1972 250 - Cold start and acceleration problems

[rgnprof]

I have only had this car a couple of months and I have had problems with it starting and lurching upon acceleration when outside temp is cold . This car has the Zentih carbs, which I believe were slightly modified for the 1972 model. Is this correct? I took it to a Mercedes garage here in Oklahoma City, and it ran better for a couple of weeks. The main complaint is upon cold acceleration from stop, the car lurches and I can hear the carbs (or something) "backfiring", or spitting. It seems the more I step on the gas pedal, the less likely I am to go anywhere! If I try and quickly accelerate, the car just doesn't go, if I accelarate very slowly, I'm fine. Once I get past 10-20 mph, I don't notice any problems and once the car warms up, it runs better. I have been reading about these carbs and I know they are complicated, but is there something I can do here? Does anyone know a good Merceds mechanic in OKC? Thanks.

Ryan

[gmask]

Originally posted by rgnprof
I have been reading about these carbs and I know they are complicated, but is there something I can do here? Does anyone know a good Merceds mechanic in OKC? Thanks.

Ryan

I'm sure you have probably read some of my posts then.. if I were in your area I'd be glad to come over and give you hand ;-)

Things to check are:

Idle speed.. If the car is still stalling after it has warmed up it could be that the idle speed is too low.

If the engine is stalling while warming up it can be helped by increasing the fast idle.

If your carbs are out of sync it can cause all sorts of problems.

You can get a tachometer for very little from practically any auto parts place and use that to see if your idle is below 700rpm.

If it is you need to sync you carbs and adjust the throttle controls on each until they are.

This is hard to explain without pictures and the one online manual I know off isn't coming through.. Get youself the Haynes manual for your car you can it from amazon.com or use doff ebay.com

Basically after disconnecting the throttle linkage and the link between the two carbs you adjust the the throttle thumbscrew on each until the correct RPM is reached and they are in synch.

You need to either use a length of hose to listen to the choke on each and when they sound the same they are synched or use a synch meter. The meter can be gotten from jcwhitney.com but you need a hood adapter from some other place I can't recall.

The fast idle screw is on the automatic choke of the front carb.. it is kind of hard to adjust because you can only see if you look up at it from below with a mirror or have the throttle pushed all the way in. Turning it clockwise will increase your fast idle during times when the engine is cold.


If this sounds too complicated then call all the mercedes shops in your area and ask them if they have first hand experience with these carbs and what they charge per hour.

Although I highly reccomend learning to do this yourself as it is a good idea to tuneup these carbs yearly or as they get out of whack. I have been doing mine twice a year. It's really not that hard once you get a better understanding of their anatomy and operation.

[gmask]

Originally posted by mpnye
If you're striving for perfection . . . can the Zeniths and get a JAM kit with 2 Webbers . . .

If he's willing to spend $800 plus to to buy an dinstall webbers then I'm sure he can afford some inexpensive tools and the time to learn how to properly adjust these carbs before replacing them alltogether.

BTW how much of an improvement in MPG do the webbers delivery?

[rgnprof]

I think the car is in great shape - odomoter reads 158,000 and when I get past 20 mph it runs great. Cruises along at 80 and you don't have a clue you're going that fast. Uses a little oil, maybe 1 qt per 500 miles and I am getting 10 - 12 mpg, in the city and once it warms up, I don't have any problems. Interior is in great shape and everything works. I paid $2200 for the car and I really love it - my first Mercedes.

Can I install Webbers? I don't know much about changing out carbs. Part of me wants to keep the car original, and just put up with problems. How much would it be and where can I get them? I have also read about putting in a Crane ignition - would that help and where can I get it for this car? I don't know that I want to spend the money right now - just weighing options.

Thanks for the feedback - this is one of the best forums I have run across!

[gmask]

>>>Keep it as simple as possable, but DON'T step over dollars to pick up dimes . . . Try this FIRST.

My point exactly ;-)

[ctaylor738]

You are obviously running lean when cold, which is causing a backfire on acceleration when the engine needs a richened mixture and not getting it.

I would guess that the shop looked at the basics like choke adjustment. Gmask has explained the importance of carb synchronization very well.

A couple of additional things to check or have your shop check.

- Top plate warping. Remove the top plate and rub it across fine emery paper taped to a piece of glass. If the sanding marks are smooth, then you are OK. If not, you can continue sanding until you get a flat surface. You may be able to compensate for some warping by using two top gaskets. The MB gaskets are a bit thicker and may do a better job than the ones that come in the rebuild kits. Several posts on this.

- Check that the accelerator pumps are working well. If you look down the carb and open the throttle quickly, you should see a good squirt of gas from the little tube.

- Make sure that the secondary throttle valves are opening correctly. They are vacuum controlled and can get stuck in the closed or partially-open position. Either of these conditions could cause backfiring.

- Do you have the tube from the exhaust to the air cleaner in place? Is the flap in the air cleaner working correctly?

