Don,

I am seeing the flat spot in the AFR. That bank goes from an erratic 12-13 to a rapid spike of 17 seemingly on cue when my RPMs hit mid 5000 range. I'll give the system a good cleaning as Jaime suggested and go from there. I'll be sure to report back as I get into the project. Thanks again for the help.

Lee

I would take a look at replacing your inline fuel filter on the bank of cylinders that is going lean. At high rpms when fuel demand is high, that inexpensive filter may have enough flow restriction in it (from years of doing its job?) to be starving that bank of cylinders? That could be the simple thing that has nothing to do with the carbs themselves.

Just a quick observation of the information you posted on jetting. PMO 40 carbs with 34mm chokes/main venturis is quite small for a 3.3L short stroke engine. I would guess that once you get the tuning corrected and its behaving well, that engine is going to hit a bit of a wall/run out of steam at around 6500 rpm. I had a 3.2 short stroke with Weber 46 carbs using 42mm chokes, large port '78-'79 SC heads, 1-3/4" racing headers. It ran really lean on low octane fuel, since it was set up to use 110 octane. It pulled hard to 7500. To get the AFR reduced, I could throw no amount of jetting at it and had to reduce the chokes & thus reduce the amount of air ingested by the carbs. After switching to 36mm chokes, I was able to corral the leanness and got the AFR in the 12-13 range consistently at wide open throttle. The downside to that adjustment was the smaller main venturis choked the airflow and the engine did not rev to 7000+ like it had previously.

Lastly, 230 air correctors are huge. A size like that is usually used when an engine is extremely rich and you need to dump more air into the mix to lean it out. 230 air correctors with 34mm chokes seems sort of contradictory to me. 34mm chokes reduces the gross air volume, but also increases the air velocity thru the throttle body, which in turn encourages the carb to pull a good amount of fuel. So a sizeable air corrector is understandable in order to get sufficient air into the mix. Nonetheless, 230 seems really high to me.

Some good reading is on PMO's website to educate you as well

PMO Porsche 911 Carburetion - Features
PMO** Carburetion & Injection
PMO Porsche 911 Carburetion - Installation (and be sure to take note of the Carb Drawing - Linkage Geometry - Venturis/Jets/Floats/Tubes - Linkage - Gasket links at the top of this page)

You can also read a boatload of 911 Weber & Solex triple carburetor knowledge at Paul Abbott's Performance Oriented website. Paul is actually 1QuickS here on Pelican. Performance Oriented

The PMO carb is patterned after the Weber 40/46 IDA and has been enhanced to use a number of the better 48 IDF parts- floats, needle valves, main jet, air corrector, emulsion tube. But the function is still basically the same as the Weber 40/46

Kevin,

Great info, much appreciated. I'll replace both inline fuel filters, check operation, then proceed to confirming float levels and cleaning the idle jets. If those simple items don't seem to fix the issue, I'll dive into disassembly and cleaning.

Question: If I do go so far as to disassemble, should I just start from scratch by installing the recommended venturis, emulsion tubes, and air correctors?

What is the consensus / opinions on what my setup should be for my engine running 200' above sea level?

3.0 converted to 3.3 using 100mm P/C
Mod-S cams
Single plug ignition - running 93 octane fuel (highest available for me)
1.75" Headers to 2-in 2-out M&K

If you disassemble, don't change the setup. It seems to work on opposite bank
Find the problem then see if you are happy with the setup. Starting from scratch could be a long process too

Agree with trond. Don't change any jets because that'll baseline you.

I think the person who made the notes was making adjustments in the right direction. The change from F2 to F11 emulsion tubes was a good change in my opinion. I'm just really surprised by the large difference between the main fuel jets and the air correctors. But if it was running well before, so be it. There's no singular perfect combination of jets for a given engine system. You could go up a bit on the venturi size, up a bit on main fuel jets and down a bit on air correctors and that would be an acceptable combination.

Just curious how you're checking the AFR on each bank of cylinders. I know how it can be done. I'm just curious how you're doing it & where in the exhaust system.

Kevin,

AFR Gauge on the dash with sensors for both outlets. Ignore the boost indicator on the Tach. Not a turbo, though I expect the car to be not terribly far away in power once tuned properly. Looks like sensors are tapped into end of headers before exhaust mating flanges. And of course a pic of the car in question since I've so rudely not attached one yet :)

https://farm1.staticflickr.com/754/2...2cc7c934_b.jpgExhaust by HKnut, on Flickr

https://farm1.staticflickr.com/746/2...546a3903_b.jpgAFR Gauge by HKnut, on Flickr

https://farm6.staticflickr.com/5722/...f58e2fc2_b.jpg20150926_173045 by HKnut, on Flickr

Nice engine and car! I have a similar AFR gauge by Innovate that I plan to use on my racecar.

Another thought is also I would suggest swapping the O2 sensors and see if the problem follows the sensor. I'm not implying the sensors have anything to do with adjusting the AFR, as in an EFI system. Just suggesting it because maybe the O2 sensor could be giving a false reading and that could lead you to investigate other things unneccessarily? It's an easy test/swap so why not.

Kevin,

Thanks, the guy who built the engine spared no expense. The AFR gauge in place of the clock is such a well thought out, useful mod. I've had he car in my possession for almost a week now and it's killing me that I can't go out and let loose with it.

That's a good thought also about checking the sensors just to make sure my readings are consistent. Certainly not going to hurt anything and knowledge is power.

