Gordo,

I need to take my PMOs out of the box and look at them. I honestly have yet to do anything with them in terms of installation and changing of jets. But looking at your picture now makes me think that the idle air corrector jets we see on the PMO are actually the common Porsche 3 bbl. Weber IDA air correctors for the main circuit? I'll take a look at the PMOs this evening, along with my collection of spare jets I have from my former Webers.

Kev

Paul Abbott is THE MAN
 

An Ah-ha moment...

I've been so focused on my rich cruise idle/progression circuit condition - that I kind of lost sight of what was going on with my main circuit.

The driving profile recommended by Paul Abbott - brought me back to looking at the whole profile, and I was surprised by my observations...

The slow acceleration profile made me realize that my rich idle/progression circuit was obscuring / masking a lean main circuit condition.

I'm now fairly confident that the slight hesitation I'm getting at transition is due to the main circuit coming in lean at lower RPMs.

The slow roll on of the throttle, 2k RPM through ~ 4.5k showed my progression circuit actually providing a pretty good AFR toward the upper end (~3k RPM), but as I slowly moved into the main circuit the AFR started to lean out - which you could feel and watch as this occurred on the LM-2.

The info on the Performance Oriented website is brilliant - should have stuck with this as a single reference from the start...

Gordo

Yes he is the man for sure. The wealth of information he provides helps SO many people work out the bugs found in tuning these carbs.

LM2 Logworks Profiles
 

Here's the AFR recordings:

Magenta is AFR / Black is RPM / bottom scale is time in seconds

This is a slow start / light throttle climb to 3rd gear, on a slightly inclined road.

The slow roll on of the throttle keeps the acceleration pump from dumping fuel and also allows a relatively distinct transition from the progression to main circuit.

Notice how the AFR starts to lean out at ~ 45 sec / 3k RPM
http://forums.pelicanparts.com/uploa...1434420164.jpg

Here's a 3rd gear run, light throttle increase on a slightly inclined road. Noticeably leaning (hesitation) at ~ 16 second point as the AFR climbs into the 15's.
http://forums.pelicanparts.com/uploa...1434420135.jpg

Never noticed this with normal driving... Even watching the LM2 / AFRs.

Normally when I would feel a slight hesitation at transition ~3k RPM, I would jab the throttle a bit and the acceleration pump would dump fuel - which masks the underlying lean condition.

Stepping the mains from current 170's to 180's. Will re-run the profiles this weekend if I get some time.

Gordo

Hmmnn...
 

Installed the 180 mains and went for a drive on the same roads I previously recorded.

No sense in posting the results - the data looks nearly identical :confused::confused::confused:

I would have expected the change from the 170 mains to the 180's to significantly richen my main circuit - meanwhile I was still leaning out into the mid 15's.

Haven't found anyone running 190's / 200's - but that seems to be where this is headed...

Gordo

main jet readings need to be done wide open throttle under load.

Transition to Mains vs WOT
 

Thanks for the feedback Aaron,

I was previously tuning using a WOT driving profile to check my main circuit jetting/air correctors - lead me to a config that's very strong / safe and drivable (for that profile). I can dump it from 1.5k RPM to WOT @ 6.8k / 1st through 3rd gear and stay in a comfortable AFR range (12's with couple of spikes < 1sec duration).

Meanwhile if I'm running around at 4k RPM (3rd gear through backroads), I go lean unless I play with the throttle enough to keep the accelerator pump actuating.

Realize carbs are a balance and trade off - but I would think I should be able to find a balance where I'm not leaning to the point of feeling some hesitation...

What would you suggest?

Thanks again.

Gordo

Sorry, I should say that Main jet check needs to be done in 4th gear and the pull should be done from low rpm to redline. The pull should be done after a short cruise in the same gear to settle everything down. You may do third, but you may thru the RPM range too quickly.

The idea is to remove the accelerator pump from the equation to get an accurate reading.

If you feel that your previous tests show 12.5/1 reading are correct, then your main jet was appropriate.

The PMO does not have the progression holes in the right place for the 3.2 with EFI cams. As a result you will have a lean spike(for me it was 2500 rpm).

I also had overly rich mixture with light throttle.

To fix I went with a big Idle air corrector (180 IIRC) and ran a bigger idle jet. This softened the vac signal to remove most of the lean spike...but not all. I had to deal with a idle mixture that was a bit rich.

It's been awhile since I did that tune. I do not have notes about the final jetting to give you, sorry.