If you like tinkering or can find a good carb guy and want to keep the car original, then mess around with the Zeniths. Otherwise, look into the Weber conversion from JAM Engineering.

I would disagree with MPNYE though, the Zeniths are pretty basic and simple carbs. The key to keeping them working well is to get the heat risers to either open and close correctly or make sure they are jammed shut. Lots of posts on this as well.

[rgnprof]

Chuck and Gmask, thanks for your replies. Chuck, I noticed that I do not have a heat riser tube from the exhaust to the air breather. I am thinking that I'm getting some "carb ice" when it's really cold which is causing my lurching problems. It's been very cold here this winter and I talked to the guy I bought the car from, and he said that he doesn't remebere there ever being a tube because it just gets too hot here in Oklahoma. Do you think this could be causing my problem? What are the consequences of putting this tube in place? I know the heat could end up causing problems as well - should I remove the tube in the summer? Also, I don't think this would have anything to do with my hard,cold start problem, would it? Once the car has been running in the morning, it starts much easier later in the day, even if it's still real cold. Thanks for your help.

Ryan

[ctaylor738]

Maybe I am getting my 280 and 250 mixed up, but I think that the 250 has a flexible tube about 2" in diameter that runs from the exhaust manifold to the air cleaner "snout." The theory is that the manifold warms the air until the engine warms up, at which point a flap in the air cleaner snout closes off the warm air.

It's not really a heat riser.

Cold starting would not be affected - but warm up running would.

To start in very cold conditions, your chokes need to work perfectly, and your accelerator pumps need to deliver a good squirt of gas to get things going.

Have you pulled the air cleaner off and observed the choke plates as the engine warms?

I assume that when you start it cold, you pump the gas pedal a couple of times before cranking, and then give it a couple of more pumps while cranking.

[gmask]

Originally posted by ctaylor738
Maybe I am getting my 280 and 250 mixed up, but I think that the 250 has a flexible tube about 2" in diameter that runs from the exhaust manifold to the air cleaner "snout." The theory is that the manifold warms the air until the engine warms up, at which point a flap in the air cleaner snout closes off the warm air.

My 71 250 was missing the hose but was set up for this.. of course in southern california it doesn't need the extra help warming up.

Are the carbs especially the rear one almost too hot to touch after the engine has warmed up? If so the heat risers are probably stuck open and the carbs are probably warped.

If you put a folded shop rag over the top of the carb while the engine is running will the engine stall? If not then you probably have a leak in the carb. If you plug the idle air bore with a ballpoint pen does the idle change? If not then you definately have a leak.

BTW.. Jaime says his online zenith manual willbe back inaction in a couple of weeks. JaimeKop.com

Here's a link to the carb synchronizers
http://jcwhitney.com/product.jhtml?CATID=5520&BQ=jcw2

Baum has the hood adpater but the links are broken on their website.
http://www.baumtools.com/

[gmask]

Originally posted by mpnye
GMASK,

How did those INAT Idle Cut Solioids I sold you on eBay work out?

I just found 2 Wiring Update Kits to install those on 250S models . . . Let me know if you're interested?

I still have them but ended up not needing them.. I'm just keeping them in case. What is a wiring update kit?

[gmask]

Originally posted by mpnye
For EARLIER models that did't have IDLE CUT SOLINOIDS, so you can install later carbs or lower bodies on your existing carbs to cure run-on in HOT temps.

Hmm.. don't think I can use them.. however I have been thinking about installing the solenoids I bought from you as my 72 has been "dieseling" since I last reset the idle to the correct speed.. the existing solenoids still click but the car sometimes runs on after I shut it off. It could be some other adjustment that I need to make but I may try the new solenoids just to see if it makes a difference.

[gmask]

Originally posted by mpnye
Check to make sure that the SECONDARIES are FULLY CLOSED.

You mean the second stage diaphragm? I have been wanting to replace those but I could get the darn things unthreaded from their posts to put the new rubber on.

[gmask]

Originally posted by mpnye
Are they frozen in the SQUARE aluminum center of the diaphragm?

Yup..

[gmask]

Originally posted by mpnye
About all you can do is loosen the jam nut, heat the aluminum with a micro-torch and spray threads with penetrating oil.

Use just enough heat as to NOT melt the rubber . . .

I left them sitting in some penetrant but I haven't gotten around to trying to unthread them sinc ethen.. The carbs have been working well enough to forget about it but I have been planning to do a more thorough rebuild and I will definately will rpelace those when I do. I missed the chance to buy some new diaphragms cheap with posts on ebay several months ago.. darn it ;-)

[gmask]

Originally posted by mpnye
check this out . . .
ROYZE

I know of them but you can get the major rebuild kits (ze-28k) from parts.mbz.org for slightly less ($38.07). I think they are in Atlanta, GA

Royze.com
ZE-11K $24.72 plus freight
ZE-28K $44.50 plus freight
I live in CA so I'd have to pay tax as well:-(