Gordo's PMO Carb Tuning

[Geneman]

Hi Gordo and guys here is my setup nothing special.. I did not carefully check the floats on mine so will do so now... however i have hard time believeing that the float level changes changes the mixture setup itys just my non engineer brain working thorugh staytuned Frank

[Gordo2]

Here's a quick shot of an AFR profile I recorded on the LM-2 today.

Top/pink line is the AFR, bottom/black line is the RPM.
The X-axis is time in seconds, Y-axis is the air/fuel ratio in pink, RPM x 1000 in black


As you can see, as I step on it/put the engine under load, my AFR continues to rise - all the way up to 16 when under a hard load

Also note toward the 15 sec point (bottom scale), at cruise - the AFR drops back down to a steady state (I'm a bit rich at idle/cruise ~ 11.5).

I'm still keeping the RPM's low with break in, only imagine it gets worse the higher I go...

Gordo

[Gordo2]

Response to Geneman - Agree, regarding float level being an unlikely culprit for high AFR's, however ensuring you have the float level correctly set appears to be a an initial step to properly set up carbs... BTW, beautiful car - looks like a pretty hot set up.

Throttle Linkage Adjustment

Today I also fixed my throttle linkage.

My linkage was too long, even with every adjustment shortened to the max. As such, I went ahead and shortened the throttle linkage rod that connects the transmission bell crank to the intake manifold bell crank:

Started by removing the threaded end piece on the rod. Its brazed on the end of the rod - a bit of heat from the Oxy/Acy torch and it came right off:



Once the end piece was removed, I cut 1" of the throttle rod off. Remounted the end piece and soldered it back in place (didn't have any brazing rod handy - solder should be strong enough for this application):



Shortening this rod by an inch gave me additional room to play with adjustments to the linkage at the carbs and pedal - worked out great.

Gordo

[KTL]

PMO float setting http://www.pmocarb.com/Images/Instructions-3.pdf

Importance of float height and all being the same- see Basic Tuning Guidlines #3 Weber Carbs Technical

Going lean on accel is a combo of main fuel jets and main air corrector jets. Increase main fuel size, decrease air corrector size. The main fuel typically has a threefold effect vs. the air corrector. Meaning a +5 larger main fuel increment (they are sized in increments of 5) is about the same as decreasing an air corrector by 15.

[snbush67]

...Your float levels from the pics above look low, shoot for the center of the glass.

Fuel pressure is a huge part of these things performing at peak. If you have in line filters after your pressure gauge your filters may be holding you back. I run 5 lbs of pressure at the gauge and end up with 3.5-4 at the carbs.

When I was running lower pressure I was chasing similiar symptoms, all were actually a result of not enough fuel.

[Geneman]

thanks shane good point i have an in line filter in the feed to the regulator. the weber tune guide says 3 1/2 pounds at carbs.. the pmo guide says 5. who knows? my guage also indicator wobbles alot .both at idle and higher .. even with the lumpy cams. ..which i am concerned about..

i am trying again based on the link that master engineer KTL posted above. those tuning instructions are infinitly better written than those that come with the PMOs (apologies richard if you read this...). cheers frank

[Gordo2]

Originally Posted by Geneman

... the weber tune guide says 3 1/2 pounds at carbs.. the pmo guide says 5. who knows?...

I have a Racor 110 filter installed just prior to the regulator - no filters between the PMO regulator and carbs on my set up.

----------------------------
Geneman,

I've only seen 3-4 PSI recommended in PMO documentation (like on the diagram of the PDF link that KTL provided).

Additionally, fellow Pelican Steve Weiner (steve@rennsport) as well as the legend Gray Clay highly recommend setting to 3.5 PSI (for any Porsche carb - PMO, Weber, Zenith...) followed by a "precise" adjustment of the float settings.

My PMO regulator gauge also bounced quite a bit when I was trying to dial down my CIS fuel pump - it's much more steady at 3.5 PSI now that I've installed a low pressure pump (I'm still using the PMO reg).

Again, I don't believe the fuel pressure or float settings are contributing to your or my issues. However, after loads of research I'm convinced that getting these configured correctly provides the best starting point to begin to troubleshoot or dial in the carbs.

BTW - I've adjusted using both the PMO method and the method as linked by KTL: same results.

Mixture Screw Adjustments and RPM

One more note (probably the most significant) - I just noticed you commented that you didn't have any change with adjustments to the mixture screws. In my response I was referring to the air correction screws that had little impact.

Meanwhile, my mixture screws had a significant impact - tightening would lean the cylinder to the point of coughing and popping, which is normal/what you are looking for when you are adjusting.

Something isn't right if you aren't noticing a change when you adjust the mixture screws...

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

My plan ahead - once I get the float levels adjusted and re-adjust the carbs I'm going to start swapping main jets if I find I'm still lean under load.

