PMO balancing

[InTheDirtCo]

Tried to post on Gordo’s PMO tuning thread to not create a new post but for whatever reason it wasn’t letting me, so, sorry if this ends up here then my attempted post on gordo’s thread also gets put up.

Seen a lot of info of people installing PMOs but the downside is a lot of it is older info that have now, dead links to websites that don’t exist any longer and photos that were originally hosted on photobucket and aren’t in the threads anymore.

Anyway, I recently bought a bank possession ‘88 911 so I got zero info on the car other than what I can visually identify. It has dual plug, MSD, PMO’s on it. Whatever life it had prior to me taking ownership, the carbs weren’t balanced at all when I received the car. It was idling warm at 1800-2000, backfiring thru the carbs, fuel pressure set to 6 psi, and VERY rough from idle to 3,000 before it did clean up some but would surge. A lot of that is now taken care of thanks to Keith @ EMPI, he talked me thru balancing them Left to Right and relieving some from linkages so the RPM didn’t climb when it was warm. When I balanced them L/R all I did was reference from the very back carb on each side. I will say that this did help drastically, however I do now notice that they aren’t balanced front to rear either and would like to take care of that.
I watched a couple YouTube’s on Webers but the balancing of each carb on the PMO seems to be different from a Weber. How does one go about balancing the PMOs front to back? I’d seen where the Webers have a shaft with a collar and set screws but the PMOs seem to be different and there are no manuals online that I can find and I hate to keep bothering Keith, if someone has photos they can post that would be great as well

[rwest]

Hi,

I have PMO ITBs, so it might be different, but on mine, there is a tiny screw with a locknut on each body that you use to balance each one.

[gamin]

On the PMO carbs the air adjuust screws are pretty much in the same location. Only things with a locknut. Basic setting is closed (in). I run mine one turn open. You need a syncrometer or a unisyn to balance the carbs. Can't be done by ear. I had backfiring thru the carbs at one time. Turned out to be debris in the carb. Internal cleanliness is absolutely essential. Also, 6 lb fuel pressure is too high. Needs to be 3-4. Literature on setup should be available from Empi. It was always available from Richard Parr.

[InTheDirtCo]

Thanks guys. I had already dropped the fuel pressure to 4psi @ idle prior to making any adjustments. I do have a synchrometer that I’d used on them Left to Right and will use on them front to back as well.
Good to know on that screw and is what I’ll do, according to the Weber tuning videos that screw was something that was a last ditch fine tune kinda thing so I was afraid to do much with it until I found out if there was another way I should had been adjusting the flow on an individual carb

[stownsen914]

First make sure your butterflies are straight relative to one another, checked at wide open throttle. Assuming they're straight, you can use the air screw as you suggest. See diagram below from PMO.

[ddinham]

When I purchased PMO’s years ago Richard included detailed setup instructions that I saved. If you want to PM me your email address. I can copy and send them to you.

Dana

[InTheDirtCo]

Perfect. That diagram is exactly what I needed.
I did run out to the garage and play with that stuff for about an hour. I do believe they may be due for a rebuild. The two that were running low on air, I cracked the jam but on and spun out, got minimal result so I ran every one of them completely in to shut the ones that were running high off as much as possible then rebalanced from there. With 4 of the 6 ran all the way in, I was able to get the one on the right side that was just a little off, in balance with the others on that side. The one on the left, not so lucky. With 2 of them ran all the way in even with the last one ran all the way out to the point of no longer making a different in air draw it’s still about .5 off of being in balance with the others. I rebalanced left to right from the “best I could get” and it’s once again leagues better than it was. The spitting back went away unless you crack it WOT then instantly let off once it returns to idle it will spit back at you but it no longer just randomly spits from a normal idle.
Think I’ll skip fine tuning any further until the winter when I can pull them and rebuild. Does have a bog when going to WOT that I’ll also try tuning out once I rebuild them and can actually balance all of the carbs correctly

[snbush67]

6 psi is way too high for fuel pressure, it needs to be regulated down to 2.5 - 3.0.

