help! 250 engine hesitation, stall

Hello,

I recently bought a 1972 250 with the dual Zenith carbs. It runs ok, but it has severe hesitation and cold starting problems. It idles pretty rough, and when I step on the accelerator, it either stalls or hesitates. If you ease into it, it willingly climbs into the high RPMs and sounds/runs ok. Anyway, its got me stumped...I've cleaned the carbs, replaced the distributor, wires, plugs, air filter, alternator, fuel filter, cleaned the gas tank...and it still runs pretty much the same! I tried adjusting the timing yesterday, but that didn't seem to help at all. What else should I look into? Is there a fuel filter screen anywhere in the fuel system? I've only replaced the filter. Could it be a weak fuel pump? Or maybe I need to adjust the distributor?
If anyone has any ideas, I'd really appreciate it. This is my 1st MB, so I'm a little unfamiliar under the hood.

Thanks!

[DANTRCAV]

Wow - with all that work you would think it would not stumble. I have a 71 250C and went through some of the same problems. Off the top of my head considering all you have done I would look at the following:

The accelerator pumps in BOTH carbs should produce a good flow of fuel the instant the throttle is moved off of idle.

BOTH carbs need to be syncronized (or VERY close) or strange things happen.

I set my timing at 10 degrees BTDC. If you timed yours by the book at 4 degrees ATDC you might try 10 just to see if that helps.

Understand that the Zenith carbs are a bear to get setup but once done they do a pretty good job and they are matched to the 250 engine. You said you cleaned the carbs, does that mean you did an overhaul with a kit? You need to get the idle screw adjustment as close as possible on both carbs. Once you have the carbs in balance then work with the idle screws until you get the same rpm drop on each carb when you plug the idle vent holes on the top. When I had the Zenith's on my car I set the idle adjustment screws 2 turns out then, once I balanced everything, worked from that setting. Another problem is that the top plate (cover) warps from people tighting the aircleaner screws too tight. You can "surface" the plates by grinding them on some 300 - 400 wet/dry sandpaper on a FLAT (i.e., glass, metal bench) surface. If you do this, file about 1/32" or so from the BOTTOM of each choke plate as they may stick if you don't (since by surfacing the plates you will have removed some material and lowered the position of the choke plate).

I replaced my Zeniths with Webers. Solved a lot of problems but I still had that off-idle stumble. I replaced my ignition system with a Crane XR700 optical system and the stumble went away.

Good luck. Let me know if you need anything else. Also, C. Taylor (no relation) on this list also has done a lot of work in this area.

Have fun..


Dan

Thanks for your reply. I'm going to try and balance the carbs today, and adjust the idle. I'm a little nervous b/c I've heard horror stories about the Zenith carbs, but I gotta try. I didn't use an overhaul kit last time, I just cleaned them out and adjusted the choke. I'm also going to take your advice and set the timing to 10 BTDC....its definetly worth a try.

I'm sure I'll have more questions, and thanks again!

[bstreep]

For what it's worth. I have a 190SL and Weber carbs. My first Mercedes (many moons ago) was a 250S with Zenith Carbs. I was a poor college student. If I had known, I'd have sprung for the Webers at THAT point in time. Now that I can certainly afford it, I wouldn't spend one more minute with the Zeniths... Also, for less than $100 you can put in a Crane electronic ignition. WHAT A DIFFERENCE it makes. Your 2nd purchase...

At times like this I'm homesick for my 250S. If I ever find another in decent shape...

[ctaylor738]

Also sounds to me like carb balance

When you put the airflow meter over the carb, make sure that you get a good fit. Also recommend that you plug the hose from the intake manifold to the air cleaner before starting. You will probably find one carb "off the chart" and one not registering at all when you start out.

This is not a job for the impatient.

[cth350]

Stick with the Zeniths. They can be made to work and work well. Do you have diagrams for all the vacuum do-dads?

Also, DON'T balance the carbs, until you have gotten the ignition to spec, the valve clearances to spec and the gas tank at least half full of quality petrol.

Follow the specs for the '72 engine....

