"Brunhilda" misfires when she gets going ; any Advice?

[Windtramp]

Greetings all.

Am a total newbie to MB's in general and especially to old girls like i have here; a '79 W123 230 Merc.(Brunhilda) This Merc i have only had for a month; have zero resources (books manuals etc) to help with maintaining her, and know zilch about them . .. apart from discovering her horrendous "drinking problem" ; and am busy sorting out a few teething problems.

After a comprehensive service and then a carb rebuild by A VERY reputable carb rebuilder (who specialises in older vehicles); who did a magnificent job on overhauling the Stromberg 175 CD, she now starts, idles, runs and sounds much more like a MB should.

About a week after the job i took her back for some minor tweaks (mixture adjustment and choke settings) to the carb, and about two days later when i took her for a run on the freeway i noticed that she now has an ever so slight misfire at higher speeds (120 Kph upwards), getting progresively worse the faster she goes. I backed off at about 145Kph as i swear the fuel companies were queing up for my business. Experimenting showed me that at any speed, but with a HIGH rpm she would misfire. At a moderate RPM when i put my foot down hard to accelerate she also misfires, but noticeably so. If i really featherfoot it i can mostly get to a good high speed (110) without any misfire, but any hard accelaration at any time will bring it on.

So before we rip into the entire fuel and ignition system, my question firstly is to this august gathering of MB lovers is:

1) is this a common problem with this model and engine? If so What is the most likely solution?

2) if the answer is NO to the above; any advice on where one should start in the long trouble shooting road?

My first choice would be to look for a potential duff plug, then a duff HT lead, then coil ... etc etc. However not knowing anything about MB's in particular, i wish to bounce this off those on this forum who know and love these patricular older MB's

Background info. She has about 217 000KM (genuine) on the clock, i have copies of a reasonably comprehensive service history, showing no particular nasties, and she was not driven much the last three years .. maybe 2000km in total, and she did stand a lot during that time. Right now she drives about 80km per day, 50/50 freeway/urban driving. Maybe this is shock to her system?

Any thoughts and advice would be hugely welcome:?

 

[dieselman]

I would folow your diagnosis approach, but given the work done would htink fuel starvation is quite possible. Have the carburettor settings and float height checked and reset.
It would also be worth checking the fuel pump delivery pressure when under load.

 

[television]

Is the cold start OK as it uses more fuel when starting.

If the fuel filter has never been changed then do change it, under the right rear wheel

 

[Windtramp]

@dieselman; was planning on taking it back to the carb guys for a recheck; one significant thing to me was that it was performing fine till i took it back for the tweaks .. your comments confirm my decision; re checking fuel pressure; i had the fuel pump replaced at the service as it was leaking pretty badly, but hey .. just coz its new does not mean its delivering full flow :neutral:; Good idea there, will get the carb guys to run a fuel flow check as well,

@television; When i took it in for a service we discovered that there was NO inline fuel filter, so i had one installed, and when they cleaned the carb out .. man was that float bowl full of junk!!!

However, i will look around and see if one of the previous owners hadn't installed another inline filter somewhere. After running so hard for the last month its quite possible another "hidden" fuel filter could be quite choked by now. Thanks for a good idea there.

Re the cold starting, no worries there, BUT it is still summer down here in RSA so i dont think it really is cold enough yet to be considered a cold start .. 15 C /60F at night ... now thats almost midsummer for you guys up north

For me the significant thing is that it ran well for about three weeks before i was able to get the carb overhauled, so IF it is a fuel filter/fuel pump thing, then surely it would have reared its ugly head before hand?

 

[Colin_P]

My money is on a duff spark plug or ht lead...

 

[grober]

Misfire is usually ignition related so you are on the right track possibly. Have these cars got manual tappet clearance adjustment? I would be looking at checking those clearances also if they are. Other possibility is that you got some dodgy fuel- did it happen shortly after the car was refuelled somewhere - perhaps not your usual gas station?

 

[Alex Crow]

you do know there is a filter in the tank don't you? it is screwed into the bottm of the tank and the fuel oulet comes straight through it.

 

[jefrs]

The carburettor is essentially the same one fitted to Volvo 240, it is very reliable.

