Bad misfire and backfiring

[antoeknee]

Hi all,

I have a UK 250sl 1967 which I have had for 15 years.
She is in pretty good shape and up until 10 years ago I used to do 2-3 thousand miles a year in her.
Unfortunately for the last ten years, kids came along, we moved to France and she was put away in a barn, in an Air Chamber, and has only had two or three brief outings a year.

The last couple of years it seemed every time I took her out, after about 35 mins she would break down, to me it felt like an electrical issue, but who knows?
I decided to  invest in a 123 ignition as I thought this would eliminate a few of the possible causes.
It was an easy installation, dropped straight in,  the car fired  up and ran pretty well, even though I did not time it with a strobee light.
I took it for a 20 min drive, was happy, then put her away again for another five months.

Yesterday I decided it was time for some tinkering with her.
It took a good thirty seconds for her to start, but once she did all seemed well.

I came across the “linkage tour" on this website and thought it was time to check things out.
The venturi valve was nowhere near shut on idle, so I followed the tour and got things almost right.

The car seemed to rev freely and I took it for a 10 min drive and it seemed good, even a  little more responsive.
I returned to my garage and planned to make some final adjustment,s but before I could, she started to idle really badly and would not rev up freely but was misfiring and backfiring.
It will hardly move and I struggled to drive in back in to its garage.

I have today  fine tuned the linkage and got both the venturi and the FI pump sitting on their stops and then both fully open.
It starts straight up and will idle badly,  but is still misfiring  and backfiring

The spark plugs are black and sooting up straight away although they are new, as is the fuel filter, the oil filter, and the water pump.
I have tried the idle air screw and the mix screw on the injection pump, to no avail.
Another thing I noticed after my drive was the water in the reservoir was not hot, is this normal? There was a little pressure when I removed the lid but it was stone cold.
Any help would be really appreciated

Antony

[kampala]

Welcome to the Forum!

There are several here who will know better, however in my experience, the 123 ignition needs to be grounded very well at it's base,  if not, your spark could be weak.   A weak spark might have been keeping the car running when your air/fuel mixture was very different and the venturi was not closed at idle but now, after adjustments, may really need a full spark.  Check that your 123 is installed on a clean mating surface. In addition, the 123 has a m4 thread underneath so that, if needed, you can add a ground wire.   When grounded properly, the spark color changed to a nice blue.  Check your spark color -- at the plug wire end -- it will provide a clue.

Don't forget to confirm the obvious --- plug wires to correct firing order.

There are several other items for you to check that hopefully those with more experience will offer up.  Obviously, you will need to clean soot off the plugs to test it and ignition timing will help rule out some items.

Best of luck and glad to hear the Pagoda is out of its bubble.

[mbzse]

.../...the 123 ignition needs to be grounded very well at it's base.../...

True. Use the M4 threaded hole on the "underside" of the distributor housing on the 123, run a ground wire from there over to the Engine block as Oz suggests
/Hans S
.

[ja17]

Sounds like a fuel delivery problem.  Do a fuel pressure check when the engine acts up. Backfiring and coughing back through the intake, usually is caused by insufficient fuel supply. A plugged fuel tank screen or a plugged fuel screen in the electric fuel pump could be the cause.

[ctaylor738]

Check to see if the water lines running to the warm-running device are getting hot.  You may have a coolant circulation problem.  If the WRD isn't getting coolant of the correct temperature it will make the poor thing run rich.

[antoeknee]

Thanks everyone for your replies/knowledge.
I now have a list of things to check.
Really appreciate this help.

[antoeknee]

Hi All,

I have added a ground wire on to the m4 thread, unfortunately it has not helped with my problem, but i'm sure it is doing some good, so I shall keep it.

I have now removed the fuel pump and the tank.

The tank was not to bad, no rust, just some small black flakes.

The fuel pump screen was 30% blocked.

Now I have the fuel pump off and on a work bench, is there a way to test it? flow pressure or volume.

All new fuel hoses, relevant gaskets, screens and filters ordered from SLS.

Should I be changing any seals in the fuel (long) pump?

Thanks

[antoeknee]

So, after cleaning the fuel tank, replacing the tank screen, rebuilding the fuel pump, replacing all fuel hoses, as well as the boot/trunk breather hoses, a new filter and 20ltr of fresh fuel, the car is now running as good as I've known it to!

When I first started her, she was still not running well, but after about five mins she cleared her throat and all was good.

I am wondering if I had had bad fuel and the first 5 mins of bad running was the old fuel that was after the fuel filter and in the FI pump??

Thanks for all the help i was given from the forum, and a huge thanks to Eric (tel76) for kindly sending me a spare three tab washer he had for my fuel pump, without which I would still be stuck.

[Benz Dr.]

I'm wondering if your coolant thermostat is working properly. I'm also wondering if the slide valve in the warm up device is stuck in the warm or bottom position. Try starting your engine again from cold and if it runs poorly until fully warmed then you have a cold running problem. If it runs OK right from cold then you probably had fowled spark plugs that cleaned up during your last drive.

[antoeknee]

Hi,

From cold it took 12 seconds to start up today, once started if ran well.

I don't completely understand the split linkage test, but I tried it today, and the revs went from 950 idle, to 1250, with the introduction of more air.
Revs dropped off with FI rod pushed down 5mm.

I have completed the Linkage setup, but have not yet done the timing, as I have never before used a strobe light.

I read that the timing should be done, before I mess around with fuel/air mixture.

Is the timing done at idle or at 3000rpm? I have read different things and don't fully understand how it is done, I have an 123ignition.

Thanks in advance