1972 250/8:
Distributor: 0 231 142 005
Dwell: 30-36 degrees (new points should be at 36)

Firing SET point at 800 RPM WITH vacuum: 4 degrees ATDC
TEST point at idle (800 rpm) W/ vacuum: 1 ATDC to 7 BTDC
TEST point at 1500 rpm W/ vacuum: 42-50
TEST point at 3000 rpm W/ vacuum: 46-54
TEST point at 4500 rpm W/ vacuum: 8-12

AFTER you have adjusted the valve clearances on the cold engine, THEN warm the car up with a short, easy drive, and set the DWELL and Firing point. When you test the dwell, the maximum allowable deviation from 800 RPM and 3000 RPM is 3 degrees. More than that, and you'll have to send it to William Durrance for a rebuild. Also, the various ignition values presume that you still have the transistorized system installed and that it is working OK.

-CTH

Well, I got the carbs running pretty well. Idle set, balanced, and I set the timing. The car definitly runs better than before, but I still get hesitation. So I started checking for vacuum leaks, and I found out that there was nothing going to the distributor! I think I may also have a problem with the vacuum advance diaphram on the distributor. I hooked up a vacuum gauge, and its not holding anything....so I assume that the diaphram is worn. Can I just replace the diaphram? Also, there are 2 vacuum inlets (outlets??) on the diaphram.....and I don't know where the the vacuum hoses go because there weren't any attached before. Does anyone know where I can find a good vacuum diagram?

I guess my main question is: if the vacuum advance doesn't work, would this be a cause for hesitation? My guess is yes, but I'm not sure.

Also, the car seems to run worse as it warms up. Is this because I'm losing oil pressure as viscosity increases? It misfires more frequently after about 5 minites of idling.

Thanks again!

Well, I got the carbs running pretty well. Idle set, and I set the timing. The car definitly runs better than before, but I still get hesitation. I was going to start to check my valve clearances, and balance the carbs, when I noticed that there weren't any vacuum lines going to the distributor! (I should have noticed this before, huh?)
I think I may also have a problem with the vacuum advance diaphram on the distributor. I hooked up a vacuum gauge, and its not holding anything....so I assume that the diaphram is worn. Can I just replace the diaphram? Also, there are 2 vacuum inlets (outlets??) on the diaphram.....and I don't know where the the vacuum hoses go because there weren't any attached before. Does anyone know where I can find a good vacuum diagram for this '71 250?

I guess my main question is: if the vacuum advance doesn't work, would this be a main cause for hesitation? My guess is yes, but I'm not sure.

Also, the car seems to run worse as it warms up. Is this because I'm losing oil pressure as viscosity decreases? It misfires more frequently after about 5 minites of idling.

Thanks again!

[DANTRCAV]

Can't help you with a diagram, sorry. When I got my 250C some of the vacuum lines were connected and some were not. After studying the situation and verifying that the various temp switches, relays and 2/3 way valves were not working either I just pulled all of them and plugged the vacuum source(s).

As far as I know there is no "kit" to fix the distributor so you will have to try to find a vacuum unit. The two inlets are as follows: The one pointing to the firewall is the advance port and the one pointing forward is the retard port. Original system w/all vacuum lines working used both the advance and retard sides at various engine speeds and conditions. Also was an early "smog controll" attempt.

When I ran by Zeniths I did NOT USE any vacuum advance I just set the timing at 10 degrees BTDC. The vacuum port(s) on the Zeniths (at least mine) are/were manifold vacuum and NOT s-port vacuum so there is/was full vacuum at the port at idle. S-port vacuum is applied as speed (hence airflow through the carb) increases which provides vacuum advance (I think). I do use the vacuum advance with the Webers but I still leave the timing at 10 degrees BTDC.

Double check for vacuum leaks and make sure that the vacuum pipes on the Zeniths are plugged at least while you trouble shoot the problems. Also, there is a hose (pipe) fitting with an orifice in the intake manifold between the carbs. This is the intake from the water/liquid separator at the bottom of the aircleaner. It is metered (has an orifice). Make sure that you still have the aluminum insert that the hose attaches to as that is what contains the orifice. If this is missing you will have a very lean and nasty engine.

As I stated before, after all the work I did I still had a hesitation problem that did not go away until I replaced my ignition with the Crane XR700. I had read on another list where an owner of another 250C had the same problem and had his distributor rebuilt and a new Bosch blue coil that solved his problems.

BWT: If you have any vacuum device that will not hold a vacuum (distributor, A/C solenoid, heater valve, etc) then, in effect, you have a vacuum leak and that will upset idle, transition, etc.

Hang in there....

