Gordo's PMO Carb Tuning

[KTL]

A main jet change is not going to adjust the entire profile up or down. It will adjust the part of the curve beyond 2000-3000 rpm where the main jet takes over. You're for sure lean so you shouldn't be reluctant to increase your main jet size.

I agree the accel pump volume can help with that lean spike. That's exactly what the accel pump does. You mash the gas and instantly the throttle plate goes wide open. The engine then starts pulling air in thru the throttle body and in turn that pulling of air activates the fuel delivery via the venturi/suction effect to start dumping more fuel into the throttle body. But the problem is the large amount of air pull that starts the process. What the accel pump does is provide a shot of fuel to account for the lag between the throttle opening and the carb being able to produce more fuel by the venturi effect.

[moneymanager]

The accelerator pumps can be a challenge. There's more than volume involved. In my case I eventually had to change RIchard's float bowl control valve choice plus do a lot of fooling with the adjusting nuts. My carbs, as initially set up, dumped way too much gas and the motor bogged seriously before taking off.

[Gordo2]

Originally Posted by KTL

... you shouldn't be reluctant to increase your main jet size...

Agree, but...

I called Mr. Richard Parr this afternoon, planning to order some jets. As we discussed the issues I'm dealing with (lean acceleration and rich cruise), he started asking me about my engine configuration - specifically interested in my exhaust.

I thought I had 1 & 5/8" headers, but he recommended I measure to confirm. Turns out I have 1 & 3/4" B&B headers, ~ 44mm OD (I knew that when I bought them years ago but forgot; they were originally advertised as 1 & 5/8" - long story ).

Richard believes that my headers are causing misleading AFR measurements. He said that the oversized headers result in slower exhaust velocities (reduced flow rates / less scavenging) which doesn't allow the exhaust system to adequately evacuate the combustion chamber while running at cruise speeds / RPM's. Apparently, this configuration can make it appear that the engine is running rich at cruise. Said he's known this for 30+ years.

Based on this, he highly recommended I replace my headers with a set that has 1 & 5/8" primaries before changing anything on the carbs.

Meanwhile, he did say I could try some 180 air correctors to richen the top end, but again, didn't think this was going to resolve my issues.

He seemed more concerned, or was more focused on, the rich cruise condition vice lean acceleration issue - I guess he looked at it as an indicator of what's going on with the exhaust and relationship to my AFR readings.

My head's spinning - while I realize a 1 5/8" exhaust is surely a better bet for my config, I really don't feel like going there right now.

Thanks Guys,

Gordo

[snbush67]

...try the spring loaded muffler baffles in the tail pipe, worked for me.

Originally Posted by snbush67

My theory is that if I put these in my tail pipes it will quiet down, restore low end torque, and maintain the high end.

I bought these on a whim. These are Auto Sensing baffles that open and close based upon the pressure form air flow.

When I went from SSI's to the larger pipes I lost a little low end torque. The claim is that these will provide the back-pressure at lower RPM's and open up for mid-high range RPM's.

The other reason I am doing this is out of consideration for others, the car sounds awesome, but it would be nice to be able to tone it down a bit.

[Gordo2]

Interesting gadget snbush - looks like a bunch of the Harley dudes use it as well.

Meanwhile, I've decided to go ahead and try some jet changes to see if I can come up with something workable while sticking with my current exhaust.

I ordered the following:

- 170 Main Jets (current - 160's)
- 180 Air Correction Jets (current - 190's)
- 50 Idle Jets (current - 55's)

I plan to swap out one set of jets at a time and make a recorded run - recreating the acceleration profiles as best I can (same road section etc).
- Start by installing the 180 air correction jets for a run/recording
- Reinstall the 190 air correction jets and install the 170 mains for a run/recording
- If required (still experiencing lean acceleration), I will try the two in combination

I will leave the setup installed that provides the best results and then see what kind of impact the 50 idle jets have.

The reason I want to try the 50's is to see if I can slightly lean out my rich cruise issue. My cruise recordings were ~ 2.8k to 3.2k RPM, which I understand falls in right in the transition area where the engine is still drawing fuel from idle circuit and the main's begin contributing to fuel delivery.

I also found that my idle mixture adjustment using the 55 idle jets was ~ 1.25 turns out to get roughly 13 AFR at idle. Although there are rules of thumb for the max/min # of mixture screw turns indicating too large or too small of an idle jet, the number of turns doesn't seem to mean too much by itself. Meanwhile, I do have the mixture screws relatively closed; this coupled with the fact that I'm rich at lower RPM's would seem to indicate the 55's are plenty large, and the 50's may be more appropriate for my engine.

