MPS Tuning Analysis Options

[Olympic 914]

I will plot the points on my MPS but I am not sure how to make them into a nice graph like "Not a Six" has posted.

Maybe I'll send him my numbers and he could make a graph out of them for me..

[Not_A_Six]

The engine configuration I am running basically mirrors Jake Rabys 2056-120 motor

2056 D-jet (of course)
KB pistons 8.6-1 comp
Raby 9590 cam
Ham RS+ heads 42x36 valves (I think)
037 ECU (stock for the ’73)
043 MPS w/stock spacer – rebuilt with tangerine kit. Initial settings to 037 MPS and adjusted from there.
No Ballast resistor
SS HE’s and a Triad muffler,
Auto Meter Wideband AFR. The 02 sensor is in the muffler
Dakota Digital CHT and oil temp
My altitude is 1000 ft. so 760/733 = 1.0368 sea level correction

Tested with a Amprobe LCR55a and a Mityvac vacuum pump.

Now my MPS settings are much richer than the OP’s and Robs, I am NOT running a ballast resistor.
Robs set-up with the 043/044 ECU doesn’t require one and “Not a Six” has one installed. As I understand this resistor (and any resistor under 300 Ohms) richens the mixture but only during the warm-up phase.
The fuel pressure is around 30, not as high as the OP’s. But I will have to check and report exactly what it is since it is an important component.

Recently my MPS developed a leak and I have ordered a new diaphragm kit from Chris.

I ran this MPS (let’s call it “B”) for three years and about 8K miles. I felt it was a little lean, during a steady highway cruise I would see 13.8 – 14.7 AFR
Settings were

0 – 1.51
4 -- 1.34
15 - 0.85

Corrected to sea level

0 -- 1.56
4 -- 1.39
15 - 0.86

So these settings SEEM that they would be way to rich but were not.



I then installed one of my spare MPS units (“C”) and it did come up as too rich across the range.
Settings on “C” were

0 -- 1.51
4 -- 1.38
15 - 0.86

Corrected

0 -- 1.56
4 -- 1.43
15 - 0 89

So I retuned my third MPS “A” and installed it. Unfortunately at this time my Wide band AFR quit so I don’t have any real numbers for it. But it does FEEL better.
Settings on MPS “A” are

0 -- 1.50
4 -- 1.35
15 - 0.83

Corrected

0 -- 1.55
4 -- 1.40
15 - 0.86


When I set the full load stop on the first MPS “B” and Later on “A” it was adjusted it at 4 points from the fully released position. Possibly this contributed to the diaphragm failure of unit “B”.
I may have to look at adjusting it further in.
Also again I will have to check the fuel pressure.

Good data. Thanks.

It's amazing how small differences in the MPS Inductance readings affect the overall AFR and driveability. I think part of that reason is that the 4 In-Hg reading in particular is super sensitive. With the 4 In-Hg inductance, you're really trying to determine (or set) the pressure P' where the diaphragm begins to lift off the part-load stop. But, that 4 In-Hg measurement is a very indirect way of finding P'.

I currently have my fuel pressure at 35 psi. But after taking more data today (see post #17), I'm planning to try the other extreme (29 psi) and re-tune the MPS as an experiment.

The choice of CHT sensor and ballast resistor affects the warmup characteristics. As you probably know, the '73 2.0 was an odd case where Porsche apparently tried to adapt the older ECU designed for the smaller engine to work with the bigger displacement. Hence the odd MPS and ballast resistor for the '73 2.0. It looks like they finally got it sorted out in '74. Then all hell broke loose in '75 with emissions stuff.

(IMG:style_emoticons/default/beerchug.gif)

[Olympic 914]

The choice of CHT sensor and ballast resistor affects the warmup characteristics. As you probably know, the '73 2.0 was an odd case where Porsche apparently tried to adapt the older ECU designed for the smaller engine to work with the bigger displacement. Hence the odd MPS and ballast resistor for the '73 2.0. It looks like they finally got it sorted out in '74. Then all hell broke loose in '75 with emissions stuff.

(IMG:style_emoticons/default/beerchug.gif)

Yes on the '73 2.0 they used the same 037 ECU as on the 1.7 tuned the 037 MPS just for that engine and put a different CHT in.