[snbush67]

Start by warming the motor up.

Turn the motor off.

Start by documenting where each air screw and mixture screws are set at.

Turn each mixture screw in, counting turns in, write each down. Turn each mixture screw in until it seats (gently, don’t over tighten), then back out 2.5 turns

Turn each air screws in, count the turns and write it down, leave the locking nut loose until tuning is complete.

Turn each idle stop screw out until you can see a gap, then turn it in 1/4 turn.

Start the engine.

Adjust throttle stop screws equally until the engine idles at 1000 rpms.

Remove linkage short arms on both carburetors.

Using a synchromter, measure the pull of each cylinder it should show around 5-6. Note each cylinder.

The next step is to balance each bank of carbs.

Using the air screws adjust each 123 cylinder to match the the highest number cylinder in the 123 bank of cylinders.

Next Using the air screws adjust each 456 cylinder to match the the highest number cylinder in the 456 bank of cylinders.


Next step is to balance left to right.

That’s simply using the synchrometer to match the 123 back to the 456 bank.

Check engine rpms, adjust the idle screws equally to get the engine idle down to 900.

Attach the 123 throttle linkage, ensuing the is no change to rpm’s. Then attach the 456 throttle linkage, again ensuring there is no change to rpm’s, you may have to shorten or lengthen the short linkage arms to be equal.

Now adjust the mixture screws for best lean.

[icarp]

go get um shane,
Yep shane really does understand the PMO carbs and set up.
If I were you I would follow everything he says .

Ian

[InTheDirtCo]

Thanks everyone! Hopefully I get a morning this week I can throw at this and see if I can’t get things a little better without rebuilding/cleaning them first. It sounds like I’m super close. I’d turned fuel PSI down to 3.5-4 a couple days back, I’ve now set the pull at 5-6 with the linkages off and had backed the idle screws off to see gap and went 1/2 turn in, which was too high and ended up right around 1/4 turn in to get a 850-900 idle. Only thing I haven’t done is get cyl 3 to pull an extra 1/2 and haven’t touched the mixture screws. I’ll try some of that in a few days

[john walker's workshop]

Be sure the float levels are correct. That's where you start.

[InTheDirtCo]

Floats are also off which is another reason to pull them for a rebuild.
There are stickers on the quarter glass from a German Auto show in Florida in 2015, I assume that’s probably from when the original builder first got the car done and assuming that these PMOs were put on it shortly before that and most likely haven’t been off the car since. Then yeah at some point went onto a consignment sell thru ZWECK to a dealer that had the car repo’d from them and I’m not sure the environment that the car was in during its time of bank ownership but I’m sure it was less than optimal and what I’m contributing the small issues like these carbs so out of wack to.

[snbush67]

Balancing instructions are assuming everything else is in order. Typically the throttle plates (butterflies) are off just a bit and as mentioned floats aren’t exact. The plates can be checked with the carburetors off the car, by shining a light from the bottom with the plates closed and checking that no light shines through, the less light the better. You can also tap the plates lightly with your finger and tell if they are closing equally by the sound, a thump means it’s closing all the way whereas a click tells you it’s not closing. The plates that click when you tap can be lightly adjusted with finger pressure.

[911 SLANT]

What size PMOs are you running? I would think 46mm on a 3.2. Whats jets do you have? Buy yourself a Synchometer to balance

[InTheDirtCo]

Sorry guys, dropped the ball on this. Had parked the car for the time being back then and moved onto another project. My life is too busy and I can’t spend too much time on one project without having to abandon it and circle back around. I did buy the sync meter and got them balanced as best as I could. There were 4 of them that came right in with everything basically at factory specs, 1 that I had to turn some of the adjustments about half out, and then one that I had to open the adjustments completely open and is still drawing a little lighter than the other 5. Ran the tank dry and done a full tank of very heavy mixed seafoam with no real noticeable result so I have full intent of pulling them off this fall and rebuilding them, cleaning everything up good while they are apart. If I can’t get it from there then I’m going to take the car up to Redstone and have them try their more experienced hand at tuning them once I get the exhaust on also.