That's my thoughts - will update once the jets arrive and I have some time to experiment.

Gordo

[Gordo2]

Mr. Parr's a great guy. He called me today to ask a few more questions about my set-up (fuel pump etc.). It was obvious that my carb tuning issue was on his mind / eating at him since we spoke the previous day.

Unfortunately we weren't able to come up with anything new to try - but I really appreciated the call back.

We discussed the camshafts (Dougherty Racing GT2-102 with what John described as "a carb friendly profile") as a potential contributor to my acceleration lean condition. I didn't have the cam specs handy to discuss in detail, but it gave me something else to look into.

The bottom line was, Richard has sold a boat load of carbs set up similar to or the same as mine are currently jetted; for a variety of engine configurations - and they normally turn out a tad rich at higher RPM...

As such, he's a bit stumped as well - BUMMER

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Another data point - here's a graph of the rich cruise condition that I'm experiencing (floating between 11.7 and 10.95 AFR at 3k RPM cruise):



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And lastly, a link to another Pelican discussing & demonstrating (via dyno) the detriments of running oversized exhaust ports: SSI's / M&K Sport Muffler vs. 1 3/4" Headers / Megaphones
Although this is on a 3.0L w/MFI he has the same cams and found a similar problem with his lower RPM air/fuel mixture going rich...

I think Mr. Parr's onto something...
Gordo

[stownsen914]

Too-large headers can definitely cause issues ...

I'm sure Richard thought of this, but one thing to consider if you're seeing a lean spike in the mid-range - you can run larger mains but go leaner (larger size) on the air correctors so you don't over-richen the top end since you don't seem to have an issue there. He is a super helpful guy. I've called him a few times in recent months as well as I'm tuning PMOs.

One question - what's x axis represent on the graph showing the lean spike. Is that seconds? Just wonder what the duration of the spike is (I'd think that's relevant for whether accelerator pump adjustment can help address it).

Scott

[Gordo2]

Originally Posted by stownsen914

... One question - what's x axis represent on the graph showing the lean spike. Is that seconds? Just wonder what the duration of the spike is (I'd think that's relevant for whether accelerator pump adjustment can help address it). Scott

Affirmative, the x-axis is in seconds.

The lean spike/curve seems to average around 7 seconds in duration before it drops back down - the upper portion of the curve well above 14 AFR.

Meanwhile, if I continue to accelerate, the AFR doesn't drop back down to an acceptable level (remains above 14 as per the previous plot where I accelerated 2nd through 4th gears).

Thanks, Gordo

[stownsen914]

Originally Posted by Gordo2

Affirmative, the x-axis is in seconds.

The lean spike/curve seems to average around 7 seconds in duration before it drops back down - the upper portion of the curve well above 14 AFR.

Meanwhile, if I continue to accelerate, the AFR doesn't drop back down to an acceptable level (remains above 14 as per the previous plot where I accelerated 2nd through 4th gears).

Thanks, Gordo

If the duration is that long, I wouldn't think accelerator pump adjustments will help. It will be interesting to see what the larger mains do.

[KTL]

I agree the long duration would indicate it's more than just the pump's squirt effect. I think a combination of the jet adjustment and the headers would make a big difference. But don't rule out the accel. pump. I know the pump jet itself has a big influence on the AFR behavior, and Richard mentions this in the technical notes on his website, but so does the cam plate influence things and how it's adjusted.

I'm surprised Richard didn't mention his new "hatchets" that he's providing to people. I got a message from him a few weeks back regarding the transition ports and the cam plates for the accelerator pump actuation. I got this message because I bought a new set of PMO46 carbs from him last December.

4th TRANSITION PORT ISSUE

Regarding Carbs sent March 16 through May 6, 2014:

Transition leanness in the 2,000 - 3,000 RPM range. This may be caused by not enough transition port mixture in this area.

Note that we used to have four transition ports but the top one sometimes would give too much overlap on the main circuit at steady cruise so we eliminated the fourth transition port. This helped cars on cruise in the 3,000+ RPM range but seems to have hurt at the 2,000 - 3,000 RPM range in transition. The 50mm carbs still have the original 4 hole pattern.

We are now experimenting by putting the fourth hole back in, but smaller. The fourth hole was .043 and the hole we are putting back is .032 for 40mm carbs and .035 for 46mm carbs. (.032 dia. is a #67 drill or .8mm .035 dia. is a #65 drill or .9mm)

We have also made a different pump cam - “the hatchet”- which gives a quicker accelerator squirt. The new pump cams are now standard equipment on PMO carbs. These are available at no cost.