The CHT used for the 2.0 is a 017 and is NLA the cold ohms for that was somewhere around 1600 whereas the 012 CHT used on the 1.7 had ~3200 Ohms resistance.

what may work, (and this is what I did on my 73 2.0 system with the 012 cht) is to add another resistor in parallel and connected to ground. I used a 2500 ohm. When cold this cuts the total resistance down to around 1200, and when it warms up and the CHT resistance goes to around 0, the extra parallel resistor had no effect any longer. Now if in your case the ECU does need to see the extra 270 Ohms of the original ballast resistor you could put it in series between the harness wire and the split of the CHT and the 2500 ohm in parallel.

Hope thats clear....



I also rigged up a 500 Ohm variable Pot to assist in tuning the warm up.

[Not_A_Six]

The choice of CHT sensor and ballast resistor affects the warmup characteristics. As you probably know, the '73 2.0 was an odd case where Porsche apparently tried to adapt the older ECU designed for the smaller engine to work with the bigger displacement. Hence the odd MPS and ballast resistor for the '73 2.0. It looks like they finally got it sorted out in '74. Then all hell broke loose in '75 with emissions stuff.

(IMG:style_emoticons/default/beerchug.gif)

Yes on the '73 2.0 they used the same 037 ECU as on the 1.7 tuned the 037 MPS just for that engine and put a different CHT in.

The CHT used for the 2.0 is a 017 and is NLA the cold ohms for that was somewhere around 1600 whereas the 012 CHT used on the 1.7 had ~3200 Ohms resistance.

what may work, (and this is what I did on my 73 2.0 system with the 012 cht) is to add another resistor in parallel and connected to ground. I used a 2500 ohm. When cold this cuts the total resistance down to around 1200, and when it warms up and the CHT resistance goes to around 0, the extra parallel resistor had no effect any longer. Now if in your case the ECU does need to see the extra 270 Ohms of the original ballast resistor you could put it in series between the harness wire and the split of the CHT and the 2500 ohm in parallel.

Hope thats clear....



I also rigged up a 500 Ohm variable Pot to assist in tuning the warm up.

I went down the road of rigging a parallel resistor with a 0280130003 CHT sensor. But, then I managed to find one of the last 0280130017 parts, and have that in the car at the moment.

If/when that -017 CHT finally dies, I think I'll end up with a setup just like yours.

I'm thinking about adding a warmup pot, too. But, I want to keep the cabin looking stock and haven't found a good place to mount it yet. I might pull off the heater light gizmo from the center console and put a pot there instead...

[Frank S]

[quote name='Not_A_Six' date='Aug 18 2020, 12:31 AM' post='2843555']
*** UPDATE ***

I was bothered by the Inductance vs Vacuum graph in my OP (the one with the red and blue lines). I couldn't explain why the red and blue curves crossed at around 7 In-Hg. I also wanted to illustrate how to determine P' and P'' from the graphs. Finally, Olympic914 asked why the graph didn't go all the way to 0 In-Hg.

So, I took some more data. I'm at 2250 ft elevation (700 Torr), and its about 90F in the garage today. The red curve in the graph below shows my actual current MPS tune. The blue curve shows the effect of turning the inner MPS screw (in isolation) one full turn CW. The blue curve is just an experiment to see what would happen to the MPS response -- it doesn't represent any kind of useful tune. I also graphed the data all the way down to 0 In-Hg to show the WOT part of the response.



At first glance, it appears that turning the inner screw CW just lowers the whole curve. But, if you "raise" the blue curve by adding an offset so it starts at .750H at 15 In-Hg like the red curve, the difference in slope becomes clearer:



P', where the MPS diaphram begins to lift off of the part-load stop, and P'', where the diaphram hits the full-load stop aren't readily visible with this particular tune because their effects on the slope are too small to see here.

As to the crossing traces in the OP, data for that graph was taken on different days with different tunes. So, maybe the effect of the outer screw and/or plug affected the values. Maybe it was a difference in temperature or barometric pressure on those days. Maybe it was a measurement error. I dunno. (IMG:style_emoticons/default/confused24.gif)

Finally, the graph in my OP appeared to show that turning the inner screw CW would make the slope of the response curve steeper and thus:

Increase Fuel Pressure + Turn Inner Screw CW => Enrich 6-8 In-Hg region
Decrease Fuel Pressure + Turn Inner Screw CCW = Lean out 6-8 In-Hg region

Today's data shows exactly the reverse -- turning the inner screw CW appears to reduce the slope of the response curve. Hence:

Increase Fuel Pressure + Turn Inner Screw CW => Lean out 6-8 In-Hg region
Decrease Fuel Pressure + Turn Inner Screw CCW = Enrichen out 6-8 In-Hg region

So, I dunno. I may reduce my fuel pressure back to 29 psi and retune the MPS to see the actual in-car driveability effects. If anybody else wants to post data from their own MPS's, that might be useful.

