Help with PMO carb issue/set up
 

Hey all;

I just picked up a 75 911 with a 3.6, stock motor except for headers and PMO carbs (also MSD ignition). Car had been sitting for a bit and when it was delivered, it was hard to start, ran rough and was constantly spitting up and out.....

I just pulled the carbs off, took them apart to see what was in them and to clean them up.

So the carb set up includes

F11's
55's
160's
190's

I was told by the seller (a casual friend who I have no reason not to trust - although he is from Texas....) the car ran fine, so I am assuming its a dirty carb issue...

Yes, the carbs were dirty and I found lots of black sticky stuff....

Got everything clean, checked to see the float bowls are at 12mm and 25mm and put everything back together with mixture screws at 2 full turns out from stop and Air mixture screws all the way in, as prescribed in a PMO article I saw.....

I adjusted linkage and played with syncro meters and felt that I got the car in a pretty good place.

Driving home, the car seems really good at 1/2 to full throttle but at 2200-3000 cruising, its not running well, to include backfiring on deceleration and at just cruising speeds, if is running rough.

If I put my foot in it (1/2 to 3/4 plus throttle it clears right up and runs strong...)

I need advice.... I'm thinking that I need to back out the air mixture screws? But I am lost at this point so any advice will be appreciated.....

Needs to be clean. Needs correct fuel pressure.
Then Colortune for the idle circuit. Basically up to 3000 rpm. Takes the guesswork out one cylinder at a time.

It is clean and it has correct fuel pressure, but thanks for reminding me to put that in!

Sounds to me like it sat for over 6 months if you had blacky sticky stuff. Degraded E10. Filter needs to be replaced and maybe tank cleaned depending how long it sat. I am assuming you did the fine tuning of air and mixture screws? If not, probably needs to be done. Not 100% sure with PMOs but with Weber’s, fine tuning is critical. Good luck. Steve

Hey Steve;

Yea, I completely took the carbs apart and they are clean and Yes I am guessing its the the mixture and air screws too, so I'm looking for wher others are on tier adjustments.

While I set me air mixture to all the way in, I'm not sure that is where they should be, same with mixture screws

black sticky stuff may be residue from rubber fuel hoses, which you need to change anyway

use EtOH rated hoses rated for carb pressures or better

use proper clamps

do NOT forget to replace hoses underneath the fuel tank

good time to replace rubber brake hoses with new rubber from the Porsche dealer (which will be TUV rated) - avoid the boy racer SS covered hoses

send Richard Parr a fax about the carbs - he'll call you back at the date/time you specify

The air correction screws are only used to balance airflow when the throttles are closed. Use your Synchrometer to set the air correction and balance with throttles closed and linkage disconnected. Start with all the AC screws all the way in. Find the throttle on each bank with the maximum airflow. That throat will leave the AC screw all the way. Turn out the other two out to match. You may have to adjust down the main throttle speed screw after adjusting airflow. AT this point you should have a nice even idle. After that you should not touch the air correction.

Next connect your throttle linkage rods. Pull the throttle and raise rpm to about 2500. Make sure both banks pull the same amount of air. Adjust linkage rods as needed until they match.

Next, set the mixture. If jetted right a starting point is usually 2 turns out from full seat. Keep the rpm about 2500. The carbs will be running on the idle circuit and engine speed is fast enough to not take forever in response. This is where you tune for lean best. A color tune helps if you do not know how to do this by ear. Turn each mixture screw (one at a time) until the idle speed starts to drop or you hear an intake spit/sputter. Open the mixture screw back up until rpm stabilizes. You want the mixture screw to be as closed as possible without a drop in rpm. Any more open and the car will run too rich and pop in exhaust.

If you have any throats that do not respond to changes in the mixture screw then you still have dirt/crud in the circuit. Clean it out and start again.

Driving the car it should pull smooth. If you have a flat spot at 2800-ish then you can open mixture screws just slightly (1/16th) until it goes away.

The 55 idle jets seem kinda small for a 3.6. But it depends on other factors. I assume these are 46 (maybe 50mm) but you don't say what size venturis.

Hey Jamie;

Thanks so much. Carbs are 50mm.

So, I did do a full clean, took everything apart, used 6 cans of carb cleaner.... every single piece got sprayed and wiped and I used carb brushes

here - https://www.amazon.com/Carburetors-Cleaner-Cleaning-Needles-Chainsaw/dp/B07D8LZ2PG/ref=sr_1_2?crid=10MBPMZJVD2UZ&keywords=carburator+ brushes&qid=1554757626&s=gateway&sprefix=carburato r+brush%2Caps%2C143&sr=8-2)

to clean every orifice and I used a magnifying glass to double check. Could more dirt have gotten in, sure, but I'm pretty sure when I put them back on, they were clean....

As to set up, I'm with you and understand everything you laid out and that is what I did - several times, until I realized the linkage itself was off center an putting leverage on the shafts, affecting the base idle.

So, after fixing the shafts, I disconnected one side and worked to balance the rear main two carbs, left and right, then I took that other linkage off and tried to balance each bank, against that first main rear carb, which for the record, had the highest vacuum.

Once I had both banks adjusted, I reattached linkage and worked to balance.. It all seemed good.

