Gordo's PMO Carb Tuning

[Gordo2]

Originally Posted by stownsen914

When you disconnected the accelerator pump linkage, did you remove it completely? Or just remove the nut from the shaft and let it move freely?

Scott,

I removed my linkage completely. Once you remove the nut, there's really no other option but to remove the linkage arm / shaft - otherwise it will just fall off. If you remove the nut, the linkage arm/shaft will pull all the way out of the small retaining block once you press the throttle. The linkage arm just swivels off of the mount on the main throttle shaft.

It's not too difficult to remove & replace the linkage arm (~35 min). The nut and arm are all that need to be removed to disable the pump - cam and other components remain intact.

Pictures of the disassembled linkage and info's on PMO's hatchet cam is on page 3 of this saga... Gordo's PMO Carb Tuning (link to pg 3 - midway down the page)



Good luck,

Gordo

[Gordo2]

I took a bit of of a break from carb optimizing to swap out my exhaust (removed B&B 1.75" system / replaced with SSI's (1.5") and Dansk 2 to 1 muffler). Back on the carbs this weekend...

-----------------------------------

F16 to F8 Emulsion Tube Swap

I replaced my F16 emulsion tubes with F8's this weekend. I wanted to try the F8's see if they activated my main circuit at lower RPM's; which would then allow me to lean out my rich idle/progression circuit a bit more.

Well, the F8's definitely enriched the mixture at low to mid RPM's - and how



The engine went rich to the point of bogging as the AFR's continued to drop into the 9 range.

Theory in Practice: The F16 e-tube diameter is 8.2mm vs the F8 at 7.5mm. As such, since the F8 is smaller, it allows more fuel to fill / reside in the well that the main jet/e-tube assembly sits in. This reservoir of fuel is immediately available to draw upon as the main circuit begins to activate and contribute - which turned out to be too much fuel at that RPM range for my engine config. The F8 essentially produces results similar to a prolonged squirt from the acceleration pump.

Bottom line - the F8 doesn't appear to be optimal for my engine config. I think the F8's are probably best suited for a larger displacement engine (3.6 - 3.8L?), or a config that really demands a lot of fuel at low to mid RPM ranges.

Good hands on learning point... If I swap tubes in the future, I think I will stick to something in an 8 to 8.2mm diameter range.

-----------------------------------

Idle Air Corrector Sizing

I'm running 55 idle jets - which have tended to be quite rich for most driving conditions.

A few weeks ago, I reamed out the PMO idle air correctors - opening them from the original 1.20 size to 1.40. This really helped with my rich idle/progression issue - taking me from ~ mid 10's to mid 11 AFR readings during low to mid RPM driving and cruise conditions.

This week, I opened the idle air correctors up a bit more to 1.50. I think this size is going to be about as good as it gets. With the 55 idle jets and 1.50 idle air correctors, my low to mid RPM driving and cruise seems to settle out in the mid 12 AFR range with a few momentary drops to the mid-10's and momentary spikes to the mid-14's.

I'm getting a little popping here and there (third gear drives down twisting roads) - but the engine is responsive and strong. I think with a little more time spent getting a good mixture adjustment, it will work out just fine.

I'm want to replace my reamed PMO idle air correctors with a manufactured set of 1.50's. The Solex main jets with M6 threads appear to be exactly the same as the stainless steel idle air corrector jets that are originally installed in the PMO's (I will call Richard to confirm):



-----------------------------------

Leaning out the Main Circuit with Smaller Main Jets

Last week, I ran my F16 e-tubes with 170 mains and 190 air correctors and found that I was too rich as the main circuit came in at low to mid-RPM's.

This week I took a set of 180 main jets, soldered them shut and reamed them out to ~ 140's and paired them with some 180 air correctors (to richen at higher RPM, smallest I had on hand).

The 140 main jets seem to be about optimal for my engine configuration (smaller than I would have expected).

Here's my normal hill climb driving profile:



The 140 mains seem to be just right - hitting mid 12 AFR's, but climbing to the mid 14's by 6,000 RPM. I'm reasonably confident I can remedy this with smaller air correctors, which should also help the main circuit come on earlier / in the lower RPM range which will help mitigate the idle circuit leaning out at ~ 2,700 RPM.

I'm not too worried about the mid-RPM idle circuit leaning (14.95 AFR before the mains jump in). Although the hill profile is a great driving profile to isolate and adjust your idle and main circuits, it's not representative of normal driving...

Additionally - I didn't notice any idle to main circuit transition hesitation with this combination (neither from a slow throttle advance or stomping to wide open throttle) - which is nice...

-----------------------------------

All set for a quick drive tomorrow AM to check out some smaller main air controller jets (soldered and reamed a set of 190's - resized to 140's to start).

It's been a productive weekend - I'm getting very close to what I would consider optimal.