PMO Setup & Wideband
 

Agree with Kevin - seems like an odd combo / setup on you PMO's for a 3.3L, but each engine is different and you've got to give them what they want (I think a couple of wise men by the name of Kevin and Paul once told me that... ;)).

Still, based on the money and effort that previous owner put into the engine, I would assume he had it running reasonably well at some point (went from "running very good" to "best ever"...). As such, I wouldn't mess with the current config just yet.

A few comments in line with Kevin's:

Venturi / choke size
The relatively small 34mm venturi (aka "chokes") will pull a strong vacuum helping draw sufficient fuel from the main jets at lower RPM / high load (dropping the pedal at 2,500 RPM...). Smaller sized chokes can improve lower RPM performance, but it depends on what your engine demands at those RPM's - which is largely related to your cam profile and cam timing.

On the flip side, smaller chokes can limit the total volume of air that can flow through the carbs at upper RPM's - which could limit power as you get above 6k RPM.

For reference on my 3.2L: I started with 38mm chokes (which Richard Parr thought was appropriately sized for my engine / config). The engine ran strong, but I had a moderate idle to main transition lean hesitation issue (@~3.5K RPM) that I couldn't seem to resolve in spite of trying various combos of jets. My 55 idle jets would start out quite rich at low RPMs and get lean at ~3.2k RPM. It seemed the 38mm chokes didn't create sufficient vacuum to draw from the mains until around 3.5K RPM.

I probably could have played around and found an emulsion tube to help bring the mains in at a lower RPM - but for my config and focus on low-mid RPM power, I opted to try the 36mm chokes. They seem to have helped resolve the transition issue and gave the engine a stronger feel / pull at lower RPM's. Meanwhile I do believe they reduced the top end HP (~200 RWHP), but I'm all about the mid-range torque...

Main Jets and Main Air Correctors
The 140 main jet / 230 main air corrector combo would seem to produce considerable leaning at upper RPM's (230's are very large - 195 to 200 was about as large as I found folks using when I researched).

Large main air correctors will help lean out the mixture at higher RPM (when your carbs are creating max vacuum and drawing from your mains). In general, main air correctors have a greater effect at higher RPM's (however they can also influence when the mains start to contribute fuel...).

The e-tubes and air correctors maintain a balanced AFR at RPM - the vacuum created within the chokes draws fuel from the mains via the e-tubes and air from the corrector which is emulsified (vaporized) within the e-tubes. Won't try to explain how the e-tubes work - they are a study in and of themselves...

Bottom line, if it weren't for the air correctors and e-tubes, the mixture would get continuously richer with more vacuum / as RPM's rise.

BTW - my change from the 38mm chokes to 36mm chokes required a significant change in my main jets and main air correctors (more vacuum...). I went from 170 mains and 190 main air correctors to 140 mains and 150 main air correctors when I swapped the chokes.

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

All good fun and learning.

Meanwhile I still recommend you stick to the basics and see if you can get it back to "best ever" condition with the configuration you currently have. Then if you decide you are ready to play with the setup / config - prepare yourself for an enduring obsession...

Good luck,
Gordo

BTW - for Kevin: my LM2 will take 2 wideband inputs, just a matter of money (Innovate is crazy on cable prices...) and time...

Also, my AFR readings seem to be correct, or completely out of whack. I either start out with the correct initial reading / calibration (22.4), or if it's slightly off - I recalibrate (button press).

I've had to do a free air calibration (remove wideband sensor & clean) about 3 times over the past ~ 2 years - but only when my readings are way off the charts and won't recalibrate with the wideband sensor installed.

I have the 4.2 wideband, understand the 4.9's are better. Not sure if Lee's wideband setup has a manual recalibration process but he may want to try this before he swaps them left to right.

A little update. Haven't gotten very far as work travel and life with three kids is tough to juggle with a hobby!

Pulled idle jets and all were very clean and open. No gunk that I could see clogging the small holes.

Float level on right side is about 1/3 up the windows. Float level on left side is 2/3 up the windows. I replaced the fuel filter on the right side with no change to float level. Fuel pressure gauge is reading 3.5 psi.

Edit: Actually took the car for a little drive instead of just letting it idle, and upon return, float level on right bank had come up to center, right on the dot and left had come down slightly just below dot in center of window. I suppose I should change both fuel filters at the same time. Will be ordering another.

Despite float levels being more correct, same issues remain.

So the car was advertised to have 1.75" headers. I put a set of calipers on them and they measured 1.485" O.D. Quite undersized for a 3.3 ss with hot cams.

I bought some 1 5/8" George's European Headers, had my mechanic play with the carbs a little and it is a completely different car. The engine just couldn't breathe with those tiny headers. It hauls a$$ and pulls like a freight train up to redline now. I can't believe the difference. Throttle response is incredible, and the sound out of those headers is glorious.

Further, the exhaust drone is completely gone and no more vibration in the 3500 RPM range as noted previously.

Thanks to all for you help, insight, and suggestions. I wish I would have doubled checked the header size from the start!

Videos after header swap for your viewing pleasure. Love the sound!

https://youtu.be/7puvzLxBuM4

https://youtu.be/ryt_Lx9vQxY

https://youtu.be/tZxRq53vQC4

I think I saw this car at Exotech, Looked nice. Mark does an excellent job!

That would be the one. Mark is a great guy.

Glad you got it running well. Enjoy before the snow hits.

Edit: didn't read it was resolved.