For my record, I have the mixture screws out 2 turns and the air mixtures all the way in

So, sitting static, in the garage, the carbs sounded great and the syncro meter was pretty much on..... It was on the way home that the light throttle flatness and roughness came out.

This was only on the lower rpm range and at minimal throttle. With a strong throttle push and rpm, everything cleaned up so my thought is that I should have adjusted the air mixture screws? Are they in too far?

What is it in the carbs that affects the light throttle, lower rpm performance of the carbs?

Also, if you are suggesting that I might still have crud in the circuit, where specifically are you referring to?

Best

Andy

btw, jamie, I bought this car from Austin Tx.....

Andy,

Did you balance using the Air correction screws? Unlikely that all 6 are all the way in.

Did you adjust the mixture using the mixture screws as I described? Does each mixture screw, when turned independently cause the engine stumble? You are basically turning off fuel to the cylinder you are adjusting. You should be able to turn off/on each cylinder by screwing in/out the mixture screw. If the engine does not respond to each mixture screw then you still have a dirty jet or passageway. Doesn't matter how many cans of cleaner went through. Did you pull the jets out of the jet holder and back flush to wash them out? Can you see light though them? Does it look like a full circular hole and not just a little spec of light?

2 turns out is a starting point. you must adjust the mixture to match your engine and its jetting. With 55 idle jets you are probably too lean with only 2 turns out. When you floor the car the accelerator jets make up for the missing fuel (its running lean) and it runs after a slight stumble. Just my guess.

Another thought, You mentioned MSD ignition. What distributor? Have you checked that the mechanical advance is not sticking? Is the distributor curved to match your engine? A 3.6 would normally have Motronic ECU to control ignition. I don't know the car just wondering what was done on it to help you get running properly.

Jamie;

To be clear, I did disassemble every part, including jets out of the holders... Really every single piece. dissembled it, cleaned it, bathed it in carb clean, took a carb brush to every part, and fed wire or a brush through every hole.... Then used a magnifying glass to see that each and every hole and passage way was clean, clear and there were no obstructions... I used a light, magnifying glass, and everything.... I feel pretty darn confident the carbs are clean... But then again, if something was in the lines or if the dual gas filters (one feeding each bank, after the pressure guage) are failing, yes it could reappear, but I am pretty confident carbs are clear.... Unless I missed a passage way....

All that said, and why I am asking, is that I balanced left and right, first throttle body, with the sync and with the linkage undone. Then I went to each bank and worked in each side, to balance each bank, but with the mixture screws and not the air correction screws, so maybe that is my issue?

I never changed the air correction screws.....

So If I understand you correctly and I think I do, I have to go back and do it again, this time adjusting the carbs to sync using the air correction screws and not the mixture screws... then play with the mixture screws to dial out lean

Now to complicate things, one of my issues was the drivers side carb, unlinked seems to run too high, even if I screwed the idle screw all the way back so it wasn't even touching the arm. so that side I set up just barely contacting the Idle arm and used that back left throttle as the baseline, although I would have loved to see that come down a bit on rpm..

What you are saying in paragraph 3 sounds best, that I could be running too lean

The car has a Marelli style twin plug dizzy. I have not checked timing yet.

Again, I bought the car, as representing it ran great and I do trust the seller, but I'm guessing that the car sat for some period as I don't know how the Texas registration tags work but ther are 2 of them on the left bottom windshield and one says 7 and 4 and the other says 15...

You are giving great advise and I'm gonna go back this weekend and take another crack at re-syncing the carbs with your suggestions and see if I missed something

Again thanks for the advice!

Although now I know why I like EFI.....

AFAIK, adjusting PMOs should be the pretty much the same as Webers.

Performance Oriented
PMO Porsche 911 Carburetion - Installation

Yes, Paul Abbott is our resident Carburetor Guru. Follow the site instructions to the letter.

Another thought. How old is the gas in your tank? This will make a HUGE difference in how it runs. If more than 2 months old its junk. Drain it and fill with clean. Modern formulations will not tolerate storage.

yea I put new fuel in it too

I had gone to Pauls website a couple days ago and it did not open for me, but it does now.
Great quick read and I'll bone up on it in anticipation for re-doing it this weekend.....colortune.... Thats a new one for me....

http://forums.pelicanparts.com/911-engine-rebuilding-forum/821976-gordos-pmo-carb-tuning.html

Good thread. Covers a lot of info.

This is the process I follow:

Performance Oriented

do you have an air fuel meter monitoring at least one of the exhaust banks? not having one makes carb set up much more difficult.

I had a small hesitation from 2600 to 3000 rpms. I replaced my plugs with 1 step hotter.....solved.

A 3.6l with 50mm carbs and only 55 idle and 160 mains seems a bit small. And it is cold out.

A New England car in April needs a lot more fuel than a Texas car.

I would suggest Your part throttle hesitation means you are lean. Right in the hole of transition ports to mains.

Adding O2 sensors help identify how far off you might be.

I’m at 60L/61R idles and 165 mains on my 3.0 with 40mm carbs. I am a little lean in this cold weather.

You might jump to 60/165 just to start the tuning process at a richer starting point.