Ignition problem/misfire: I can't drive 55 - PeachParts Mercedes-Benz Forum

gmask · #1 06-12-2003, 11:44 PM

Okay.. a problem developed as I was barrelling back from las Vegas and since I haven't needed to do much highway driving I am only now getting around to solving this one.

basically when I try to drive over 55 the car hesitates.. I guess it's a misfire although at first I thought it was a vacuum leak.

From what I've read this could be because my distributor is worn and I can't really set the point gap anymore.

The plugs all look good and all seem to be getting a spark.

How should I isolate this problem?

Thanks

Adrian

ctaylor738 · #2 06-13-2003, 11:07 AM

Recommendations:

- Check your dwell, and make sure it is holding steady at about 34 degrees as you bring the RPM up to 3000 or so.

- Check the timing and make sure you are getting the right amount of advance at 3000 RPM (it's in the book)

- Make sure your carbs' secondaries are working right

- Make sure your fuel pump is putting out enough gas, fuel filters clean, including the little buggers in the carbs.

gmask · #3 06-13-2003, 12:03 PM

Quote:

Originally posted by ctaylor738
Recommendations:

- Check your dwell, and make sure it is holding steady at about 34 degrees as you bring the RPM up to 3000 or so.

- Check the timing and make sure you are getting the right amount of advance at 3000 RPM (it's in the book)

- Make sure your carbs' secondaries are working right

- Make sure your fuel pump is putting out enough gas, fuel filters clean, including the little buggers in the carbs.

Alas my dwell readings are next to nothing?? The cam is really worn on my distributor and my readings are usually only 10-20 degrees. I was thinking about having Randy rebuild mine (god rest his soul) but now I guess I should consider doing the petronix conversion myself or fork it out for a new one.

There was actually a problem with the secondaries .. the vacuum passages in both were clogged so I swapped them out with the lower sections off some other carbs I had. When they were clogged I was getting hesitation at lower speeds but that seems to be fixed now.

The fuel pumps seem to be fine..I get a good strong stream from both. These filters you mention where are they located in the carbs? What are they made of?

I have the feeling that it is probably the vacuum advance.. so I'll check that first.

ctaylor738 · #4 06-13-2003, 01:19 PM

You gotta fix the distrbutor if that's all the dwell you're getting - you are probably not able to generate enough of a gap to fire the plugs at higher RPM. Either the Pertronix or the Crane is pretty painless to install.

There are/should be little filters where the fuel lines meet the carb bodies.

gmask · #5 06-13-2003, 01:25 PM

Quote:

Originally posted by ctaylor738
You gotta fix the distrbutor if that's all the dwell you're getting - you are probably not able to generate enough of a gap to fire the plugs at higher RPM. Either the Pertronix or the Crane is pretty painless to install.

There are/should be little filters where the fuel lines meet the carb bodies.

LOL.. well amazingly I have put about 10k on this car since I first checked the dwell and those have allways been the readings. GO figure?? Although I think the last time I tried to set the gap I left it a bit close... don't know why it decided to become a problem after it drove fine for awhile.

I'll look in the fuel lines for the filters.. I guess it's wire mesh filter?

Guess I'll go with the Petronix.. it certainly is cheaper than a new distributor.

Thanks

Adrian

gmask · #6 06-13-2003, 01:33 PM

http://www.pertronix.com/2003%20PerTronix%20Catalog.pdf

Okay here's the thing with the pertronix.. which kit should I get??

They list one for a 1970 250c 280 which I'm thinking is the closest.

skalie · #7 06-13-2003, 01:46 PM

Don't know sweet sod all about Mercedes petrol engines, but I once solved a similar problem on an Opel by replacing the capacitator in the distributer.

When they die it makes the car run rough at a certain speed.

Cost of parts.........about one dollar.

Time to repair.........about a minute.

Worth a shot, assuming that there is one to replace.

gmask · #8 06-13-2003, 01:51 PM

Quote:

Originally posted by skalie
Don't know sweet sod all about Mercedes petrol engines, but I once solved a similar problem on an Opel by replacing the capacitator in the distributer.

When they die it makes the car run rough at a certain speed.

Cost of parts.........about one dollar.

Time to repair.........about a minute.

Worth a shot, assuming that there is one to replace.

Hmm.. I dunno.. but something broke or got knocke dout of whack when I was driving back from LV.. I thought I made it out of there with my wallet but I guess not???

From the diagram I have the internals appear to be mostly mechanical and no capacitors. Anywho.. I'm goign to check the advance in a few minutes and see what's up?

gmask · #9 06-13-2003, 03:30 PM

Well so far I have found that the gap was non-existant. The car seems to be better although it is making a "chaining/clicking" noise when under load.. I'm not sure but I have had problems getting the distributor cap on straight in the past so I'm hoping that's the noise maker.

So now i'm going to check the advance..

gmask · #10 06-14-2003, 12:21 AM

Advance looks fine now I have set the gap back to spec.

ctaylor738 · #11 06-14-2003, 08:54 AM

Probably what happened was that the rubbing block on the points wore down and decreased the gap. When this happens, the points also open later, which retards the timing. What if you tried a new set of Bosch points which comes with the now hard-to-find grease for the lobes on the distributor shaft?

I think that the 250C/280 Pertronix or Crane unit would be fine. All the six-cylinder models used the same basic distributor. There are a huge number of posts on this conversion, probably second only to our beloved Zeniths.

gmask · #12 06-14-2003, 11:37 AM

Arggh.. my beloved Zeniths are still giving me heck..

On the rear carb I must still have a leak somewhere.. The only way I can get them synced is to disengage the lead to the automatic choke and crank it around so that the choke flap is closed. Otherwise the RPM's bounce all over the place and I pretty much cannot get it it to settle between 700-900 RPM. After closing off the rear choke flap it settled around 800rpm. The idle is still rougher than I would like but it runs.

I think there must be a leak in the base because I have sanded and the top and replace the gaskets in the upper part of the carb.

I dunno.. I have almost bought webers but something tells me I should solve this leak first because I'd feel silly if I got the new carbs and still had the same problem.

ctaylor738 · #13 06-14-2003, 02:04 PM

That is the classic symptom of a vacuum leak. You are getting way too much air because of the leak and so you need to close the choke to reduce the amount of air. I used to have to put a rag over the rear on one of my former carbs to get it to idle.

Have you tried spraying carb cleaner around the base of the carb to locate the leak?

I have parts carbs if you need another base.

gmask · #14 06-14-2003, 03:53 PM

Quote:

Originally posted by ctaylor738
That is the classic symptom of a vacuum leak. You are getting way too much air because of the leak and so you need to close the choke to reduce the amount of air. I used to have to put a rag over the rear on one of my former carbs to get it to idle.

Have you tried spraying carb cleaner around the base of the carb to locate the leak?

I have parts carbs if you need another base.

Yeah that's pretty much on the mark.. I haven't replaced the gaskets in the lower part yet but I have new gaskets so that's next on the list of things to do.

#15 06-14-2003, 05:32 PM

two things i want to add:

on my 250 i ended up using the crane unit because petronix was incorret, 2nd is the little box under the battery tray is the ignition
control module , turned out that mine was intermittant so I could bypass it with the crane. works very nice.