If you are going to open up the fourth transition port hole in your carbs note that there is a drill dimple at that location. A new 4th transition port can be carefully placed using a jet drill.

To check whether the carbs have three or four transition ports you have remove a transition port cover. The transition port covers are the 11mm diameter brass slotted plugs situated directly above the mixture screws. The (6) mixture screws are the ones with the springs, cup washers and o-rings.

Note that if there are 3 transition port holes then engines above 2.4 liters would be probably be better off with 4. There is a drill dimple at the #4 spot even if there is no 4th hole. This location would be the highest of the pattern, “highest” meaning toward the top of the carb. Also note that the transition port covers have a gasket and it’s almost impossible to get the gasket properly placed unless the carb is on its side.

The exception to all this is the smaller 2 to 2.4 liter street engines. If the carbs have four transition ports then those engines are better off with three transition ports. We have a free transition port plug kit and we are now recommending the 3rd hole, from the bottom, be plugged.

Best Regards,
Richard Parr @ PMO

I'm not saying this is your problem. But it's applicable to the situation and worth considering. Here's a picture of the new cams and the note he includes with them.

[Gordo2]

Originally Posted by KTL

... I'm surprised Richard didn't mention his new "hatchets" that he's providing to people... which gives a quicker accelerator squirt. The new pump cams are now standard equipment on PMO carbs. These are available at no cost.

Thanks Kevin - great point out.

I found a thread discussing cam lever evolution and the "hatchet" this weekend, but it was after I spoke with Richard, and I didn't realize that version of accelerator pump cam is now standard on new PMO carbs.

Here's the thread discussing various accelerator pump & linkage components (to include the "hatchet"), for reference:
Weber accelerator pump linkage help please

I wouldn't mind giving the "hatchet" cam a try (coupled with the air correction or main jet swap). I'll see if I can get in touch with Richard tomorrow.

I guess Richard's focus on getting my engine configuration correct / optimized (i.e. swapping the 1 3/4" headers for 1 5/8") vice swapping carb parts is based on the idea that changing my carb configuration is essentially an effort to offset a less than optimal exhaust configuration - and although I may be able to find a carb setup that is workable, it probably won't be optimal. As such, I may end up swapping jets, and still have the rich cruise issue, or some other undesirable condition (hesitation at some RPM range etc.).

Meanwhile, I find it somewhat surprising that my current carb configuration seems to be considered a standard or baseline for most engine configurations. I fully expected the need to change jets to optimize the carbs for the engine - otherwise I wouldn't have extensively researched jetting or invested in the LM-2 as I was building it. Furthermore, I would still expect the need to optimize the carbs with a set of 1 5/8" headers - after all, the engine isn't stock (outfitted with performance cams, pistons etc) and there doesn't seem to be a one-size-fits-all carb setup.

I guess his main point is that my current carb configuration typically provides a reasonably good starting point - normally producing a safe, somewhat rich mixture condition for most engines & operating environments (3.0L to 3.2L), which indicates my engine configuration isn't quite right, and I should address this first.

FWIW, those are my thoughts on why Richard didn't recommend or mention the "hatchet".

Thanks again, Gordo

[BURN-BROS]

Kevin is correct in regards to the progression circuit issue.
I had to tune a stock 3.2 with PMO's a while back. My customer's issue was similar to yours

I could never fully remove the lean spot, but I eliminated 90% of the drivability issue.

I increased the size of the idle air corrector to weaken the signal....this eliminated the overly rich condition during cruise.

I was then able to increase the idle jet size to help the lean spot.

I tried adjusting acc pump qty, but it did not help. It was not quantity, it was when the pump would start that was the issue. I needed fuel to start as soon as possible and I bet with the new "hatchets", I would have been able to reduce the problem even further.

I hope this helps.

[KTL]

Great hands-on experience from Aaron. Thanks a bunch for chiming in with your observations in tuning an engine of similar size.

For reference, I had Weber 46s on the engine when it was a 3.2SS and they performed pretty good in that I was able to get the jetting in the 12-13 range with 40mm chokes, 60 idles, 170 mains 160 air correctors if my memory serves me. My AFR numbers were good enough to get me started tuning w/out being concerned about leanness.

