'71 250 Running Problems

[MercedesBenz250]

Hi, My 1971 Mercedes 250 has a problem that has been re-occuring, it seems like everytime I get her to Idle correctly in park and in gear she will run like that for awhile, lately the car has been doing this a lot, she will idle somewhat okay in park but when she is put in gear the oil pressure drops and the engine dies. I don't know what to attribute this too. I have advanced the ignition and played with the idle on the carbs but this keeps happening, I just put on new points,cap,rotor,wires,and plugs. The carbs also backfire. I am wondering if I should just buy some webers for the car and install those because I have heard that these zenith carbs can be troublesome and I've had a lot of work already done to them.
Thanks

[MunichTaxi]

Oil pressure will drop when you put in gear (10 psi is minimum at idle, in gear).

Try replacing the return springs on the carb linkage. Sure, the gas pedal will have a stiffer feel, but the linkage will return to the same location.

Have you synchronized the carbs, at idle and at 1500 rpm?

[MercedesBenz250]

Thanks, I know the oil pressure will drop but from idle (45) to gear it drops to about 15-17 and stutters. As for the springs I can try that and as I don't have a carb synchronizer I should probably hunt for one, but I don't know if I want to spend money on fixing it when I can just put the money toward the webers.

[MercedesBenz250]

Also, where can I get a tach and how would I hook it up to the car?
Thanks

[cth350]

There is a vacuum dashpot that bumps the idle slightly when you're in gear with an automatic. Sounds like it needs adjustment or repair. Look for Jaime's website. The manual there is still for sale by mercedes at 1-800-for-merc. -CTH

[MercedesBenz250]

Thanks

[ctaylor738]

WADR to CTH, the dashpot should only kick in when the AC is engaged, or when you are maneuvering at low speeds and using the power steering. Your idle setting should be good under normal idling in gear.

I would suspect a plugged idle circuit, or failing idle solenoid. There are lots of posts on how to check and fix these problems.

[230/8]

250:

Here's my two-cents....

Let's assume you have tried to clean the carbs and there is nothing literally plugging the idle circuits. Since you indicate having a good idle until it drops into gear, this may be a safe assumption. And let's also assume you have checked that the idle solenoids are working by removing the wire to each to check that it shuts off the fuel and makes the engine stumble and stop.

With those assumptions behind us, and your statement that you'll need to get a balancing tool, actually it's two tools, let's further assume you have not gone through the correct procedure to set idle and balance the carbs. I believe this is your basic problem, because I am going out on a thin limb here and further assuming you have no obvious tune-up issues of timing, plugs vacuum leaks, etc. Sooooo...in order to get you running until you can get the proper tools,

I recommend you look for the vacuum idle regulator attached to the rear carb. It is a large dashpot with a plunger that presses against the rear carb linkage. It retracts and extends in response to changes in engine vacuum, such as when you drop it in gear or turn on the AC. Once you locate it test it for operation and adjustment by simply removing the vacuum hose while the engine is running. The plunger should extend a bunch and contact the rear carb linkage. This will raise engine speed. If it works, reconnect the hose and start as follows:

1. The little plunger itself is actually a small screw threaded bolt fitted into the spring/plunger assembly. With the engine running, adjust this screw to gain about .040" clearance between it and the linkage.

2. Now, place the car in gear and see if it dies. If so restart and while in neutral adjust the spring nut so the screw plunger pulls away from the linkage. Get as much clearance as you can, but there is a limit because vacuum is limited. You may be able to get 1/4" clearance. Once you have it, readjust the screw itself, without changing the spring tension, to reset the .040" clearance at the linkage. If you don't have a feeler gauge, use a business card folded over once.

3. Place car in gear and see if it dies. It should stay running and be good for you until you can get proper balance tools and a tachometer to set idle speed and carb balance. If it stalls, then you really need to get the proper tools so you can go through the steps to set idle speed and balance the carbs. You can try to evenly adjust the links on each carb to raise idle speed, but this will definitely throw them out of balance, with all the problems that entails.

Idle speed and balance MUST be proper in order to accurately set the vacuum idle regulator, but what I have described is a stop-gap that should keep you running until you get tools. (Side note: don't let anyone convince you that balancing Zeniths can be done by listening through a rubber hose. Not true, no matter what anyone may tell you about their personal successes. Once you have the proper tools you can experiment and verify this for your self.)

If you want to own one of these cars you will have to get the tools and develop the skills to care for it yourself. Very few of us are still around who understand the old cars and carbs. Also, you really need to get a decent manual. The factory CD ROM is available for $19.99 from MBUSA classic technical literature. It is a bargain and has all you need to know for your model. Also the much maligned Haynes and Chilton are actually pretty good.

Tools are available here:

http://www.baumtools.com/

You will need the plastic bonnet and uni-syn for Zeniths.

Good luck,

230/8

[MercedesBenz250]

Thanks, I'll check it out, but I think I am still planning on switching to webers, but I will still need the tools. I'll play around with the carb like you said
Thanks Again

[chrismalm]

I had a similar experience with my 71 250 dying when I put it into gear and it turned out to be worn/broken mechanical advance weight springs in the distributor. As the distributor cam was also worn, I replaced the whole unit and it idled and accelerated great afterward. Your carbs are also prone to warping which could cause idling problems. I have the webers on my 72 250C and I prefer the original carbs if they're set up and working correctly. My car now has a touchy throttle that I have to baby or it "leap-frogs".

Chris
72 250C

[MercedesBenz250]

Thanks, I'll check into that, would that explain why when I advance and retard the ignition when I first start her up and then again when she's warmed up that she runs better?

[MercedesBenz250]

Also, where can I buy these springs and how difficult are they to change?
Thanks

[MercedesBenz250]

Why don't you like the webers as much as the original ones, I've heard they're much simpler to work on and more reliable also not to mention better performance, is there any truth to that?

[MercedesBenz250]

I was looking at my distributor and it looks pretty worn around the cam where it contacts the points. Where could I find a new distibutor I searched around google but the most inexpensive one was $482

[M_Anker]

Regarding the distributor, try Jim Row at Buffalo Machine Works. He rebuilt one for my 280SEL and it's perfect now. He can machine the cam so it works properly, or find you a really good core to rebuild from. This guy is a perfectionist, and I highly recommend him!

Buffalo Machine Works