Subscribed...
I`m on vacation right now and don't have access to my MPS tuning data, will post later (3weeks from now).

1. Make sure that Full Load Stop does not influence your data. So remove Full Load Stop to record Part Load and Part Load to FullLoad tranition area. Latest data look much better/correct.

2. IN Chart A, P" arrows should go up and down versus left and right, I think

3. Fuel pressure should be limited to a maximum of 33 PSI as Injectors do not operate reliable above (stay open at max duty cycle, causing very rich mixture at above 5200 RPM)

4. Yes, engine leans out at higher part load RPM and if you compensate you will end up pig rich at coldstart, idle and blow max. torque (3000 - 3100 RPM) and overrun, rest can be tuned almost perfect.
So, I think you need to touch ECU internals (uplift the idle pot. capability and tue the RPM range between idle and max. torque and keep us posted...)

5. I'm just trying to gather data for volumetric efficiency for almost the same engine with Megasquirt and to translate/mimic it to D-Jet ECU later

6. I think the 0043/0044 ECU will be easier to tune as you have a better mechanical range of the MPS available

Take care,
Frank

[Not_A_Six]

Awesome insights! Thanks, Frank. Please see my comments below in italics.

Cheers. (IMG:style_emoticons/default/beerchug.gif)

...

1. Make sure that Full Load Stop does not influence your data. So remove Full Load Stop to record Part Load and Part Load to FullLoad tranition area. Latest data look much better/correct.

I believe that the Full Load Stop (Let's call it the "FLS") should only affect the curve in the region from P'' to 0 In-Hg, unless there is some subtle effect at other pressures that I don't understand. What say you? (IMG:style_emoticons/default/confused24.gif)

2. IN Chart A, P" arrows should go up and down versus left and right, I think

The position of the FLS sets the mechanical distance that the diaphram can move between the Part Load Stop (PLS) and the FLS. You can find that actual distance by measuring the number of FLS turns from its limit at the PLS. The screw has a 1mm pitch, so a full turn CCW from the PLS corresponds to 1mm of diaphragm travel. IIRC, pbanders estimated the stock distance to be around 1.3mm (on some MPS variant, at least). FWIW, thanks to the Tangerine kit, I can turn the FLS with a hex key, and I count hex "flats" (1/6 of a turn) to determine the distance when tuning.

On Chart A in the OP, this diaphragm travel distance corresponds to the delta-P between P' and P''.

3. Fuel pressure should be limited to a maximum of 33 PSI as Injectors do not operate reliable above (stay open at max duty cycle, causing very rich mixture at above 5200 RPM)

Thanks. Yet another reason to try 29 psi again.

4. Yes, engine leans out at higher part load RPM and if you compensate you will end up pig rich at coldstart, idle and blow max. torque (3000 - 3100 RPM) and overrun, rest can be tuned almost perfect.
So, I think you need to touch ECU internals (uplift the idle pot. capability and tue the RPM range between idle and max. torque and keep us posted...)

I've finished my ECU mod and put it back into the car. I'll post about it after some testing. (IMG:style_emoticons/default/smile.gif)

5. I'm just trying to gather data for volumetric efficiency for almost the same engine with Megasquirt and to translate/mimic it to D-Jet ECU later

Very cool! Please keep us posted.

6. I think the 0043/0044 ECU will be easier to tune as you have a better mechanical range of the MPS available

Take care,
Frank

[McMark]

I'm thinking about adding a warmup pot, too. But, I want to keep the cabin looking stock and haven't found a good place to mount it yet. I might pull off the heater light gizmo from the center console and put a pot there instead...

Check out the trip odometer reset knob location... (IMG:style_emoticons/default/shades.gif)

[Frank S]

Awesome insights! Thanks, Frank. Please see my comments below in italics.

Cheers. (IMG:style_emoticons/default/beerchug.gif)

...

1. Make sure that Full Load Stop does not influence your data. So remove Full Load Stop to record Part Load and Part Load to FullLoad tranition area. Latest data look much better/correct.

