High fuel consumption?

[Oreo]

Hi All

My 2 litre 115 engine with 175 CDT carb is over the factory stated 12 litres/100kms. I average about 15-16 l/100km. That 30% over. I have tried everything to lower this including:
1) Switch to electronic ignition
2) Installed new needle/jet in the carb
3) Lowered the carb fuel bowl level
4) Valve adjustment
5) Choke mechnism swithes off in 5-10 mins

I really can't think of anything else to do, then I spotted in an MB article that mileage can vary up to double from the traffic conditions and driver habits. However, the funny thing is even when doing all the above there was scarely any improvement in the mileage. What am I missing?

Only other clue that I have is that it is harder to start when hot. Is this normal evaporation or boiling fuel?

Regards
Oreo

[Mark DiSilvestro]

Have you tried leaning the mixture adjustment. On US W115 gas engines, the Stromberg 175 CDT mixture adjustment is on the main jet, on the bottom of the carb with the idle cut-off solenoid attached. Loosen a 17 or 19 MM locknut and rotate the whole assembly. Keep in mind, on the Stromberg, this adjustment affects fuel mixture throughout the whole range, not just at idle speed. Make sure the damper reservoir on the top of the carb is filled with proper amount of oil. Also adjust your idle speed as low as possible to improve city mileage.
Retarded ignition timing will increase gas consumption and check if your distributor vacuum unit is working properly. Even if your ignition timing is OK, if you can advance your timing a couple of degrees, without causing pinging, this can slightly improve power and mileage.

When I had my '72 220, it got only 12 MPG on my first highway trip - pretty horrible for a 4 cylinder! Carburetor adjustment got that up to 23 but still only about 15 MPG in the city. My 220 was automatic. A stick-shift would get maybe 2 - 3 MPG better.

Happy Motoring, Mark

[Tomguy]

16l/km, I'd kill to get that (14.7MPG)!

12l/km is 19.6

Just quick conversion for you guys (gotta love TI graphing calcs! )

[nachi11744]

Hello,
What chassis, what year....??????
If it is still the correct, original carb, it is worn out AND the choke and/or vacuum actuated fuel return is leaking, causing high fuel consumption.
It always helps if you let us know what chassis, if it is a 230G, I'd say you are doing OK with 16l/100km.
BTW, if you do sort it out, please do let the forum know the remedy.
Have a good week

[Oreo]

Hi

Mark:
Thanks for the tips. I went thru that whole excercise on timing. I'm at about 15 BTDC now, but timed at 4K rpm to max advance. Idle mixture is as lean as it can go at idle and rpm is at about 900 due to a/c conpensation (drops to about 750). So I changed the needle and jet to be sure mixture is correct at the higher rpms. I have 20W50 oil in my damper but I want to switch back to ATF but somehow the ATF keeps disappearing very fast. Is there a leak on my carb piston cover (I got new O ring)? I understand the damper oil only acts as an accelerator pump?

Nachi:
I once got a max of 600kms on 65 litres of gas and probably running a little on vapour and a prayer as I couldn't find a gas station on the NS highway past KL. Was on my way to Cameron Highlands from Singapore. That's almost 11 litres/100kms! Mine is a 79 2 litre 123 chassis with the 115 engine. Recently I replaced the diaphram for the fuel return valve - but how does this work? Is it like a fuel pressure regulator? My choke mixture is as lean as possible in the mornings and switches off after 3-4 kms so I don't think its the choke. Actually the carb is in good condition as it was replaced not long ago by the PO.

As a matter of fact, my spark plugs were reading almost to the lean side when I decided to open them up to 0.40. Recently I lowered the fuel bowl level about 1 cm and it made a big change in the feel. The car is now very smooth on idle, acceleration and cruise and feels almost like a fuel injected car. But sadly this hasn't shown as a FI engine on the fuel gauge. But will report back if there's any improvement. I'm still open to all suggestions but I'm more resigned now to the driving conditions. Almost bumper to bumper home for about 7kms on a 15km one way commute.

