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Hey guys, I have a 1965 230S benz, that is driving me nuts. I have had it for 18 years, I bought it to see what made them tick, stripped it down to bare metal, went through the engine, trans. etc. repaired the rust, etc. to restore it. My problem is with the Zenith Carbs 35/40 INAT on the thing, I think. I had them rebuilt by “proffesionals” and I still keep having the same problem, which is I can seem to get the thing to start off the key, without using starter fluid. Once I get it running though it idles OK and runs smoothly until I try to drive it. Then it dies, and I go through the process with the starter Fluid again. I think it is the Automatic chokes, but I’m not sure. No one arround my area, that I know of has the expertise on the older models. I have two other engines and transmissions I can put into the car, one being a fuel 1961 fuel injected engine, but hasn’t been ran since I took it out of the other car. My question is I guess is there any other way to modify or change the carbs to something that will perform better?

I’m not much of a Benz guy but I used to mess around with Zenith and SU/Stromberg type carbs since they were used on SAABs, Datsuns, British vehicles, etc… Since this is ancient history I’ll have to think about this a bit.

Just clarifying something. Ignition timing correct? Advance weights in the distributor not frozen?

I would have to say yes on the timing part, if the chokes don’t interefere with the timing advance on the the distributor, and the distributor is not frozen. Thanks for responding.

Normally you should set the timing with the vacuum advance hose disconnected, so the choke should have no effect. Have you searched for manuals? Here’s one that includes Zeniths
Also, have valves been adjusted?

p.s. - google for ‘mercedes forums’ you’ll find discussions better-suited for this older model.

Do you let it idle for a few minutes before driving off?
My Chevelle of that same year had the problem if I didn’t let it sit idling for a good 5 minutes or so before driving off, it’d die on my as soon as I put it into gear.

I’m going to back up here and state that I’m wrong about the carburetion; dead wrong. For some reason I was thinking of side-drafts and after taking a look at one of my old manuals I see this car uses 2 down-drafts.
All carburetors, just different methods of operation.

Normally if a carbureted engine falls flat when the accelerator is depressed there is a fault with the accelerator pump circuit. This can be verified by removing the air cleaner and looking down the carburetor throats while working the carb. linkage by hand with the engine off. You should see gasoline being sprayed out of some discharge tubes each time you operate the linkage.

Assuming timing is close, advance weights are free, and gasoline is being sprayed from the discharge tubes along with the engine even being moderately warmed up, the part about starting fluid is throwing me a bit.

Is the engine compression and camshaft timing known to be correct? What I’m getting at here is that if there is a problem in this area it’s possible that manifold vacuum (which controls all) may be off enough that it does not have the ability to draw gasoline up through various emulsion tubes, etc.

You state the engine idles Ok. Does this mean smoothly or is it a bit ragged?

Seems like I read a while ago that these old MBs use engine vacuum for a lot of accessories - a bad vacuum connection would be something to consider?


This looks very complex.

I think those Zeniths are one reason people like fuel injection so much on the older MBs.

Vintage Mercedes guys have a ton of experience with that sort of issue. Try a M-Bz forum like these:

I repair older mercedes for a living and I have two in at the moment with exactly the fault you describe. Both are Fintails,one is a 220S with a 280S engine and the other is a 230S .Both are in very good condition but other wise undriveable because of the running fault. While I have some 40 years experience with Mercedes I must admit these are not the easiest carbs to tune.
First,ensure you have an adequet fuel supply.Fuel pressure from the pump must be 2-4bar.
With the aircleaner off,and the engine cold the choke flaps should be closed.
On the side of each carb is a lever which operates the acelerator pump.Push the choke flap aside so you can see down into the carb then lift the lever.
Lift it slowly so it takes you 2-3 seconds to move it through it’s arc.
It should squirt fuel in a strong stream from the first movement to the end.
If not,three elements are at play.One the pump piston is worn and the washer is not sealing. 2/ the ball valve on the bottom of the acelerator pump is blocked or jammed ( shake it to see if it rattles,rattles are good ) .
3/next to the pumps ball valve is another brass plug,this is the acelerator pumps pressure valve,again check this not blocked.
Now with the pump squirting a full Audible stream ,you must now make fully certain that it is spraying at the side of the venturi bore and not straight down or at the device in the center on the venturi. The tube can become blocked over time. You can reverse flush them with spray type carb cleaner and a plastic straw.
The tube is brass so can be carefully adjusted so it aims in the correct direction.
Now with the carbs squirting properly,check the vacuum device on the side of the rear carb,this is designed to open the throttle in the event of stall or near stall situation.
With the engine idling,have some one try the trans and power steering while you watch the vacuum device. As revs drop it should extend and push the throttle open,thus maintaining the engines speed.
If it doesn’t work, the engine will stall .
it is does work and engine revs still drop ,the vacuum diaphragm is leaking or is not recieving vacuum ( check the line and power brake line for leaks)
insufficinet vacuum can be caused by the aircleaner to manifold hose leaking too,it’s a 12mm Dia hose between the carbs.
To set the clearance ,start by having the engine at running temp(180 average) .Turn the engine OFF.
The diaphragm should have extended out,and should be pushing against a lever on the side of the carb.
Check that this lever is not sloppy on it’s pivot. if it is ,fix it.
Now this lever pushes a small spring loaded valve on the carb,thats the float vent. There is screw stop. that is a idle speed adjustor.
The lever should allow 1 mm maximum movement of that valve.
Adjust the vacuum device so it holds that lever at the 1mm positon on the float chamber vent.
Now,start the engine and do not rev it up!.
With it idleing,let it tick over for a few minutes.
The vacuum device shoud have pulled basck so there is a clearance between the lever and the device.
Adjust the spring tension ( 19mm nut) but tighten or loosening the nut and counter holding the nut close to the diaphragm.You need to get it to within .10mm clearance. You should now be able to get the car running clsoe enough to drive.
You still need to check carb synch etc but that generally doesn’t cause stalling just bad fuel mileage etc .

Is there sufficient fuel getting to the carburetors? I ask this question, because I had a similar problem with my 1978 Oldsmobile. I had to prime the carburetor to start the engine. When I looked down the throat and pulled the accelerator linkage, the accelerator pump wasn’t pumping fuel. However, it would pump fuel once the engine was started. The problem wasn’t the carburetor at all–a neoprene section of fuel line down by the gas tank had deteriorated and the pump was sucking air. Once the car started, the pump would pump quickly enough to fill the float bowl and keep the car running.

I don’t know whether your car has a mechanical pump or an electric pump. If it has an electric pump in the fuel tank (unlikely if the car is a 1965), then what I have said doesn’t apply. However, if it has a mechanical pump or an electic pump that sucks fuel from the tank, a crack or hole in the fuel line might be a suspect.

Since I had great difficulty rebuilding a 1 barrel carburetor with a manual choke on my 1950 Chevrolet pick-up truck some 37 years ago, I always try to eliminate other problems first. You can have carburetion problems, but this doesn’t mean that the problem is with the carburetor(s).