MFI barametric cell shims

[kwikt 911]

Anyone know the shim thickness under the barametric cell on a 72 with MFI is. Also was wondering what the shim(washer) thickness on the thermostat is. Should be 1 thick and 1 thin, not the symbol looking washers. Thanks!!!

Kevin

[Zeke]

doesn't it vary with altitude location? That's what Grady has said. Seems I remember that he added shims to richen up the mixture. So, at sea level, you'd need something if I read him right. Sorry I can't be more specific.

[kwikt 911]

That's what I'm thinking Milt, but how many? I've been playing with this for a while and it does make a big difference adding shims. More shims= richer at sea level, no shims= lean mixture. Just wondering if there is a set shim thickness. What about the thermostat washer/shims?

[jcge]

There are no standard number of shims

The compensation rates are

Thermostat = 0.09mm per °C
Barometric cell = 0.58mm per 100mm Hg

These rates are measured at each device respectively, (ie shimming rates, not at the rack).

Measure the effect of your compensation efforts with a lambda meter

Ultimately, this is no substuitute to getting your pump & regulator set up and calibrated on a pump dyno.

Regards
John

[Zeke]

Lots of articles here and on the Early S BBS on the T-stat ass'y. It wasn't until recently that I've noticed the compensator shim business.. Did you read the complete MFI thread by David what's-his-name? PM Grady Clay, he knows.

I think we do assume there are non standard shims unless the factory just pulled a number of 1000 ft. elevation as a catch all. Wish CIS worked that well.

[kwikt 911]

OK,,,,,, maybe if I remove the side cover and get some sort of measurement where the half moon slot is. Or drive the car while adding and subtracting shims to see what works best. Like I say it does make a huge difference in performance. The 1 pump I have has 2 thin shims and the other pump has a very thick shim and 2 thin shims. Of course being at sea level the thicker shimmed cell works better.

Kevin

[Grady Clay]

Kevin,

The shims under the aneroid are Factory set to have the air pressure compenstion proper for the conditions at assembly. This allows the aneroid to operate corectly throught the range of atmospheric pressures and altitude. This is not something you normally ever adjust or change.

For a given engine (T-E-S and displacement) you first adjust the mixture with the 'main mixture adjustment' behind the Allen screw on the MFI pump. When the main mixture is close to correct you can then adjust the idle mixture. You must readjust the idle every time you adjust the main.


Where the idea comes from changing the shims under the aneroid is when dramatically changing the diplacement of an engine: for example from 2341 cc (2.4) to 2808 cc (2.8).

Best,
Grady

[Zeke]

Well, that make the most sense. After all, it is a compensator, so why would one change the setting? But, he never got his answer.

[David E. Clark]

Originally Posted by milt

Did you read the complete MFI thread by David what's-his-name?

I prefer to be called David (it's not my job) Clark. Here are the two threads:

Ultimate MFI resources thread; and

MFI Message Board Index

Hope they help!

[kwikt 911]

So does anyone want to measure there shims in their perfectly running 72 911E? If I go with the thin shims it really pops alot unless my 911 has been that far off all these years(22).

Kevin

[Zeke]

Originally Posted by David E. Clark

I prefer to be called David (it's not my job) Clark. Here are the two threads:

Ultimate MFI resources thread; and

MFI Message Board Index

Hope they help!

Sorry, David. I was having a senior moment. You DO deserve a lot of credit for establishing that thread. At least a full name.

[911st]

MFI is a reference system that has no idea how much air flow there is at any time.

I am rusty on MFI but things like the altitude can and the thermostat act on the position of the injection pump rack which determines how much fuel is metered at any time. Messing with shims is like shortening the linkage rod to the pump. It is a short cut for just adjusting the pump correctly. Over time a pump can go out of spec and external adjustments may no longer be sufficient.

Add to that possible air leaks about the throttle plate shafts and there comes a time when the TB's need to be rebuilt and the pump calibrated on a bench.

From my limited info, messing with the height of the altitude can makes no sense. One might get lucky and stumble on something close to the correct air fuel ratios but it is not the right way to do it and you will loose the intended correction the altitude can provides.

[Scott Clarke]

Sounds like Kevin is trying to figure out if someone else has messed with the shims.

Also, changing shim thickness seems to make a lot of sense if you have changed the motor. i.e. displacement. Mr Clay has posted about this previously. I'll be tinkering with mine in the near future to get my pump thinking about 2.5 liters instead of 2.2.

[kwikt 911]

Update!!!! Well it seems I was looking in the wrong place(shims). My mfi problem lies with the mfi pressure to the injectors. While disconnecting the fuel lines to the injectors on the T pump, I noticed sudden burst of gas from the lines. Makes sense(pressurized). Now when I installed the E pump and ran it, I loosed the same lines and no pressure just a dribble of gas. That explains the hot start problem the became progressively worse as time went on. So does my pump need rebuilt$$$ How much$$$ Maybe just the sealsSo now when I install the T pump again I hope I can get it adjusted correctly without all tha backfiring.That's were I came up with this shim stuff. Both pumps have different shims. Back to square one. Oh I did try something crazy which made the car run extremely fast with the T pump. I set the barametric linkage arm pin in the hole instead of resting on top. My God the low end and midrange. But it about fouled my sparkplugs and looking at this setup I believe it makes the barametric cell useless since the linkage is almost stationary.

Kevin

[orange911]

Originally Posted by kwikt 911

Now when I installed the E pump and ran it, I loosed the same lines and no pressure just a dribble of gas.

Kevin

Kevin, what ever happened to the E pump? Did you figure out why there was no pressure in the lines?

[kwikt 911]

Well,,,, changed my T pump to E with a space cam(thanks to Jbell). The car runs great!!! The E pump is still sitting on my work bench with a newer E space cam and I cleaned and checked the injector check valves that are in the top of the mfi pump. I figure one of these days I'll install it back on the car and see how it runs.

Kevin