PMO carb help

[Djdiplomat]

Hello! Recently bought a 76 911 2.7 with PMO carbs and xdi ignition(black box). Issues I'm having: Car does not increase idle at cold start and when it warms up I'm getting misfires and has a lopey idle (not drivable). I have found out in the records it has the following carb setup:
Main-135
Idle-55
Ac-205
Emuls-F3
Venturi-34
Pre at- 4.5.

Window in carb fills 1-2mm above halfway, psi around 6. I also pulled plugs and can see they are black and wet.

Where do I start? Thanks in advance.

[snbush67]

Where are you? There might be someone close by that can help you.

Start here - Turn the psi down to 2.5 or 3. The 6 psi is way to high.

Floats may be a little high but not too bad, but you have to take the top of the carburetors of to adjust and it’s real easy to tear a gasket, so unless you have replacement gaskets ready to go it’s best to leave it for now.

You’re going to need to check the balance of the carbs and that done with a synchrometer.

Research at the following links, and ask questions here.

PMO Porsche 911 Carburetion - Installation

Performance Oriented

Tips on Installing PMO Carbs

[stownsen914]

As noted above, the pressure is high. Usually 2.5-3.5 is recommended.

How old are the sparkplugs?

If it were me, I'd start simple. Adjust the pressure, put in a new set of plugs, and try again. If there's still an issue, start the detailed troubleshooting. The links mentioned above are excellent resources.

[LJ851]

There is no cold fuel enrichment or cold idle speed adjustment on PMO carbs.

[Nditiz1]

Was this a recently running system or had it been sitting? It sounds like the jets are clogged.

[Djdiplomat]

Thanks for the tips! Car was sitting for sometime new owner picked it up in 2018 and had a shop go through entire fuel system including new tank. The carbs were cleaned, floats set and valves adjusted. Per the prev owner car never ran right and shop kept blaming the carbs. I just fired it up this morning and fuel pressure is more like 8! Thinking a kink in return line? Plugs do not look great either so I will do that as well.

[chiroracer]

FWIW I ended up with 150 mains for my 2.7 with soles cams and rs pistons(8.5)

[3literpwr]

I recently installed a set of 46mm carbs on my car and found that any low speed stumble, misfire or lack of smoothness was caused by dirty idle jets. Easy to clean. 10 min job.

[snbush67]

Originally Posted by 3literpwr

I recently installed a set of 46mm carbs on my car and found that any low speed stumble, misfire or lack of smoothness was caused by dirty idle jets. Easy to clean. 10 min job.

Agreed, OP should clean the idle jets. But please consider that dirty idle jets isn’t the only thing that can cause low speed stumble, misfire or lack of smoothness. Maybe you didn’t mean to type the word any.

[Djdiplomat]

Going to tackle this fuel pressure 1st. I think I have the old regulator (pic link below) and I see PMO has an updated version. I was thinking to find one of those or just buy an adjustable one.
https://ibb.co/pXryw1h

[snbush67]

Are you sure the gauge that you have isn’t adjustable? Look and see if there is a slotted screw, most likely on the side opposite of the inlet hose.

Posted your picture from the link you provided:

[Djdiplomat]

Thanks! No adjustments from what I see. That's not really not a fpr from what I read, it just regulates the pressure to the return line


QUOTE=snbush67;11777143]Are you sure the gauge that you have isn’t adjustable? Look and see if there is a slotted screw, most likely on the side opposite of the inlet hose.

Posted your picture from the link you provided:

[/QUOTE]

[stownsen914]

I think I've seen that pressure gauge arrangement before. early PMO kits came with them. And correct, granted going from memory here, but I think it's just a distribution block with FPR no function.

Two additional questions:
1. It's a little hard to tell just from this pic, but is one of those fuel lines a return line?
2. What kind of fuel pump do you have?

[Djdiplomat]

Yes one is a return line. I have a pierburg pump. Another thing that I thought was weird I have two individual fuel filters right before the connection to the carbs.

Originally Posted by stownsen914

I think I've seen that pressure
gauge arrangement before. early PMO kits came with them. And correct, granted going from memory here, but I think it's just a distribution block with FPR function.

Two additional questions:
1. It's a little hard to tell just from this pic, but is one of those fuel lines a return line?
2. What kind of fuel pump do you have?

[Locker537]

Originally Posted by Djdiplomat

Yes one is a return line. I have a pierburg pump. Another thing that I thought was weird I have two individual fuel filters right before the connection to the carbs.

The individual fuel filters is common. Redundancy, assuming you also have a filter elsewhere, like before your pump, is a good thing.

[stownsen914]

If you have a return line and still getting 8-9 psi, it suggests a restriction or FPR in your return line. Possibly even a kink in the return line as suggested? Are you sure there's no FPR between the gauge block and the tank?

[Djdiplomat]

I am not sure... I am going to put it up on the lift these days and follow the fuel lines and see what I find.

Originally Posted by stownsen914

If you have a return line and still getting 8-9 psi, it suggests a restriction or FPR in your return line. Possibly even a kink in the return line as suggested? Are you sure there's no FPR between the gauge block and the tank?

[Djdiplomat]

Car is in the air and followed the lines to the gauge block. From the pump on I see no filters or fuel pressure regulators. I did confirm all lines have been replaced (have a service record from 2018). I'm guessing my block is bad...tempted to order one of these if I can find one. https://www.************.com/Porsche-911-1977_p-2057-fuel-pressure-regulator-pmo-with-screw-on-fittings.aspx

[jpnovak]

Those original PMO blocks used to come with a line clamp for the return line. Usually when unrestricted the straight flow would be around 1psi on the gauge. You would tighten the line clamp to get your 3-3.5psi max pressure.

The new blocks are really nice and have an integrated adjuster screw. A worthwhile upgrade if you can find one.

[stownsen914]

jpnovak is right, forgot about that. A simple clamp to restrict flow in the return line and (in gross fashion) raise the fuel pressure.