Best of luck,

Gordo

[JJ 911SC]

Gordo/Frank

Are you guys using "dry" fuel pressure gauge?

Mine did that until self destruction.

I installed a liquid filled one which took care of that. Mind you I'm on EFI and the gauge is a 0-100.

[Gordo2]

I adjusted the PMO Carb floats today and cleaned up all the jets and passages while I was at it.

I purchased my carbs used, and they sat for ~ 2 yrs after I bought them - possibly containing varnish and gunk that could contribute to the lean condition I've been experiencing.

The level of effort, and difficulty was minimal - I took my time with the first carb (~3hrs total) and the second went much faster (~1.5 hrs total). I've never worked on PMO or Weber carbs, but I am reasonably carb savvy from work I've done on other carb'd vehicles, motorcycles and small engines. If you've ever taken apart a carb - the PMO isn't any more difficult or different in principle.

Documented and photo'd the process to hopefully help others adjust floats, or clean their PMO carbs in the future.

PMO Carburetor Disassembly:

Straight forward - filter assemblies removed:


Throttle body removed from intake manifold (removed for cleaning - removal's not required for float adjustment):


Removed the top plate (gasket needs to remain with the top plate during top plate removal):



PMO Carburetor Cleaning:

Once I had the main components removed/disassembled, I began to remove each jet and screw to clean the components and passages that each item controlled. I sprayed the removed parts and throttle body passages with carb cleaner; I let it sit for a minute and then blew everything out with compressed air.

An example of parts removed, sprayed and cleaned (mixture adjustment screw, air correction screw and passage access screws removed):


PMO Emulsion tube and main jet removal:



PMO Float Leve Adjustment

Once I had the carb parts and passages cleaned up, I turned focus toward adjusting the float levels.

Prior to disassembly, I had noted (with a rough sketch) each float level height (in the window) and upon disassembly I found that each float that was low was clearly not adjusted to spec (floats measured greater than the 12mm spec).

I used PMO's instructions to adjust the float level http://www.pmocarb.com/Images/Instructions-3.pdf, the diagram is a bit lacking - hopefully the following pics will help out...

I started by laying the top cover upright on my workbench. The top cover doesn't need to be perfectly upright/level - I placed the speed-square in the photo for reference/bearing:


With the cover upright, you can lightly press the floats toward the top plate to get a feel for the how they depress the spring loaded float needle valve. The needle valve spring should (will) not compress under the weight of the float.

Here's a close up showing the float tab in contact with the float needle valve:


My float levels were low and the measurements corresponded, coming in > 12mm. This places the floats deep in the float bowl, allowing them to rise and close the float needle valve before the float bowl is filled to the half way mark.

Here's photo showing my initial measurement of a float (mic adjusted to 12mm, float hangs at ~ 15mm which makes the float level low):


Float removal is straight forward: remove one of the retaining clips and slide the pin out of the body of the float pivot/top assembly:


After some trial and error, I was able to bend the float level tabs as required to measure 12mm. I also had to play around with the upright tabs to get the correct float drop to 25mm (2 needed adjustment)

Photo of the tabs that I bent to adjust the float levels.


The tab perpendicular to the float adjusts the height, the upright tab adjusts the maximum drop or depth that the float can lower into the float bowls.

I reassembled this evening but didn't pressurize the fuel system.

Will update tomorrow as I check the float levels, re-adjust the carbs and give it a test run checking my air/fuel ratio.

- Gordo

BTW JJ, I don't have a fluid filled gauge - but now I want one, thanks.

[Gordo2]

Ran the fuel pump this morning and found I had 2 floats that were not right - float levels in 2 bowls are now on the high side.

Back apart for another adjustment

Will re-measure and adjust the ones that are high based on best guess height to get them level vice the 12mm spec (assuming they still measure 12).

Gordo

[snbush67]

It looks like you are right on track.

IRT fuel sight glass; Have you checked the level of your motor, for where your car is parked? If it isn't level that may contribute to the readings. My carburators are always filled higher on the front glasses while parked on level ground.

[Gordo2]

Yea!

After a few try's my float levels are finally good...

I did a bit of tuning, but didn't completely sync to my satisfaction, so I didn't record another A/F run.

Meanwhile, I did take it on a short jaunt and my A/F ratio is definitely responding better - It's not climbing with RPM the way it was before I took the carbs apart.

I think the improvement probably had more to do with the cleaning than the float adjustment, but both efforts were worthwhile.

The below photo (taken last night) probably shows the primary cause of my troubles: I had a small chunk of material banging around in one of my float bowls (probably some tubing that made its way through the lines from my fuel line installation):




Here's A few tips/takeaway's on my PMO float adjustment:

1.) Take measurements from similar locations on the top plates and edge of the floats to compare the front and rear float measurements. As it turned out, 12mm does put the float level mid-window, however its difficult to precisely bend the tabs and accurately/consistently measure 12mm.