Considering Chokes / Venturis
 

Thanks again Aaron -

Do you recall where you picked up the idle air correctors? I want to give that a shot, but if they are only available through PMO - I need to Fax Mr. Parr and get back in touch to close the loop.

The main jet driving profile you mention is where I'm running into the lean conditions (climbing to high 15's).

I was previously (and incorrectly) running a WOT driving profile that consisted of slapping the pedal to the floor, smiling and making sure my AFR's were good (which they are - that's where I'm getting the 12 numbers...).

BTW, the cams are GT2-102's.

Due to the limited impact that the 170 to 180 main jet change had upon AFR in the 3-4.5k RPM range, I'm wondering if my PMO chokes/venturis are to big to be responsive in this region.

In short - I'm debating swapping my 38 chokes out for a set of 36's.

I'm after mid range torque vice high RPM HP - so I would think the 36's might be more accommodating of my goals.

Any thoughts on how this swap might affect performance?

Thanks again, Gordo

Wait until the mains are squared away before buying the air correctors.

Do you have a jet reamer kit?

Jet Reamer
 

I've considered buying a jet reamer but it seems I'm getting uncommonly large on the mains (current 180 mains are still lean at 4.5k RPM - higher seems out of whack...).

Also prefer to keep the jets I'm acquiring in original form (identification & resale).

Jet buying has been a no brainier @ $30/set, the vents at $100/set make them less attractive from a why not try it standpoint.

Thanks again, Gordo

Gordo, thanks for posting the details. I have 50 PMO carbs on the 3.6 in my 74 911 with the stock 993 cams. The tuning out of the box has been perfect once I balanced the carbs and got the mixture correct.

Now I'm building a higher compression twin plugged 3.0 with GE80 cams and plan on 46 PMO carbs for my 904 replica. Your tuning guide will be very helpful, as may be your collection of jets and bleeds for sale!

Progression Circuit Observations
 

As a refresher - this is all about optimizing. The vehicle's a screamer, but I think it can be better in a few areas...

I replaced my 50 idle jets with the 55's this afternoon. Went back to being way rich through the idle/progression circuit, but significantly toned down the leaning that occurs during transition to the main circuit (2.8k RPM ~ 3.5K RPM).

Again, with 180 main jets I'm still finding the engine is leaning out from ~ 3k RPM through ~ 5k RPM. I think above that range it's smoothing out to ~ 12's.

My theory is that the velocity of air flowing through the 38mm venturis/chokes in this RPM range is not sufficient to create the needed vacuum to draw an appropriate amount of air/fuel mixture through the main circuit. As RPM's climb above this range, and the velocity / vacuum increases, the air/fuel mixture flows through the main circuit limited only by the jets and correctors. Also understand that this is directly influenced by cams - which as some folks on this board have expressed, are not the best profile for carbs (yet to be determined as far as I'm concerned...).

As such, I believe if I swap the 38's for 36's - my mid range RPM / transition performance will improve. Will order the 36's this week and test my theory next weekend...

-----------------
Thanks mreid - do you have a wideband on your 3.6 (caution - leads to obsession)? Good luck with the 3.0; sounds fun. As for the spare parts - I'm a Porsche parts hoarder, just never know when I might need that stuff again (I can never leave well-enough alone...).
-----------------

Gordo

Hi Don,

Why not throw a set of 200 mains in it before you apply your theory?

I was running 195 or 190 mains on my 3.2 with Weber 46. I'd give the 200 a try before you go to the 36mm venturis. The smaller venturis will certainly have more air velocity and theoretically pull more fuel. But I don't think you're grossly too big with the 38mm venturis. Smaller venturis will also choke the top end power a bit.

I know you're not going for peak power. But the smaller chokes do indeed put a limit on top end breathing and you can notice it. When I went from 42mm down to 36mm, because 36mm was all I had at my immediate disposal, I could feel the engine run out of breath at ~7000. Before with the 42's it would zing up as far as I dared push it. The tradeoff was definitely more low end grunt because the smaller venturis had higher low rpm velocity and therefore the fuel "signal" was stronger.down there

PMO 2.00 Main Jets
 

Aaron,

I considered that, but didn't find anyone else running that big of mains (or at least not on a 3.2L street engine...).

Also, I was surprised that my move from the 170 to 180 mains provided no noticeable change in AFR at low to mid main circuit RPM's (~3k to 5k RPM).

As such, I'm not sure the issue is associated with main jet sizing as much as it is a matter of venturi flow or vacuum - that's my theory for what it's worth.

I hope to be able to get a call into Richard Parr or Paul Abbott today to see if they can provide some guidance.

