Author Topic: Intermittent bad misfire (Read 23954 times)

jaymanek · « **on:** June 06, 2011, 09:36:03 »

Hi Gentlemen,

I have a 1969 280 SL Euro.

As you may have read I have just rebuilt the head on my car and was feeling very proud of myself until yesterday.

The car was running great after I did the head.. last week i decided to set everything up... idle air/fuel, ignition timing etc.

Yesterday the car started misfiring badly whilst cruising, if i dip the clutch the car would cut out. However the problem dissapears as quickly as it comes... Upon restarting each time the car would run fine again for some time... then the problem would reappear...

I have set the timing to 30deg with vac hose connected as in my haynes manual. Initially I had set it to 30 with hose disconnected as per instructions on here, but that must be for US cars or maybe earlier cars as my car did not like that setting one bit.

Something to note is that before I set the timing up, it was running well but was in the region of 50 degrees at 3000rpm... The car seems to run stronger the further the ditributor is turned anti clockwise...

Dwell angle is still around 33, where I set it last year... I realise it should be 38 but i struggled to get it any closer and seemed to run fine...

Apologies if the above doesnt make sense, but im not that used to older ignition systems!

Any ideas please?

p.s. most of the system is new - Distributor cap, rotor arm, condensor, points, plugs & leads... the only old parts are the coil and ballast resistor as far as im aware.

jaymanek · « **Reply #1 on:** June 07, 2011, 15:28:22 »

Well I changed the coil last night with a new Beru replacement and set the timing to 35 degrees at 3000 rpm instead of 30... went for a decent test drive and no problems... holding my breath but i have a feeling it will come back to bite me!

Cees Klumper · « **Reply #2 on:** June 07, 2011, 17:00:04 »

It does sound like an ignition problem to me, although once I had forgotten to tighten down the bracket that holds the accellerator linkage across the engine. I had tightened the bolts by hand and there was a slight amount of play. Well that caused very similar symptoms as when the ignition is not right. So you may want to just double check all the connections in your linkages and make sure they're tight.

P.s. I have also found that when spark plugs have fouled because of whatever cause, this can render them bad, and cleaning does not help, at which point they have to be replaced. So putting in a fresh set of plugs even if the current ones are not old and look ok, may just save a lot of other diagnostics and trial and error. If they are not the problem, you can always put the old ones back in and save the new ones for the next service.

jaymanek · « **Reply #3 on:** June 07, 2011, 17:29:50 »

Thanks...

Now that you mention it.... I also adjusted all the throttle linkages last week, could there be a relationship here?

I printed off and followed the instructions on here, and got the car idling perfectly - the previous owner had set the linkages so at idle neither the fuel distributor nor the air intake were quite in the closed position... a common thing from reading posts on here...
The issue I have in this department now is that the accelerator pedal seems to stick slightly at the beginning when i depress - i guess i need to lubricate my air intake flap.

ja17 · « **Reply #4 on:** June 07, 2011, 18:21:37 »

Hello Jaymanek,

The sticky accelerator pedal could be caused by the venturi flap sticking in it's bore. Adjust the small slotted screw on the venturi a bit to keep this from happening if this is the case.

Otherwise the linkage rod from the cross over rod to the linkage pivot on the intake manifold may need adjusted so that it is not resting on its top dead center.

jaymanek · « **Reply #5 on:** June 08, 2011, 08:39:12 »

Thanks, Ill do that tonight and report back.

mdsalemi · « **Reply #6 on:** June 08, 2011, 11:25:31 »

Jaymanek,

Hope you found your problem. Though not entirely applicable here, there is a general penchant to blame fuel systems for electrical issues...

...which is why an old tongue in cheek service manual I had once for a 1960s Austin said that "Carburetor is a French word meaning 'It's the electrical system, stupid'!"

The same book also had, as part of the points adjustment, a directive to remove the rotor and place in your front pocket. When the instructions then called for reassembly, it said, "Now, remove the rotor from your front pocket..."

jaymanek · « **Reply #7 on:** June 12, 2011, 11:32:08 »

Well ive done several trips now and all seems well..

I changed:

- Coil pack - Standard Beru Replacement
- Adjusted dwell angle to 38 deg, it was at 33 deg previously as I couldnt get it spot on before.
- adjusted timing to 35 deg at 3000rpm (vac connected) as opposed to 30 deg in the book
- New distributor cap (old one was recent from the dealer anyway)
- New rotor arm (old one was recent from the dealer anyway)

I think the most likely real cause was the coil pack and maybe the timing.
Car pulls really well now with no hesitation or spluttering...

Lets hope it lasts!

Benz Dr. · « **Reply #8 on:** June 12, 2011, 18:49:47 »

Sometimes the distributor isn't indexed properly. The drive gear below the distributor can be turned so that the rotor will point to number one on the cap when the crank is at TDC and both valves are closed on number one cylinder. If the drive gear is out one tooth, the marks on the front balancer will probably be off as well. The car will appear to idle well at 40 degrees when it's reality it's at 5 degrees BTDC ( just an example, yours may be some other number ) You can set the distributor anywhere you want and as long as it's time the engine will run. Number one will strobe at a different place on the crank unless everything is indexed correctly.

You can index the distributor or live with it.

jaymanek · « **Reply #9 on:** June 13, 2011, 09:04:41 »

Thats an interesting thought Benz Dr.

I will look into this although im reluctant to change anything now that its running good and starting on the button from cold or warm..

graphic66 · « **Reply #10 on:** June 14, 2011, 04:33:09 »

Some great advice is "don't fix it if it isn't broken".