Well I wasted the engine shortly after rebuilding it and never really got to dial it in completely and consider it finished. So my point is that I thought PMO 46 would be the hot ticket to make the carbed engine even better. Sold the Webers for a good price and put that toward a set of new PMOs. Richard couldn't have been nicer. Talked thru my configuration (downsizing to a 3.0L to avoid too much hp for my class & weight, WebCam 120/104 cams, large port SC heads, 10.5 comp pistons, 1-3/4" MSDS headers, close ratio trans, basically a dedicated mild racing engine) and we settled on a jetting selection based on his experiences & balanced that with what I had used in the Weber46. Then I get the works of art in the mail and look at the instructions. It says typically 3.0L-up race engines should use PMO 50 carbs. What????

What i'm getting at is Richard and I discussed things in detail for quite a while and he did not take issue with what I was proposing to build around the 46 carbs. He was willing to listen and not be absolute about what I should go with. If 50s are a must for 3.0-up, I think he would have steered me away from the 46. The fact that my engine build is pretty mild, I think he felt the 46 would suffice. So each situation is going to vary a bit, especially when you start talking about different cams, displacement, etc. Nonetheless the guy has a world of experience and resources to help get you pointed in the right direction. Guys like he and Paul Abbott have more Porsche Weber-based carb knowledge in one brain cell than most of us could ever hope to have in our entire brain!

[Gordo2]

Thanks again for the input guys.

My jets arrived late this afternoon - looking forward to stepping through the swap / run / record / analyze and adjust process tomorrow.





I also got in touch with Mr. Parr on Tuesday; he was kind enough to separately ship me a set of the "hatchet" accelerator pump cam levers, which arrived today as well. I told him I would provide feedback on how they influenced my acceleration AFR #'s.

On to the effort at hand...

Replacing the Stock PMO Accelerator Pump Cam Lever with the "Hatchet" Cam Lever:

Although it would have been easier to swap the accelerator pump cam levers with the throttle bodies removed, I decided swap them leaving the carbs installed.

I took this approach since I wanted to try to limit/control variables as I swapped parts and analyzed how each component influenced my air/fuel ratio (AFR). If I removed the throttle bodies, I would have felt compelled to re-synchronize the carbs (and I didn't want to get sidetracked playing around with the throttle linkage attempting to perfect my sync again ). This also allowed me to remove the assembly without changing the settings on linkage adjuster nuts.

The original accelerator pump cam levers were easy to remove - I unscrewed the flat head screw that holds the lever in place, rotated the shaft/arm down toward the engine and removed the entire linkage assembly (cam lever and actuator shaft) as a complete unit:


Accelerator pump linkage assembly as removed. Replacement "hatchet" cam lever at the bottom of the photo:



Here's a close up photo showing the original accelerator pump cam lever laid on top of the replacement "hatchet" profile cam lever for comparison:



There's a small clip that holds the cam lever to the linkage shaft/arm (other side of the square block in the above photo) - I removed the clip, swapped the cam and washer (provided with the cam) and replaced the clip.

Reinstalling the accelerator pump linkage would have been much easier with the throttle bodies removed, but not absolutely necessary if you don't mind working blindfolded . Be careful not to drop the screw or small thin washer - they are not magnetic and are difficult find / pick up if they drop in hard to reach places (yea - that accounted for ~ 1/2 hr )

Important Note
When reinstalling the accelerator pump linkage/cam lever - be sure not to over-tighten the retaining screw; it can easily bind the cam lever keeping it from rotating. I used a bit of Loctite blue on the screw before I re-installed it to ensure it didn't back out.


"Hatchet" Accelerator Pump Cam Adjustment Note

Here's a shot of the new cam installed:



On the original configuration, the accelerator pump linkage adjustment nut was tightened/positioned to allow actuator arm to loosely rest in the knee of the cam lever (no slack).

With the new cam installed, and the adjustment nut remaining in the same/original position - I noticed that the actuator arm is slightly engaged / moving up the cam.

As such, although I originally planned to leave the adjustment nuts alone, I will loosen the nuts to allow actuator arm to rest freely in the cam knee, while ensuring I don't have any excess slack/play in the linkage (similar to the original setting).

Will do a run tomorrow, taking a LM-2 recordings to see what kind of impact the new accelerator pump cam lever has on my AFR's. Will post my results as I get them.

Thanks again,

Gordo

[toddu]

Going to 50 idles? I dropped from 60 to 55 and my car is a whole different car. My engine was tuned on the dyno, but was definitely rich on the bottom end. Awaiting my 14point7 iDash to confirm AFRs yet, but it's much more lively around town now.

Todd

[Gordo2]

Folks,

I recorded a few hard acceleration runs today to with the accelerator pump "Hatchet" cam lever installed - same roads as I've run for previous recordings, no other carb adjustments.