Thanks,

Gordo

Hi Don,

There are so many things that influence main jet size and yours looks like it demands more.

Mains are gross adjustment
Emulsion tubes provide the fuel curve
main air correctors trim the fuel at upper rpm and can influence when the mains comes on.

Kevin posted a solid main jet size for his build, and that was with a Weber 46. The PMO can flow more air at WOT than the Weber, and if the demand is there, will require more main jet.


Your approach is to limit air to get the 180 main to work for you. In other words, you will effectively limit horsepower in order to achieve your main jet size. And you will be successful going that route. Please don't take this as criticism as you may find the 36mm venturi a better fit for your cam choice. I just want to point out that I put the appropriate main jet in according to engine demand.

Ideally, I would have both venturi sizes available, tune both of them and see which one is a better fit to both the cam and driving style.

Great Gouge
 

Love this board... thanks again for the feedback and recommendations gents.

I considered my options again this afternoon - larger mains vs. smaller venturis.

Great thoughts and considerations, but decided I would give the 36mm venturis a try (ordered via Richard Clewett late this afternoon).

I figured the 190 or 200 mains would ultimately require an air corrector reduction (to avoid an overly rich WOT condition - my sustained WOT already tends to dip into the 11's with the 170's). Additionally, the cost associated with this route starts to get close to what it takes to swap the venturis..

Also considered the impact that the 36mm venturis would have on upper RPM range performance, but rationalized that I'm not really in that RPM range too much. It's a street engine that I rev limit to ~ 6.8K RPM (ARP rod bolts, & balanced rods, but stock retainers and springs).

Lastly I gave a lot of thought to Kevin's comments regarding the performance impact of going from 42mm to 36mm venturis and how this is a significant jump as compared to my proposed adjustment going from 38's to 36's. With this minor change, I'm primarily concerned that it may not produce an appreciable/noticeable performance difference.

I truly enjoy this stuff & look forward to getting the venturis and posting how they turn out. If I find I'm still lean during transition - thanks to your feedback, I'll jump to the 190's or 200's without a second thought.

Bottom line, as Aaron mentioned - I'm slowly approaching the ideal situation of having all sorts of combo's to play with and find what ultimately works for my engine/configuration.

Again, thanks for your help and guidance - keep it coming.

Gordo

How to install new venturis / chokes in PMO carburetors
 

Ordered a set of 46/36mm venturis (AKA chokes - from Clewett Engineering (PMO distributor), Richard Clewett's a great guy BTW) to swap with my current 38mm chokes. Arrived yesterday - installed today.

A few photo's detailing the install, which was quick and easy.

---------------------------------------
How to install new venturis in PMO carburetors

It's possible to swap the chokes with the carbs installed but I chose to remove them so I could access them on a workbench (cart)
http://forums.pelicanparts.com/uploa...1435461206.jpg


After the top plate is removed (described earlier in this thread) - remove the acceleration squirters; they position and retain the venturis in the throttle bodies. Unscrew the jet / bolt (yellow oval) that holds the squirter in place and the secondary venturi and squirter simply slides up and out of throttle body:
http://forums.pelicanparts.com/uploa...1435461759.jpg

Once the secondary venturi is pulled out, you can reach in and slide the primary venturi up and out of the throttle body.
http://forums.pelicanparts.com/uploa...1435462627.jpg


Insert the new venturis and reassemble the carbs - done in about 1.5hrs.
---------------------------------------

Unfortunately I didn't get to drive the car after installation - the weather was miserable.

In spite of the poor weather I had a great day due to my opportunity to chat with Paul Abbott - he was incredibly knowledgable and helpful, discussing carb theory and tuning. He's the go-to guy if you need some work done on you carbs, and his website is top notch Performance Oriented

Looking forward to tomorrow for some driving, logging and further experimentation.

Gordo

36mm Venturi, 180 Main, 180 Air Correctors, 55 Idles
 

Another day, another learning point...

The 36mm chokes ran quite well - feeling stronger at mid RPM. Difficult to tell how they perform to redline - my mixture is now sickly rich up there.

Took a recording today that provides a great picture of whats going on with the transition from idles to mains - my mains are clearly kicking in at ~3.9K RPM.

http://forums.pelicanparts.com/uploa...1435536033.jpg

Did some other runs showing I'm still really rich at lower RPM's and cruise - didn't expect, or see any AFR changes on the idle circuit with the smaller venturis.

Next steps - smaller mains and bigger air correctors, and sourcing / playing with some idle air correctors.

Gordo