inexpensive device will allow you to precisely tune and verify the mixture under load, at idle, just driving around, etc. You don't have to install sensor bungs if you have a single-out exhaust, the Innovate people (Klatinn, the inventor, posts here) have designed a tailpipe clamp for temporary installation.

LM-1 Installation in Early 911- MFI

With this device you should be able to set the mixture as lean as possible for smooth running and good transition. I remember it by 5-6-7-8, i.e. 5-6% CO at idle, 7-8% at 3000 RPM, but those numbers are very rich, you may be able to do better.

See also here for more excellent information: Ultimate MFI resources thread

Good luck!