Appears the new accelerator pump cam lever is reducing my lean acceleration condition.

Full throttle 2nd to 3rd gear run:


Still overall lean while accelerating hard, but the initial lean spike is definitely toned down considerably (was climbing to 16+, now remaining below 15 AFR).

Next up, swapping my air correctors and recording.

Reply to Todd: yep, smaller idles to see if I can lean my cruise a bit - they will be the last jets I swap in my step-by-step process.

Gordo

[Gordo2]

Next up - I swapped the 190 air correction jets for 180's and did another run.

Main jet assembly removal:


Air corrector jet location in the main jet assembly:



The 180's had a great effect - bringing the hard acceleration lean curve down below 14 AFR:

Hard acceleration in 2nd gear:



Hard acceleration shifting from 2nd to 3rd:


Getting there but still a bit lean on hard acceleration.

Next step, swapping the 190 air correctors back back in and replacing the 160 main jets with the 170's to see how they impact the curve.
Gordo

[JJ 911SC]

Look like you are on the right track.

One change at the time should narrow it down.

[Gordo2]

Main Jet Change (larger)

I returned to the original 190 air correction jet and changed the 160 main jets to 170's.

I took some recordings; will try to post the results tomorrow (the Innovate Logworks software isn't Mac compatible).

Observations:

In general, the larger main jets (in combination with the "hatchet" cam levers) had the following impact on my lean acceleration issue:

- AFR's are below 14 during hard acceleration (but still climb to mid-14's very briefly).
- AFR's during moderate acceleration (not hard enough to engage the accelerator pump) appear to be good / not rising above 14 AFR, but close.

I was happily surprised to find that the larger main jet had no noticeable impact on my cruise AFR's - I was expecting the larger main jets to compound my rich cruise issue. My cruise remained rich hovering in the 11 AFR range.

Idle Jet Change (smaller)

I swapped original 55 idle jets for a set of 50's. The change was more drastic than I expected - my idle lowered to the extent that I couldn't make it down the driveway.

I attempted a quick idle screw adjustment to get me on the road - making minor adjustments until my idle AFR's were back to ~13 (the 55 to 50 idle jet change resulted in the AFR going from ~12 with the 55's to ~ 15 with the 50's ). Rapid/poor idle mixture adjustment attempt may have contributed to the following observations:

Observations

- The idle jet change necessitated readjusting my idle mixture screws
- The engine is leaner initially on hard acceleration
- The engine is excessively lean during moderate acceleration (mild throttle application / little or no accelerator pump contribution, AFR #'s climb to the 15's).
- My 3k RPM cruise steadied up in the high 12 AFR range (much better, but unacceptable due to the above lean conditions)

Key Observation Apparently cruise at ~ 3k RPM is primarily supplied / influenced by the idle circuit and idle jet size, with no observable main jet influence.

Question

Do idle mixture screw adjustments influence the AFR's throughout the RPM range that utilizes the idle circuit (idle to ~ 3k RPM)?

I think I've read that it should, but my adjustments only seem to influence the AFR's at idle (~ 900 RPM). I've leaned the idle (via idle mixture screws), and observed little impact to the cruise AFR's (rich). In other words, once the RPM's climb above idle, the AFR seems to be entirely dependent on idle jet size, with little or no influence by the idle mixture screw adjustments.

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My plan of attack for next weekend: Leave the 170 main jets installed and couple them with the 180 air corrector jets - I think this will bring me to mid-13 AFR during acceleration. I will also attempt to get a better idle mixture adjustment with the 50 idle jets. If I find it's still lean at low RPMs, I will return to the 55 idle jets and go from there.

Enough for one afternoon/evening - more next weekend.

Thanks again for the help and encouragement.

Gordo

[Gordo2]

Update with a graph from the LM-2 wideband.

All of the previously posted / above plots were run with the 55 idle jets. The combination of the hatchet accelerator cam levers and the 180 air corrector jets nearly resolved my lean acceleration issue but I was still running rich (~11.5 AFR) at cruise (~ 3k RPM).

I pulled out the 55 idle jets and replaced them with 50's and found that they leaned out the acceleration way too much.




Another oddity from this weekend - somehow the float levels on the right bank of carbs (pistons 4-6) are now too high (viewed while the vehicle is sitting in the exact same garage parking space where I originally adjusted the floats to mid-point).

No clue how that happened - possibly associated with the new accelerator pump cam levers and linkage adjustment (grabbing at straws - doesn't seem possible since the float tabs are the only thing that should change the fuel level in the float bowls...)???

Gordo