Trouble with PMOs

den912 · 08-30-2021, 11:26 AM

I’ve recently bought a pair of PMO carbs, I’ve had them fitted to my 1970 911, I can’t get them to run correct, I’ve had them fitted by carb specialist then tested on a rolling road, they run good till I’ve done about 100 miles then the plugs on cylinder 4&3 get sooted up and won’t fire leaving the car running on 4 cylinders ( this has happened 3 times)the engine was completely rebuilt less that 2500 miles ago, not sure what to do next
Thanks Brian

john walker's workshop · 08-30-2021, 11:34 AM

A list of all the jet and venturi sizes would be a good start. Are all the float levels correct in the sight glasses?

den912 · 08-30-2021, 11:41 AM

I’m not technically minded I’m afraid, the carbs are as they came from Richard Parr at PMO, they where set up and ran really well then after a 100 miles odd you can feel the car lose power and not run right, I have a set of PMOs on my 914-6 and there fantastic but these just won’t run right after a bit ☹️

RetroSC · 08-30-2021, 12:04 PM

No offense (and I actually mean that), but if you're not able to check the float levels in the glass window of the carb, I'm not sure what anybody could suggest here that would even remotely help.

You need to take it to a good mechanic and explain symptoms. Sounds like you're rich on those cylinders. Wrong float levels, weak ignition, low compression, jetting.. who knows.

den912 · 12:26 PM

No offence taken the float levels are even on both sides and I've had the car at several different Porsche specialists, compression was checked and all was ok the car was tuned and again ran well for a 100 odd miles then next time I took her out she had what feels like low compression, I then took her to a carb specialist who found that the plugs on 3&4 where sooted, he changed the jets to make the car run less rich and more lean but the problem has returned, the guy who built the engine is a well respected Porsche Mechanic but twice Ive lost compression on the same cylinders

RetroSC · 08-30-2021, 12:49 PM

Have you ever had a full compression check on all 6 cylinders?

den912 · 08-30-2021, 01:03 PM

The first time I took it on a dyno the tech guy said the car was running on 4 cylinders ( something is suspected) I took her back too where the engine had been rebuilt and the guy did a compression test and all was ok, he said the guy on the dyno had moved the 123 electronic dizzy and this is what caused the low compression, he reset the dizzy and the car ran fine for a bit then went back to that feeling of low compression, took the car to another dyno guy and he said the plugs on 3&4 are sooted up and changed the jets so it ran less rich but again it's gone back to feel of a car with low compression, just wondering if any of you guys think there might be a problem with the engine and not the carbs

john walker's workshop · 08-30-2021, 02:10 PM

I wonder if the float valves are drooling.

proporsche · proporsche's Garage

do you have in your fuel system PMO fuel press regulator?

Ivan

Hi_Fi_Guy · Hi_Fi_Guy's Garage

The rule of thumb when diagnosing engine troubles is to do things in this order to avoid chasing your tail or making additional problems.

COMPRESSION
- Test for compression
- Verify valves are adjusted correctly as this is part of compression

If compression Is verified and the problem persists then move to the next item in the list.

IGNITION
- Verify static timing and full advance
- Verify coil, distributor and points, or in your case electronic ignition is working correctly
- Inspect plugs and plug wires. Verify spark plug gaps. Verify spark as needed.

If ignition is verified and the problem persists then move to the next item in the list.

FUEL SYSTEM
- Verify fuel pump, pressure, and flow are correct
- Clean/replace fuel filters as needed and inspect fuel lines and fittings
- Verify accelerator and carb linkage are set right and operating correctly
- if everything else checks out only then is it time to make changes and adjustments to carbs.

If you are not capable of doing it yourself then pay a pro to do it. Just remember no amount of tinkering with carbs will fix an issue elsewhere. It might mask the problem for a while but it will come back. That’s why you need to use the process of elimination to verify everything else before you throw time and money at the carbs.

Gordo2 · Gordo2's Garage

My guess is you have something intermittently wrong with your ignition system - wires shorting out or something causing those particular plugs not to fire.

You engine runs fine for a little while - so it seems you have the carbs properly configured (jetted) and adjusted. Hard to imagine how a carb adjustment (mixture), configuration (jets) or failure could cause only a couple of plugs to foul.