A misfire may be caused by the spark breaking down under pressure at higher revs, i.e. the electrical ignition system, not the fuel (carb).

It starts and runs at low/medium revs? - assume timing is right.
It breaks down at high revs or on power? - look to the HT side - plugs, leads, distributor cap, rotor arm. I'm assuming you have electronic breaker-less ignition so don't have points and condenser.

Plugs erode, fur up inside or track down the porcelain.
HT leads and caps loose their insulation.
Distributor cap gets dirty or cracks - the HT tracks down/through it.
Rotor arm corrodes.

The Volvo 245 greatly enjoyed triple-electrode plugs giving it better pick-up and smoother mid-range.

 

[Mic]

It took me ages to resolve a problem with identical symptoms on my MG TD in my student days when I didn't mind getting my hands dirty. Eventually tracked the problem to slight wear and hence movement under load of the drive shaft to the rotor arm which in turn was effecting the spark.

Mic

 

[Windtramp]

Wow, gentlemen i am really amazed by your willingnes to assist a total stranger like this. Thanks for all these really usefull leads. @alexander patient , nope i did not know about that filter in the tank, as i said i know nothing about these cars :Oops:; but that is one worth checking out for sure. Weekend was hectic but i was able to buy another set of plugs, but not yet able to fit them due time constraints.

Will let you all know how it goes trouble shooting this week.

Thnaks once again for your advice.

 

[Windtramp]

Problem solved

Hi all.

First, greetings for the new year, trust you all had a good festive season and news years celebrations.

After a long year of trying various things to sort this problem of intermittent misfiring out, i finally got it solved. After replacing HT leads, plugs, dizzy cap and rotor, after checking timing, gaps and carb i was starting to look somewhat fearfully at removing the fuel tank filter .... rusted solidly in place .... SO i swung by a private workshop for some advice from the Father/Son team who have their own vehicle repair business, specialising in MB's. They are both ex MB Mechanics, but that's another story.

The problem lay with the coil and resistor. I replaced both and haven't had had a hiccup for 3 months and 3500km now. Before i was getting intermittent problems almost daily towards the end.

Apparently the W123 230 (in RSA at least) uses an 8 volt coil. The 12V system is reduced to 8V by a ceramic resistor block on the LT supply line to the coil. The previous owner of my vehicle had bypassed the ceramic resistor block (because it was faulty) and fed 12V directly to the coil. Of course the system worked, but only for while. Due to the extra voltage the coil was now breaking down. And when she was hot from a fast freeway run, heavy slow traffic, or running at high revs, the coil would stop working. After a few seconds (up to 45 seconds) it would have cooled down enough to work again. Sounds wierd, but that seems to have been the problem.

I sourced another second hand coil and resistor block, wired it up correctly this time and she runs without any intermittent problems at all.

So thanks to all those who gave their input. Appreciate all the advice

 

[WG M-B]

Great to hear your baby is back to her best

 

[Alex Crow]

These older cars often used a different ballast, or none at all during startup (when batt voltage dips to circa 9.5v anyway) to aid starting.

If someone removes the ballast, and you are not familiar with the setup, it is very difficult to guess what is wrong!
Guess you were lucky to find someone to look at it that actually knew what should be - glad you have it sorted now.

 

[Windtramp]

Great to hear your baby is back to her best

You bet ... really nice to be back on the road without concerns.

 

[Windtramp]

These older cars often used a different ballast, or none at all during startup (when batt voltage dips to circa 9.5v anyway) to aid starting.

If someone removes the ballast, and you are not familiar with the setup, it is very difficult to guess what is wrong!
Guess you were lucky to find someone to look at it that actually knew what should be - glad you have it sorted now.

Alex you are so right ... i spoke to a few different people this side, but none really knew these older models. But this father/son team i mentioned seemed to know these models pretty well. I think the dad did his apprenticeship on these models back in the 70's so their number is on my speed dial now They did also warn me that if the coil replacement did not work, the next logical place to look would be that fuel tank filter .... And having being a coastal car there is a better than even chance that filter would be pretty siezed up by now ..

Glad this worked .. got off lightly it seems.