220SEB Only Firing on 3 Cylinders

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220SEB Only Firing on 3 Cylinders

Post by hcsodh » Thu 30 Nov, 2017 3:08 pm

Hi guys,

I recently purchased my first older style Benz - a partly restored 1961 220SEB sedan. The car looks fantastic in white with black/red leather, it's very clean and I'm very anxious to finally get this thing on the road - it has been sitting in the garage now for 2 months. I'm a bit of a backyard mechanic and I have a lot of experience with newer Benz's and electronics. There isn't much I haven't been able to sort out myself in the past but this really has me stumped.

When I picked up the car it had a problem with the generator and wasn't charging. Had the generator reconditioned and have checked/tested all the wiring to the voltage regulator (it is the old big black box type). I do believe the voltage regulator may have been connected incorrectly when I got the car, and in the process of diagnosing the problem one of the condensers attached to the regulator *exploded* and the regulator itself had some smoke emerging from it. Since then I have disassembled the regulator, checked/cleaned the points, verified the connections, removed the faulty condenser and now all seems to be good on that front - the charge light in the dash goes out when the car is started and the battery is reading just under 14v with the engine running.

Since this charging/regulator issue was resolved, the engine runs very rough and is only running on 3 cylinders. I have been checking this by attaching a timing light to each of the ignition wires. Cylinders 1-5-6 are fine, 2-3-4 have no spark.

So far I have replaced the cap/rotor with brand new Bosch parts, brand new genuine condenser, points look to be in good shape and I have gapped them to 0.35mm. Ignition wires are Beru and appear to be in great shape - have swapped them around to no effect. Have cleaned/checked all the wiring to the distributor and the coil is getting a solid 12v. If I pull a lead from a working cylinder the spark is blue and will jump a good 5+mm so not sure it's a weak spark or coil related.

I have been scouring the forums looking for ideas and I'm really stumped. It was firing on all cylinders BEFORE we started stuffing around with the generator/voltage regulator so I'm not convinced it has anything to do with the distributor itself. It all looks pretty clean in there, the advance rotates freely and the shaft springs back when turned by hand. The dizzy shaft does have some vertical play and while I'm not sure this is normal, it was running fine before. Plugs are NGK BP6ES from memory and were gapped around 0.28".

I've got a cheap chinese coil on order just so I can rule this out, but I don't understand how it could be the coil when half the cylinders get spark. I've also been toying with the idea of just installing one of those Pertronix electronic point sets but I'm not convinced this will fix it either at this point.

If anybody has any ideas on where to go from here I would be very grateful for your assistance. I've got some engine mounts and various bushes and things to do over the next few days so that'll keep me busy, but would really love to finally take this beast out on the open road.

Thanks in advance

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Re: 220SEB Only Firing on 3 Cylinders

Post by dynamike » Thu 30 Nov, 2017 10:18 pm

I'm sure you'll get more qualified responses but as a first thought have you checked all cylinders are getting fuel injected to them / at the right time?
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Re: 220SEB Only Firing on 3 Cylinders

Post by TheMadRacoon » Fri 01 Dec, 2017 12:36 am

Is it worth checking your spark plug leads are connected in the right order?
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Re: 220SEB Only Firing on 3 Cylinders

Post by Tony From West Oz » Fri 01 Dec, 2017 2:08 am

Since this charging/regulator issue was resolved, the engine runs very rough and is only running on 3 cylinders. I have been checking this by attaching a timing light to each of the ignition wires. Cylinders 1-5-6 are fine, 2-3-4 have no spark.
OK, what could cause this?
  1. Distributor Cap
  2. Spark Plug leads faulty at Dizzy cap end
  3. Distributor cam
  4. Dry joint / burnt or broken wire in distributor (still touching except for 3 cylinders).
It can't be the coil as this works for the 3 working cylinders.
It can't be the fuel injection as there was no spark at those 3 cylinders that are not firing.
It can't be air or exhaust as this is the same for all 6 cylinders.

Check for:
  • loose / broken wire on points plate
  • Check for faulty dizzy cap cracks or evidence of arcing
  • Replace the spark plug leads for the 3 non-firing plugs
Anyone else have ideas?
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Re: 220SEB Only Firing on 3 Cylinders

Post by Bartman4800 » Fri 01 Dec, 2017 11:20 am

This is a weird problem, because the ignition for a M127 only has one set of breaker points (are you sure you have the original dizzy?).

The fuel pump however does have a fuel output for 2 sets of 3 cylinders.

Assuming your assessment about having spark/no spark is correct, how about this option:

Check your breaker point gap (or even the dwell angle if you have a meter).
I have seen breaker cam lobes that were completely worn out (fiber follower must have worn out - steel on steel)
The points might close on some lobes, but not on others. You can check them for conductivity as each of the 6 positions off cam.
You already changed and gapped your points, so that cannot be it.

I threw out my worn dizzy and replaced it with a 123 ignition (not from a W123 :book: ) which is completely free of mechanical components.
But you should be able to make the engine run pretty decent with the normal ignition - considering it is not too worn out.

Spark plugs should be set at 0.8 mm; .28" = more than 7 mm :drool:

There is one thing you can do to eliminate your dizzy cap:

Attach the HT clamp of your timing light to the HT lead coming from the coil.
On slow idle you should be able to match the fires of the engine with the flashes of the light. If your engine misfires evenly it goes duf chaf duf chaf out of the exhaust.

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Re: 220SEB Only Firing on 3 Cylinders

Post by Mercmad » Sun 17 Dec, 2017 5:37 pm

The coil may be the original (generally a fat looking black unit) and the points might be open too far. Replace the condenser with A new one, Charge the battery and set the points really closely. The correct setting has a gap so small you can barely see it open .Try it again . You said you fried the regulator, these can be repaired actually and regular 220SE's have a common unit. 300SE's have a rare high out put unit. Did you polarise the generator before starting? If you didn't,that is what fried the regulator. A non working recently overhauled generator will do this as soon as you try and start the car. never forget this important step!. Now, i am thinking the lack of spark is simply lack of voltage getting to an old coil which is why i suggested charging the battery right up (13 volts on test) .If the coil is old replace it with a new one because as asoon as it gets hot it will fail, only to start working as soon as it cools down.
Another thing, have the injectors ever been out? are they installed correctly ? many years ago i had a car come to me with a similar problem, it refused to run right and they even rebuilt the head. The injectors on 1 2 and 3 were facing up the manifold instead of at the intake port... Make sure the brake booster hose is tight and the booster isn't leaking vacuum.

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