Gordo

[Gordo2]

Noticed this for the first time when I recently swapped my idle air controller jets:



All 3 screws on this carb are fully tightened down (2 of 3 shown in photo), but the heads of the screws don't seat completely flush with the accelerator nozzles.

On the other carb, all 3 screws tighten and seat appropriately - as you would expect them to.

I removed a screw from each side and compared; they are the same length / depth.

It seems like this carb has a manufacturing defect - insufficient depth of the holes that the accelerator nozzle screws thread into.

Here's the other carb for reference:



Anyone ever seen anything like this?

Thanks,
Gordo


Edited Update

I figured it out tonight. The side that fits properly has brass washers (seals) under the nozzle fitting / in the hole that the nozzle fits into - the side that doesn't is missing these brass washers. The brass washers raise the nozzles just enough to allow the bolt to pinch and seal them to the body of the carb.

I wonder how long they've been like that...

Its possible that the washers came out when I installed the 36mm chokes. I always work on the carbs on a cart with a tray that I line with white paper towels - specifically for the purpose of finding any "spare parts" as I'm re-assembling things - but things sometimes have a habit of disappearing when you are working with small parts.

As is - this carb's accelerator nozzles weren't working properly. Some fuel makes it's way through the nozzles, but some just gushes past the nozzle and up through the gap between the fitting and the screw (most likely still gets sucked down into the carb - but definitely not as a measured squirt).

Guess I will contact Mr. Parr to ensure I get the right size - correct thickness is essential to a proper seal.
------------------------------------

[rbogh901]

Originally Posted by Geneman

Hi Gordo and guys here is my setup Frank

Nice thread.


Are you just mounting the gauge with sheet metal screws into the cross beam on a plate or what?

[toddu]

The PMO regulator (with gauge) comes.with a mounting bracket. Three attachment points if I remember correctly. Think it even came with sheet metal screws. Or maybe check out rivnuts as a little more elegant (maybe) solution. Either way, remember to paint your holes.

Todd

[Gordo2]

Originally Posted by rbogh901

Are you just mounting the gauge with sheet metal screws into the cross beam on a plate or what?

Thanks rbogh - I made a mount out of sheet metal. The one supplied with the PMO reg fit but I wanted to raise the reg so that the gauge was viewable / elevated above the Xdi coil pack.

Gordo

[xjoegtix]

Subscribed. Really helpful


Sent from my iPhone using Tapatalk

[crashtest22]

Hey Don,
I have been referencing this page for the last week or so
I have the same build except I used JE pisons on nickies cylinders...

What was your final setup that worked the best in the mid range.

[Gordo2]

Originally Posted by crashtest22

...What was your final setup that worked the best in the mid range.

Still running my PMO's with:

F16 E-tubes
55 Idle Jets
1.40 (slightly oversized) Idle Air Corrector Jets
140 Main Jets
180 Main Air Correctors

Been with this setup since finalizing original tuning - and still running my wide band. The AFR's are good and I'm still satisfied with performance across the range, but the tendency to tinker is still there. At some point I would still like to play around to make the F8's work.

Good luck,

Gordo

[lrodri64]

Are you still with the 36 venturies?

Regards

[Gordo2]

Originally Posted by lrodri64

Are you still with the 36 venturies?

Regards

Yep - still running the 36's.

[hienz]

I have a nice pairs of Weber IDT which do not have any Auxiliary Venturis.
Are all Webers must have Auxiliary Venturis?

[crashtest22]

Originally Posted by Gordo2

Still running my PMO's with:

F16 E-tubes
55 Idle Jets
1.40 (slightly oversized) Idle Air Corrector Jets
140 Main Jets
180 Main Air Correctors

Been with this setup since finalizing original tuning - and still running my wide band. The AFR's are good and I'm still satisfied with performance across the range, but the tendency to tinker is still there. At some point I would still like to play around to make the F8's work.

Good luck,

Gordo

Excellent...
thanks for the post.
I will get to what you have and see where we are.

Though I don't have the F16 tubes but F11
I remember you had used the F11's at one point
what was the outcome with those?

I have the mains, idle air, idle jet, and 36 venturies

[Gordo2]

I don't recall trying the F11's, however at a glance (without looking at specs) - they appear to be very similar to F16's.

My carbs came with the F16's E-tubes - which seems to be Mr. Parr's (PMO) go-to E-tubes because they seem to work well in most applications.

I tried the F8's because I wanted the transition from idle/progression to mains to begin at a lower RPM (my engine was going lean during idle/progression transition to mains). I was battling a hesitation problem that was due to leaning ~ 2.5 - 3k RPM.

The F8's definitely promoted a lower RPM transition to mains, but also resulted the mixture becoming very rich across the entire main circuit (3k & up RPM's). I could have corrected this using different main and main air corrector jets, but I was pretty close to getting to a good point using the F16's with the main and main air corrector jets that I had available. As such, I ended up optimizing using the F16's E-tubes.

