Issues with car sputtering/running rough after warmed up

[kelseytara]

Hi all, resident newbie here. Acquiring my grandfather’s 1969 280 SL (automatic), which has been in storage for three or so years. It was running fine when put into storage, but now it’s struggling and it’s been in and out of the shop all week. The mechanic is reputable and passionate about old cars. He’s cleaned out the fuel pump and flushed everything, test drove it and it ran fine. My parents picked it up (I’m three hours away) and it ran beautifully for ten minutes on the way home before sputtering. Took it back to the mechanic, he worked on it further (attaching the work order of what he did here), again, test drove it up the highway and back for over a half hour and it was fine. My dad picked it up and had issues two blocks into the drive. Given all of this, I bring it to the group: does anyone have any ideas? Attaching info from my dad as well.

[Cees Klumper]

Sounds like there is crud in the fuel tank that obstructs fuel flow after a brief time running. It settles to the bottom of the tank when the car sits for a while, then upon restart it starts all over again. That doesn't explain why it seems intermittent but it is my first guess. Doing the full 'linkage tour' (see technical manual) would be a good idea as well.

[Benz Dr.]

Cold heat range on Spark plugs.
Probably has carbon core coil wire on new wire set.
As Cees pointed out, fuel starvation.

All this for $195.00 USD an hour..........

[kelseytara]

Cold heat range on Spark plugs.
Probably has carbon core coil wire on new wire set.
As Cees pointed out, fuel starvation.

All this for $195.00 USD an hour..........

So for the spark plugs, do you mean that the type he put in isn’t ideal? I looked up another thread and saw that using resistor spark plugs isn’t recommended, and it appears that the ones that he put in are resistor.

And what is the issue with the carbon core wire?

I’m not particularly knowledgeable about cars but trying to learn what I can as issues come up.

[rwmastel]

The core (wire) of the spark plug wire can be copper, carbon, etc.

[Benz Dr.]

So for the spark plugs, do you mean that the type he put in isn’t ideal? I looked up another thread and saw that using resistor spark plugs isn’t recommended, and it appears that the ones that he put in are resistor.

And what is the issue with the carbon core wire?

I’m not particularly knowledgeable about cars but trying to learn what I can as issues come up.

Plugs should be hotter than 7. Maybe 9.   Carbon core coil wire will have too much resistance which cuts your high tension voltage back a lot. Engine may have the appearance of running rich but may be low voltage.


Sounds like you could have a plugged strainer in the bottom of your fuel tank.

[ctaylor738]

Agree with the clogged fuel tank strainer, which can be checked by removing the fuel gauge sender and shining a focused light at the flower pot.  Also check the filler neck for rust.

Bosch WDC7 or NGK BP5ES plugs are my recommendation.  And copper core plug wires.

Good luck,

CT

[ja17]

It is also important to check the fuel screen built into the intake fitting of the electrical fuel pump. This often overl-ooked fuel strainer is the first thing to become restricted.

[mdsalemi]

Bosch WDC7 or NGK BP5ES plugs are my recommendation.  And copper core plug wires.

Discussion of plug wires and resistance is frequent. A little research will yield the following:

1. It doesn't matter WHERE from point A (distributor) to point B (plugs) the resistance is; it's the total resistance between these points that matters.
2. Our cars were initially specified with copper core wires (~zero ohms resistance) and Beru or Bosch resistor plug connectors (1K ohms).

Now, if you look at some of these facts you'll see why the issues of "use copper core" and "non-resistor plugs" comes up.

1. The resistance in an NGK resistor plug is ~5K ohms.
2. The resistance in carbon core wire varies greatly, but can be 3K-7K ohms PER FOOT.

So, if you use the inappropriate resistor plug, combined with the inappropriate carbon core wire, #6 cylinder may have over 10K ohms resistance.

The resistance in the ignition circuit is basically there for RFI/EMI Suppression. The resistance partially attenuates high-frequency RFI generated by the spark. This prevents interference with your radio, engine computer, and other onboard electronics. Since only the later Pagodas had any electronics the effect would be mostly on AM radio. Resistance also helps limit peak current and extends the duration of your spark. I guess a longer spark means better combustion?

FWIW in 2011 or so, I removed the Crane XR700 capacitive-discharge ignition system, and replaced it with the Pertronix Ignitor 1864LA. I had a custom wire set made with Beru copper core wire, and Beru 1K shielded connectors. I used their black Flamethrower II coil. Everything went in smoothly save for a little file-work on the Ignitor unit to sit properly on the base plate of the -051 distributor. Once that was in, and the engine timed, it has been essentially untouched now for close to 15 years. Yes the current instructions on Pertronix call for suppression wire but I do not believe my instructions from 15 years ago said that. It's possible that they had some failures of the sensor assembly due to extreme RFI/EMI. But in 15 years of solid and more importantly, consistent operation I think I'm OK.

[thomasw]

Discussion of plug wires and resistance is frequent. A little research will yield the following:

1. It doesn't matter WHERE from point A (distributor) to point B (plugs) the resistance is; it's the total resistance between these points that matters.
2. Our cars were initially specified with copper core wires (~zero ohms resistance) and Beru or Bosch resistor plug connectors (1K ohms).

Now, if you look at some of these facts you'll see why the issues of "use copper core" and "non-resistor plugs" comes up.

1. The resistance in an NGK resistor plug is ~5K ohms.
2. The resistance in carbon core wire varies greatly, but can be 3K-7K ohms PER FOOT.

So, if you use the inappropriate resistor plug, combined with the inappropriate carbon core wire, #6 cylinder may have over 10K ohms resistance.

The resistance in the ignition circuit is basically there for RFI/EMI Suppression. The resistance partially attenuates high-frequency RFI generated by the spark. This prevents interference with your radio, engine computer, and other onboard electronics. Since only the later Pagodas had any electronics the effect would be mostly on AM radio. Resistance also helps limit peak current and extends the duration of your spark. I guess a longer spark means better combustion?

FWIW in 2011 or so, I removed the Crane XR700 capacitive-discharge ignition system, and replaced it with the Pertronix Ignitor 1864LA. I had a custom wire set made with Beru copper core wire, and Beru 1K shielded connectors. I used their black Flamethrower II coil. Everything went in smoothly save for a little file-work on the Ignitor unit to sit properly on the base plate of the -051 distributor. Once that was in, and the engine timed, it has been essentially untouched now for close to 15 years. Yes the current instructions on Pertronix call for suppression wire but I do not believe my instructions from 15 years ago said that. It's possible that they had some failures of the sensor assembly due to extreme RFI/EMI. But in 15 years of solid and more importantly, consistent operation I think I'm OK.

I can step in here and confirm, I have the same setup on my W128 since +15 years now without any problems or need for adjust anything regards the ignition timing.