You could have crud in your fuel tank & lines that are making it into the carbs, clogging up a couple of jets causing poor mixture and incomplete firing - but I don't tend to associate that condition with fouled / sooty plugs.

Good luck,

-----------------------------
Added thought - quick way to see if your plug wires are shorting / grounding out: park the vehicle somewhere completely dark, lift engine lid, fire up the engine and see if you can see any sparks jumping.

snbush67 · snbush67's Garage

Ignition - Is the 123 installed and adjusted right with the right coil? It is my understanding that they do do well with the stock coil. It’s interesting that it can be calibrated from multiple positions make sure it’s locked down tightly and recalibrate it.

Also your #3 and #4 plug wires might be the issue, try changing those out and see if the problem goes away.

Also as Mr Walker posted above you can post the specs from your carbs and that might help with a diagnosis.

Tom_in_NH · 08-30-2021, 05:40 PM

I'm with Gordo. Runs fine for 100 miles and then starts to misfire sounds like a coil heating up and partially breaking down. Could also be ignition wires for the same reasons, as SNBUSH suggests.

JackkTheMackk · 08-30-2021, 06:25 PM

If this only started after the PMO install and not after the engine rebuild, it sounds more like a carb issue than ignition. Can you let us know if this issue only started after changing carbs?

Cylinders 3/4 running rich leave me thinking floats, as those two cylinders share float bowls with the accelerator pumps and the other 2 bowls are shared between 1/2 & 5/6. However it seems odd this happens only after 100 miles.

Will 3/4 kick in when you hit the main circuit?

Since you have verified compression, up next is to verify float levels. Might be worth it to pull the carbs and make sure they measure at 12mm/13mm height with the gasket in place and then proceed to thoroughly clean the bodies like a maniac 10x before installing.

Tom_in_NH · 07:18 PM

The idle air jet passageways get blocked with incorrect installation of the air cleaner bases/gaskets, or perhaps using the wrong air cleaner bases/gaskets. I think the problem occurs if they are switched from left bank to right bank. Can't remember which................

john walker's workshop · 08-30-2021, 08:52 PM

I believe the idle air jet blockage issue was addressed with the later top gaskets and air cleaner bases having holes on both sides now. Worth checking though.

VFR750 · 08-31-2021, 03:24 AM

Are you sure it isn’t oil fouling? If the rings did not fully seat you could be oil fouling.

Can you post pictures of the plugs?

Did someone else mention doing a leak down test?

D911SC · D911SC's Garage

Quote:

Originally Posted by Gordo2

My guess is you have something intermittently wrong with your ignition system - wires shorting out or something causing those particular plugs not to fire.

You engine runs fine for a little while - so it seems you have the carbs properly configured (jetted) and adjusted. Hard to imagine how a carb adjustment (mixture), configuration (jets) or failure could cause only a couple of plugs to foul.

You could have crud in your fuel tank & lines that are making it into the carbs, clogging up a couple of jets causing poor mixture and incomplete firing - but I don't tend to associate that condition with fouled / sooty plugs.

Good luck,

-----------------------------
Added thought - quick way to see if your plug wires are shorting / grounding out: park the vehicle somewhere completely dark, lift engine lid, fire up the engine and see if you can see any sparks jumping.

My old SC used to have PMO's. I started to get similar symptoms and it was a bit of a mystery until we discovered that the fuel tank had been previously repaired. The internal coating had started to delaminate and was clogging up the outlet.

A new fuel tank and clean lines sorted the issue. I can't remember if we had fouled plugs or not.

ed mayo · 08-31-2021, 05:21 AM

Cross fire in distributor cap? 4 and three are adjacent in the firing order

VFR750 · 08-31-2021, 05:58 AM

If you put fresh plugs in, and it runs well, pull them out after 20-30 miles and take a picture of them. If you are oil or fuel fouling, 3,4 should show signs of early fouling.

As mentioned make sure your plug wires are good and some amount of separation. This is a good-to-do thing to help reduce the chances it is electrical.

New cap and rotor?

If you are leaking oil past the rings you will get good compression. A leak down test will push air past the rings.

You may also be leaking through your valve stem seals. This will do the exact same thing as leaking ring wrt fouling, but will not show up in a compression test or a leak down test. I know this because it happened to me.