Benz Dr. · « **Reply #11 on:** June 14, 2011, 14:03:20 »

Quote from: graphic66 on June 14, 2011, 04:33:09

Some great advice is "don't fix it if it isn't broken".

If I looked at everything that way I'd be out of work. Basic maintenance is all about fixing things that aren't broken. My philosophy is, '' I can make it work better . ''
After doing this for 33 years I tend to see things most people miss. Not everything and not right away, but I do see them eventually.

jaymanek · « **Reply #12 on:** June 15, 2011, 09:31:23 »

Unfortunately the problem came back...
It seems to start when driving at high speed, then the spluttering will gradually get worse until stopping and restarting the car.

I still feel the issue is ignition related but if it was timing then wouldnt the problem always be there?

I changed the condensor and points last year (mb items) but maybe I should change them again just for completeness?

Any ideas?

ja17 · « **Reply #13 on:** June 15, 2011, 13:15:46 »

Hello jaymanek

Make sure you have the correct coil and ballast resistor or you may be burning out other ignition items.

jaymanek · « **Reply #14 on:** June 15, 2011, 13:24:40 »

------

see below

jaymanek · « **Reply #15 on:** June 15, 2011, 14:06:31 »

Just a thought but all the parts ive ordered have been for non transistorized ignition.

Mine is a 69 with cast iron distributor, am i correct in thinking this is non transistorized?

jaymanek · « **Reply #16 on:** June 15, 2011, 18:05:26 »

Ok I dont think my car is transistorized as I cant see any control units under the battery and the wire from neg of coil goes to condensor directly.

The new coil I have been supplied says it is 1.8 ohm and the ballast resisitor says it is 0.9 ohm.

This doesnt seem to tie in with any of the figures in the techical manual!

My car is a 1969 euro car.

From what I can see I should have a coil that has 0.4 ohm resistance and a ballast that has 1.8 ohm.

However the ballast was supplied by Mercedes and the only other one they list is the one for transistorized systems which i believe is a 0.6 ohm.

HELP! What should I have.?

Im wondering that i may have too low resistance which is giving the condensor a tough time so is causing the misfiring under load I am experiencing. Or am i on the wrong tracks?

ja17 · « **Reply #17 on:** June 16, 2011, 01:38:35 »

Hello,

Sounds like your coil calls for the 1.8 ohm ballast resistor. Change your ballast resistor, ignition points and condensor and the problem should go away for good unless the coil is damaged also.

clembeauch63712 · « **Reply #18 on:** June 16, 2011, 03:53:20 »

Hi All; I too had a similer issue.. Sara Jane would run 15 mins and quit. If i hit the coil with 12v it would run till i took the 12v jumper off and stall put it on again and wold restart..I examined the coil to find it cracked accross the top and was leaking oil from it. I ordered a non resistor coil for $15.bucks from Rock auto, installed it and all is well runs better than ever..I also rebuilt the WRD and now doesnt quite shut all the way but only very light suction at the filter port.. Had an issuer with the inj. pistons - frozen and that was my first fix - watching Joe's posting of recent help.. I also had an inop rack solinoid: But I called H&R and Hans a very willing to help and nice man advised me to tweek the black bakelite rear cover slightly and peen connections and i got luckly it appear to be working again.. She start well & Idles in cold run at about 1100 for 5minute warmup and then drops to my setting of 700 rpm .. couldnt be better.. Regards & thanks all for your help and convidence to geter done.. Clem Beauchemin in CT. PS had a great weekend in ME at Gernold's Shop..

Benz Dr. · « **Reply #19 on:** June 16, 2011, 05:15:31 »

Diagnostics is a science all to itself. One that can take a while to learn and becomes much harder the more the models of cars you work on. If you only work on one type of car, you can get pretty good at figuring things out.

jaymanek · « **Reply #20 on:** June 17, 2011, 14:53:39 »

Today have changed:

Ballast resistor - 1.8 ohm
Points
Condensor

I think my ignition system is all new now! Dwell angle set at 38 degrees, managed to get it spot on first time!

Fingers crossed.

jaymanek · « **Reply #21 on:** June 19, 2011, 10:19:43 »

Still got the problem, it only seems to do it after thoroughly hot..

Any ideas anyone?

ja17 · « **Reply #22 on:** June 19, 2011, 13:24:54 »

Hello jaymanek,

Remove the spark plugs and let us know how they look. If the engine is running lean the plugs can short out internally causing a high rpm misfire, especially when the engine is hot.

A fresh set of NGK spark plugs can cure the problem with little expense.

mdsalemi · « **Reply #23 on:** June 19, 2011, 15:20:38 »

...and that would be NGK BP6ES I believe...that's what I've got in mine, a '69.

jaymanek · « **Reply #24 on:** June 20, 2011, 11:21:19 »

Hi,

Plugs are all new, I got them from Mercedes to avoid the resistor plugs issue. Leads etc all new too.

Went for another long drive this morning and still the same..the car just gets bogged down after driving around 4-5 miles at speed and when the revs fall, it will just cut out...
The issue seems to build up over a couple of miles, i.e. there will be some intermittent spluttering which gets worse until the car misfires badly and has no power.
A small rest and all is fine again for a little while.

I listened to the fuel pump and that seems to be buzzing away as it should.

Its got to be something simple!?

I have disconnected the distributor advance hose to see if that makes any difference but havent had chance to drive it far enough yet.

News:

Author Topic: Intermittent bad misfire (Read 23954 times)

jaymanek

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graphic66

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clembeauch63712

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