I believe that the Full Load Stop (Let's call it the "FLS") should only affect the curve in the region from P'' to 0 In-Hg, unless there is some subtle effect at other pressures that I don't understand. What say you? (IMG:style_emoticons/default/confused24.gif)

2. IN Chart A, P" arrows should go up and down versus left and right, I think

The position of the FLS sets the mechanical distance that the diaphram can move between the Part Load Stop (PLS) and the FLS. You can find that actual distance by measuring the number of FLS turns from its limit at the PLS. The screw has a 1mm pitch, so a full turn CCW from the PLS corresponds to 1mm of diaphragm travel. IIRC, pbanders estimated the stock distance to be around 1.3mm (on some MPS variant, at least). FWIW, thanks to the Tangerine kit, I can turn the FLS with a hex key, and I count hex "flats" (1/6 of a turn) to determine the distance when tuning.

On Chart A in the OP, this diaphragm travel distance corresponds to the delta-P between P' and P''.

I'll post some data when I'm back home, can continue this part of the discusssion based on data then.

3. Fuel pressure should be limited to a maximum of 33 PSI as Injectors do not operate reliable above (stay open at max duty cycle, causing very rich mixture at above 5200 RPM)

Thanks. Yet another reason to try 29 psi again.

4. Yes, engine leans out at higher part load RPM and if you compensate you will end up pig rich at coldstart, idle and blow max. torque (3000 - 3100 RPM) and overrun, rest can be tuned almost perfect.
So, I think you need to touch ECU internals (uplift the idle pot. capability and tue the RPM range between idle and max. torque and keep us posted...)

I've finished my ECU mod and put it back into the car. I'll post about it after some testing. (IMG:style_emoticons/default/smile.gif)

Perfect, Thanks!

5. I'm just trying to gather data for volumetric efficiency for almost the same engine with Megasquirt and to translate/mimic it to D-Jet ECU later

Very cool! Please keep us posted.

Will do that, but will be a whole project as I need to build a test stand to mimic all input parameters and have to calibrate the output data against MS data as good as possible.

6. I think the 0043/0044 ECU will be easier to tune as you have a better mechanical range of the MPS available

Take care,
Frank

[Not_A_Six]

*** UPDATE ***

Based on the data from post #17 and Frank S's suggestion, I lowered my fuel pressure to 29 PSI -- basically going from the high extreme to the low extreme to try to exploit the differences in the slopes of the curves to compensate for an over-lean condition I was seeing under moderate load (about 8 In-Hg).

I re-tuned the MPS and adjusted the idle knob for the following AFRs:

Hot Idle: 12.0
Hot Cruise 13.0
Hot WOT 11.5-13.5

Notes:

1) Today was 90F out, so I may make some further adjustments if/when I can test at a cooler temperature.

2) My idle AFR is a bit richer than I would like, but I'm running into a heat-soak issue after dropping to idle immediately after a hard WOT run, where the car would sometimes stall.

3) My "cruise" AFR is still a bit rich at low load to keep the mix from going too lean at moderate load (about 8 In-Hg). Note that this tendency to go lean at 8 In-Hg, pretty much across all rpms, was what prompted me to start this thread and explore the MPS slopes.

4) This current MPS tune reflects changes I made to the VE curve inside the ECU, as I was having a problem where the AFR would go too lean at around 4600 RPM, WOT. The WOT AFR still varies across RPM, and is a bit lower than I would like at 3000 rpm (11.5) and a bit higher than I would like at 4600 rpm (13.5), but it is much improved after modifying the ECU.

5) Driveability is excellent. It idles well. There is no noticeable surging or flat spots across all loads and RPMs.



The yellow curve in the chart above is my latest MPS tune, after lowering the fuel pressure to 29 psi, and modifying the ECU.

Note that the yellow curve has the highest slope of any so far in the region from 5-15 In-Hg, and represents my best effort yet of reducing the lean spot at 8 In-Hg.

On the yellow curve, you can also see the "knees" corresponding to P' at 5 In-Hg, and P'' at 3 In-Hg. (P' is set by turning the inner and outer MPS screws together. P'' is set by turning the Full-Load Stop Plug screw.)

EDIT: These MPS inductance values are at 700 Torr; they have not been corrected to sea level.

I'll start another thread with details on my ECU mod.