Dan


[Edited by DANTRCAV on 02-15-2001 at 10:26 AM]

[cth350]

The 1971 model had some fancy cross-over circuitry for retard and advance. The schematics exist and they're not too difficult to deal with. I'll scan them and email them to you.

Email me directly (cth@Mbz.org) so I can reply w/ the attachments.

-CTH

[ctaylor738]

My parts book shows a replacement vaccuum advance unit. Figure 85/195, Group 15/2, part 114. You should be able to get it from a dealer, or you might try Star Quality on the web.

Also recommend that you connect the advance to a straight vacuuum source and forget the rest of the lines, diaphragms, etc.

You might also check to see if your secondaries are opening correctly. They are vacuum-controlled as well.

250c4ever

[bachlauter]

I just bought a 1969 mercedes 250 ALSO with the zenith twin carbs and my car won't start! I bought it in a non-working state, but I had no idea what a nightmare this would be. First off, I don't know much about cars anyways! Yikes! So, my car cranks really strong and well and sounds like it is just aboout to start, but doesn't. A mercedes mechanic diagnosed it as having clogged carburators. I took the top off one of the carbs and it has good clean gas in it. I pulled out one of the jets and it has a nice hole in it. The previous owner says that when the car warmed up, he witnessed the plates open to a certain degree, which I thought indicated that they worked. I don't know what to think. The mechanic who looked at it said he tuned them to be in sync, but I'm not so sure because this guy usually works on the modern mercedes' and isn't very old. So PLEASE anyone wiht any suggestions or thoughts on this HELP ME ASAP!!! Christy

[cth350]

You have just put your foot in it. Carbs are complicated critters, but with patience, YOU will figure it out. I did, and I'm hardly a champ mechanic.

OK, those were the words of encouragement. Now, comes the work.

First off, if you mechanic balanced the carbs, he had to do so on a running engine. Yes? Alternatively, he just set things up so that there's a chance of getting it running. [Please pick one.]

Second, do you have brand spanking new, COPPER plugs in the car? If not, toss whatever's in there. DON'T get the bosch super platinum things, they are NOT recommended for your car.

Third, have you verified that the dwell is correct and that the timing is about right?

Fourth, have you adjusted the valve clearance?

DON'T even think about skipping one of these steps.

Finally, with ONE shot of ether down the primary of each carb, does the engine catch and run for about 2 seconds?

If yes, then we can start messing with your carbs.

-CTH

PS. Oh, while you have ALL the plugs out, do a compression test and report back the results for each cylinder. If they were indeed old plugs, which ones are black, which ones are wet, which ones are grey, which ones are brown.

[bachlauter]

WOW! Thanks for the great reply AND the words of encouragement!! :-) Ok. I'm NOW on a search for COPPER plugs! Everyone I've spoke to so far thinks I'm crazy. Of course I didn't think the local "pep boys" would have them, but they'd never HEARD of them. SO, I called Mercedes thinking this must be some specialty item and they don't know what I'm talking about either. The mercedes people say they have the plugs for may car, but they are basically regular plugs (except they cost $3.00 a pop!) So, should I go with the mercedes people and get THOSE plugs or do YOU know where I can get these COPPER ones you speak of!
Can you believe I'm only at phase one of your plan??!!
Also...the mechanic who supposedly put the carbs in sync, did it with the car NOT RUNNING. So.....I still have yet to do the compression test and the ether test. BUT...isn't ether illegal? Will I be able to get it?? I have starting fluid WITH ether in it, but no pure. Hmmmm. Let me know!!
Your the best! Christy
Also...could you tell me how to hook up a remote starter? One end goes on the starter and the other end goes???? (boy do I need help!)

[ctaylor738]

Any good European-oriented auto parts place will have the Bosch copper plugs - look in the Yellow pages.

It is impossible absolutely positively impossible to sync up the carbs with a dead engine. You basically need to work through the ignition problems before worrying about the carbs.

There are two small wires posts on the starter solenoid. Get about a six foot length of wire with aligator clips on each end. Clip one end to one of the small posts and touch the other to the positive terminal on the battery. If the starter does not engage, try the other small post.

When you try to start the car with the carbs out of adjustment and the engine timing suspect, you may get a backfire through the carbs. This can cause a small fire, in the carb, so be sure you have a rag to smother it and a fire extinguisher handy. This is not a big deal if you know what to expect.

Good luck.

Chuck