I found that the 36 vents (vs 38's) also promoted greater vacuum at lower RPM, therefore drawing more fuel/air, which also helped offset my idle/main transition hesitation issue.

Keep in mind - the engine's camshafts have a significant influence on carburetor's required settings / jetting. My setup was tailored (via a studied trial and error process) to match the requirements of my cams (DC's - GT2-102 with a "carb friendly lobe").

Good luck - enjoy the learning curve.

Gordo

[crashtest22]

Originally Posted by Gordo2

I don't recall trying the F11's, however at a glance (without looking at specs) - they appear to be very similar to F16's.

My carbs came with the F16's E-tubes - which seems to be Mr. Parr's (PMO) go-to E-tubes because they seem to work well in most applications.

I tried the F8's because I wanted the transition from idle/progression to mains to begin at a lower RPM (my engine was going lean during idle/progression transition to mains). I was battling a hesitation problem that was due to leaning ~ 2.5 - 3k RPM.

The F8's definitely promoted a lower RPM transition to mains, but also resulted the mixture becoming very rich across the entire main circuit (3k & up RPM's). I could have corrected this using different main and main air corrector jets, but I was pretty close to getting to a good point using the F16's with the main and main air corrector jets that I had available. As such, I ended up optimizing using the F16's E-tubes.

I found that the 36 vents (vs 38's) also promoted greater vacuum at lower RPM, therefore drawing more fuel/air, which also helped offset my idle/main transition hesitation issue.

Keep in mind - the engine's camshafts have a significant influence on carburetor's required settings / jetting. My setup was tailored (via a studied trial and error process) to match the requirements of my cams (DC's - GT2-102 with a "carb friendly lobe").

Good luck - enjoy the learning curve.

Gordo

Must have been another pmo thread I was reading about the F11 tubes.
I did install the 36 vents but have not swapped for the 140 -145 mains yet.
I do suspect we will be on the 140's though.
we do have the GT2-102 cams as well.

[rs-vic]

The PMO carbs are based on the Weber IDF floats and jets. You should try the F11 emulsion tubes, they work excellent in the IDFs.

[911 SLANT]

Very helpful. Thanks! I'm running 46mm PMOs on my 2.8 with MOD S cams 1.5 headers and GT3 muffler. Was have hesitation during transition also from 2800 to 3500. Running with
185 air correctors,155 mains, 130 Idle air jets, 55 Idle jets and F16 emulsion tubes. Running also Innovate duel AFR gauges. I ran from Idle to 4000 12.5 to 13.8 readings. After talking with Richard I went with this setup.
175 air correctors
155 mains
110 Idle air jets
55 Idle jets
F16 E Tubes
This brought down the hesitation to 2800 to 3000. Very drivable and fun to drive. AFR numbers basically the same. At least now I can put 500 miles on motor before dyno to fine tune.

[sandiego]

My AFM runs around 11.2 between 4000-5000 RPM on the dyno. Is there a rule of thumb on how the AFM might change with one size decrease in the main, ie from 175 to 170 for example?

[InTheDirtCo]

Hey guys, going to bump this thread back up due to some of this info having been dead links now from the age of some of this thread. Hopefully my questions will also help others in the future. I’d been talking with Keith at EMPI and he helped me drastically on this but I hate taking up his time and obviously it’s the weekend now as well. So, I acquired an ‘88 911 3.2 a couple of months ago. It was a bank possession car so I got zero info on it other than just visually ID’ing parts as I come across them. I drove it home and it is a reliable car but it did run “rough” to say the least below 3,000 then would come out but surge at cruising RPM, it would also idle around 2,000. It has been converted to dual plug MSD and PMO swapped. Keith helped me take care of the high idle and that was simply the linkage not being adjusted right, doing that I was also able to balance them somewhat left to right and that helped things sooooo much. Started off with the left bank was drawing around 8 and the right bank was around 5 so they were way out of adjustment.
I have them averaged L/R now at 5-5.5 and is as mentioned, night and day from where it was when I first drove it.
My current issue and question, since PMO doesn’t have any diagrams or manuals online that I can find now, is how do you adjust the draw independent between carbs 1-3 and 4-6 of each bank? I’ve watched a few videos on webers and they show having a collar on them with set screws to adjust each one but I’m not seeing that on the PMOs, so I’m a little lost on that aspect. Like mentioned, I have them averaged L/R currently but that was simply to get it to run “better”, now I’m trying to actually go thru them and set them correctly by doing each one to 5-5.5 and then balance L/R. Currently have two on the L bank that are at 5.5 then the last one is at a 3.5, and then on the right I have one that is a 7, one is a 4 and the other is a 5 so they’re in desperate need of a balance front to back

[silver911rdb]

Hi- I’m on vacation now, but if you pm me your email address, I believe I have all the PMO documents saved. I can send them to you. I also struggled to tune my PMO’s but found the original documents along with the Weber tuning handbook to be a good resource.