Cheers. (IMG:style_emoticons/default/beerchug.gif)

[Olympic 914]

*** UPDATE ***

Based on the data from post #17 and Frank S's suggestion, I lowered my fuel pressure to 29 PSI -- basically going from the high extreme to the low extreme to try to exploit the differences in the slopes of the curves to compensate for an over-lean condition I was seeing under moderate load (about 8 In-Hg).

I re-tuned the MPS and adjusted the idle knob for the following AFRs:

Hot Idle: 12.0
Hot Cruise 13.0
Hot WOT 11.5-13.5

Notes:

1) Today was 90F out, so I may make some further adjustments if/when I can test at a cooler temperature.

2) My idle AFR is a bit richer than I would like, but I'm running into a heat-soak issue after dropping to idle immediately after a hard WOT run, where the car would sometimes stall.

3) My "cruise" AFR is still a bit rich at low load to keep the mix from going too lean at moderate load (about 8 In-Hg). Note that this tendency to go lean at 8 In-Hg, pretty much across all rpms, was what prompted me to start this thread and explore the MPS slopes.

4) This current MPS tune reflects changes I made to the VE curve inside the ECU, as I was having a problem where the AFR would go too lean at around 4600 RPM, WOT. The WOT AFR still varies across RPM, and is a bit lower than I would like at 3000 rpm (11.5) and a bit higher than I would like at 4600 rpm (13.5), but it is much improved after modifying the ECU.

5) Driveability is excellent. It idles well. There is no noticeable surging or flat spots across all loads and RPMs.



The yellow curve in the chart above is my latest MPS tune, after lowering the fuel pressure to 29 psi, and modifying the ECU.
[color=#3366FF]
Note that the yellow curve has the highest slope of any so far in the region from 5-15 In-Hg, and represents my best effort yet of reducing the lean spot at 8 In-Hg.

On the yellow curve, you can also see the "knees" corresponding to P' at 5 In-Hg, and P'' at 3 In-Hg. (P' is set by turning the inner and outer MPS screws together. P'' is set by turning the Full-Load Stop Plug screw.)

I'll start another thread with details on my ECU mod.

Cheers. (IMG:style_emoticons/default/beerchug.gif)

This thread is great.

Did you happen to break out the difference in dropping the fuel pressure and the latest MPS tune, before making the ECU mods?

Waiting to see what you can change inside the BOX...
I have an 044 ECU sitting in the wings but I'm not sure I want to go through the route with retuning the MPS for that one..

[Not_A_Six]

This thread is great.

Did you happen to break out the difference in dropping the fuel pressure and the latest MPS tune, before making the ECU mods?

Waiting to see what you can change inside the BOX...
I have an 044 ECU sitting in the wings but I'm not sure I want to go through the route with retuning the MPS for that one..

In a perfect world, I would have made just one change at a time. But, alas, I made the ECU mod, dropped the fuel pressure, and went out for an MPS tuning drive.

But, theoretically, the MPS tune and ECU tune should be pretty independent:

The MPS tune affects AFR vs Manifold Vacuum.

The ECU mod (at least what I changed) affects AFR vs RPM.

Cheers. (IMG:style_emoticons/default/beerchug.gif)

[914_teener]

I.m a little confused with parts of your posts particulary where you say that you made changes to the VE of the engine by altering the ECU. Brad anders is an EE and I have a minor in Mechanical Eng. So I.m dangerous with electrical circuits in that I know enough to sound like I know what I.m talking about. So I.m curious to know, because I love to learn.

What do you mean by this and what did you change in the ECU?

[Not_A_Six]

I.m a little confused with parts of your posts particulary where you say that you made changes to the VE of the engine by altering the ECU. Brad anders is an EE and I have a minor in Mechanical Eng. So I.m dangerous with electrical circuits in that I know enough to sound like I know what I.m talking about. So I.m curious to know, because I love to learn.

What do you mean by this and what did you change in the ECU?

I'm an EE also. After studying Brad Ander's work, I took the plunge, pried open the box, broke out the soldering iron, and changed component values. I'm composing a thread to describe the process. (IMG:style_emoticons/default/ohmy.gif)

(In the end, I only needed to change the value of a single resistor - R161 - to change the "Wfm4" part of the VE (SC) curve.)

EDIT: Here is the link:

http://www.914world.com/bbs2/index.php?act...=0#entry2844727

(IMG:style_emoticons/default/beerchug.gif)

[Olympic 914]

Got my MPS graph done

0 = 1.52
4 = 1.34
15 = 0.83

Fuel pressure at 29



For comparison sake, the above graph with the range Not_a_Six used.