2.) The floats can be slightly twisted or canted - this can throw off your measurements. Measure the float to the top plate from multiple points on the edge of the float - if you find that it is twisted, you can bend it back in shape.

3.) Be prepared for trial and error. I removed & reinstalled my top plates a total of 3x to get to the point where I was satisfied with the float levels.

Looking forward to fine tuning next weekend.

Reply to snbush67 - Thanks, I considered how level the vehicle was each time I checked the float windows - mine were pretty far off...

Thanks, Gordo

[stownsen914]

Very timely to see this float adjustment! I am about to do exactly this on my 911 racecar that has PMO 50s on it.

Quick question - I've heard both from Richard Parr and read on another thread her on Pelican that it's important to have the car running (or at least have just run it) when measuring float level.

I have to say I don't see how running the engine in my driveway would make an appreciable difference in the float level, but I want to make sure I do this right! Is it confirmed that the engine needs to be run to accurately measure float level for PMO carbs?

Scott

[moneymanager]

If Parr says do it, I'd do it. He knows more about his product than most of us will ever know.

[Gordo2]

Originally Posted by stownsen914

I've heard both from Richard Parr and read on another thread her on Pelican that it's important to have the car running (or at least have just run it) when measuring float level. Scott

Scott,

I too read the same from instructions before I started the project and thought "how's even possible?"

Meanwhile I think if the instructions read "view and assess the float levels with the engine running" would make a lot more sense.

I don't think there is anything float related that you can actually measure on the PMO's with the engine running...

Make sense, or did I misinterpret your question?


- Gordo

[stownsen914]

Thank you for your response.

Rereading my question, I see I was't so clear. What I am actually wondering is whether it's necessary to measure the fuel level in the float bowls while the engine is running. I'd think if the floats and needle valves are doing their jobs, the fuel level in the float bowls should be the same whether the engine is running or not.

[snbush67]

The reason you check the window for fuel level with the engine running is because there is an upper and a lower limit, by observing the window under running conditions you can ensure that the level doesn't get to high, and more important, that it doesn't get to low. A few ticks up or down from center is fine and normal.

As you run the engine wide open the level may be lower, and then as the engine returns to idle the. The fuel level will return.

[Gordo2]

Although I still have some basic tuning/adjusting required, I went ahead and drove around to take a couple of LM-2 recordings last night.


Basic Carb Set Up

The carbs are currently set up a bit lean at idle (~13.5 AFM), and sync'd at idle - but considerably out of whack for sync at 3k RPM (I need to play with the linkage/intake manifold mounts...).

That's the context for the following observations from the recordings:


LM-2 Recording Observations

Third gear run up from ~ 1,800 RPM to ~ 5.5k RPM:

1) Hard acceleration seems to be still producing a lean spike - climbing to ~ 16 AFR.

Coming off the 3rd gear acceleration run, dropping into 4th gear (and slowing to ~ 60mph... ) to check AFR at cruise:

2) Fourth gear cruise at 2.8k - 3k RPM tends to go rich - hovering in the 10 to 11 AFR range.



I'm off tomorrow - so I will get the carbs sync'd up correctly and richen up the idle into the high 12 AFR range and make some more runs.

Hopefully the adjustments will give me a profile that I can make sense of.

With regard to the above situation, I would think that a larger main jet would reduce the acceleration leaning issue but further richen my cruise issue.

We'll see. Any thoughts?

Thanks - Gordo

[toddu]

Drop airs to 180?

Todd

[Gordo2]

Well - the carbs are adjusted as well as I can get them WRT idle mixture and synchronization (idle and 3k).

I was getting some hesitation as I accelerated from 1k ~ 2.5k RPM. I found that adjusting the Electromotive XDi initial advance from 10 ° to 15° really smoothed it out. I adjusted the XDI 3k advance accordingly to keep total advance around 38°.


Here's plot from an Innovate LM-2 recordings that I took of while shifting 1st through 3rd gear, applying moderate throttle (brisk acceleration, but not all out):

(Logworks max AFM smoothing used, otherwise the AFM appears as huge spikes):


Climbing into the mid 15 AFM range - thinking a step or two up on the main jet would bring this leaning condition under control.


Here's a recording that I took during a hard/full throttle, 2nd gear acceleration run:


This is disturbing - I don't see how a main jet change is going to correct this. If I understand correctly swapping a main jet will only adjust the profile up or down.


I'm thinking my next troubleshooting step will involve confirming that the accelerator pumps are putting out the correct volume. I assume that they should be dumping fuel immediately as I mash the pedal - which would cover this short/hard period of acceleration.

More to follow...