BTW Torr will be affected by the current barometric pressure, in addition to altitude.

Just to throw another factor into the equation.

http://www.leaktestingspec.com/Content/Cal...ic_Pressure.htm

[rjames]

I know I'm late to the party- this thread is over a year old, but I finally got around to picking up an inductance meter (The same Wavetek LC55 that Anders used, not the newer 'A' version) and measured my MPS (043). I had previously calibrated it by measuring AFR only and the car drives great and AFR #s are good, although admittedly I'm not sure I'm setting AFR for part load correctly- not sure where in the RPMs/what speed (& flat or incline) I should be focusing on for part load AFR measurement. The shape of the curve from 15 in. Hg to 6 in. Hg makes me think my adjustment might be off.


I also have a spare MPS (let's call it MPS #2) and decided to calibrate it for the 'stock' numbers per Anders site (and adjusting for elevation). After calibration it was too lean and the car wouldn't start. I then adjusted it to match MPS #1's inductance settings. Afterwards, it started and idled ok but would stall on acceleration. I double checked the settings with the meter, and sure enough both MPSs matched. (IMG:style_emoticons/default/confused24.gif)
After adjusting the outer screw independent of the inner screw (and readjusting the inner screw accordingly), i got it to idle and accelerate without stalling.
I need to drive the car with #2 installed and adjust for AFR. Once I've done that I'll be curious to see what the inductance #s are on it. I don't understand why 2 MPSs with the same inductance settings would lead to different AFR#s.

[rjames]

Speaking of MPS tuning...

How does one know when the outer screw is adjusted correctly given that part load calibration can be obtained by adjusting either the outer screw or the inner screw (when they are turned independent of one another).

I've seen other posts refer to the outer screw as used to adjust transition from part load to WOT, but don't see that mentioned in Anders information.

[adolimpio]

Speaking of MPS tuning...

How does one know when the outer screw is adjusted correctly given that part load calibration can be obtained by adjusting either the outer screw or the inner screw (when they are turned independent of one another).

I've seen other posts refer to the outer screw as used to adjust transition from part load to WOT, but don't see that mentioned in Anders information.

Although turning the outer screw independently of inner screw will affect part-load, you would not want to do this because it will also affect full-load.

The proper sequence is to turn the inner screw, independent of outer screw to set part-load with outer screw against part-load stop (~15 inHG). Then turn the outer screw together with inner screw to set full load (~4 inHg). Then use the cap screw (full-load-stop) to set the maximum travel at WOT (~0 inHg).

[rjames]

Speaking of MPS tuning...

How does one know when the outer screw is adjusted correctly given that part load calibration can be obtained by adjusting either the outer screw or the inner screw (when they are turned independent of one another).

I've seen other posts refer to the outer screw as used to adjust transition from part load to WOT, but don't see that mentioned in Anders information.

Although turning the outer screw independently of inner screw will affect part-load, you would not want to do this because it will also affect full-load.

The proper sequence is to turn the inner screw, independent of outer screw to set part-load with outer screw against part-load stop (~15 inHG). Then turn the outer screw together with inner screw to set full load (~4 inHg). Then use the cap screw (full-load-stop) to set the maximum travel at WOT (~0 inHg).

@adolimpio

*EDITED*
Thanks. Ok, what was throwing me off was trying to figure out where the starting point for the outer screw should be since at 15 in.HG moving both the inner and outer screws independently of one another will change the inductance reading. I suspect a few folks noted for their MPS knowledge have figured out the starting point measurement of where the inner screw needs to be in relation to the stop plate at 0 in. HG. Would be great to know what that measurement is as it would save a LOT of time when rebuilding an MPS if you don't have a good starting point.

One also has to take into account that after calibrating for inductance, the final step of installing the full load stop screw will also change the part load inductance #s.
Following Ander's excellent write up can get you in the ball park, but there's also some nuance to getting it set right so the car runs decently even before you can start to make micro adjustments to get the AFR fully dialed in.

[adolimpio]

I suspect that you're losing vacuum because you've backed out the outer screw to the point where its o-ring is no longer sealing against the diaphragm.

[rjames]

I suspect that you're losing vacuum because you've backed out the outer screw to the point where its o-ring is no longer sealing against the diaphragm.

Thanks- that occurred to me as my head hit the pillow last night and validated that had indeed happened it when I took it apart this morning.