Regards
Oreo

[nachi11744]

Hello,
OK, W123 with a M115, listed fuel consumption is about 14.5l/100km. I have a 1976 W115 with M115 and the BEST I have got is 10l/100km and the WORST 16.5l/100km, usual is around 14l/100km in traffic and about 12l/100km on the highway. The W115 and W123 are not economical cars, the W123 having been designed before the 1973 energy crisis.The 12.5l/100km figure that you have is the DIN 70030(IIRC) spec road test, 2/3rds maximum speed OR 68mph(110kmh) on a LEVEL road with 1/2 fuel tank and two persons on board at 68F(20C)temp etc etc........ get the picture!
Check CO% at idle and 2500rpm, should be about 1.5-3% at idle and approx. 0.5-1.5% at 2500rpm. The choke is a mini carb all by itself, the needle may not be seating and leaking, to verify make a gasket for the choke housing that does not have the fuel transfer holes(use the original gasket as a pattern) and then drive with the choke completely blanked off. I am wondering why you need a choke at all at 90F ambient temps! My W115 starts up and idles(a bit low)just fine from a cold start without any choke. I have found from experience that the unleaded fuel around here does not *color* plugs like the late, great 98 octane leaded fuel.
The one thing that you do need to do is get the float level to specs, it may have been incorrect before your adjustment, but the level is 16-18mm with the ball pressed in at the float needle valve, measured at the front(thick)end of the floats.The intake has to be completely free of leaks, Stromberg carbs seem to be very sensitive to air or vacuum leaks
The fuel return valve is effective only when the vacuum is high enough to pull it open, my understanding of it is that around 1/2 throttle, it is closed off and no fuel returns, but if it leaks, it can suck fuel thru the small vacuum hose into the intake manifold.
The damper only needs to have the fluid in the damper shaft, the bit the damper piston lives in, if it is leaking out of that, there is a hairline crack in the depression chamber. Stick with engine oil, it will give you *slightly* better acceleration, if you want to call it that in a 200 !
Hope that is of some help, if it is any consolation, the M115 has a well deserved reputation as one of the toughest gasoline engines ever to come out of Germany, along with the BMW M10 and the Opel CIH engines.

[j9fd3s]

Originally posted by Oreo
Hi

Mark:
I have 20W50 oil in my damper but I want to switch back to ATF but somehow the ATF keeps disappearing very fast. Is there a leak on my carb piston cover (I got new O ring)? I understand the damper oil only acts as an accelerator pump?


Regards
Oreo

sort of, air is lighter than fuel, so when you push the pedal down fast the fuel lags behind the air and the engine leans out and it will stumble. most carbs add extra fuel via and accelerator pump, but the constant depression carbs do not. they have the damper which is designed to open the carb more slowly. the cool thing is you can tune it by varying the wieght of the oil. one of my other cars is a triumph tr3, and the racers run no oil in there because they dont want any dampening at high rpm. bascially the lower rpm you're running the engine the thicker you want the oil to be.

too thin and you get a nice flat spot
too thick, i think its just slow

[Oreo]

Hi All

Firstly on the topic of damper oil. I had a discussion with someone about this and the gist is that a heavier oil will give a richer mix for acceleration. The thicker oil slows down the piston upward travel and thus create as smaller venturi which equals higher air velocities and hence more fuel being sucked in. If the piston travels up too fast as in the case of having no oil, the venturi is lost. In fact I know when my oil is gone when I start to bog when accelerating. j9fd3s is that what you are getting at?

Nachi:
If I can get 14l/100kms I'll be happy. Currently I'm at your worst figure 16-17l/100kms.

I haven't explored the choke end of the carb yet. Is the gasket you're talking about the one that looks like the Dali impression of Mickey Mouse? Currently at the idle mixture I have now, without the choke the car will idle too low in the mornings (25 Celcius). Choke cut off is within 5-10 minutes and rpm will drop down a little low. When fully warmed, it will go up a little to the correct speed. Unfortunately, I don't have access to a CO meter. Where is this choke needle you mentioned? I never saw one before.

I have to top up the damper oil every 10-15K kms or so. I thought that was normal. But I do find red oil residue in the bottom of the piston. I think this is oil/fuel residue. Is this normal? The housing looks very good, all rubbers are new. The only hole I can see is in the oil cap. The oil, I gather, is sucked thru (vent in the cap) the capillary tube to the piston. What if I crimp the tip of the capillary tube just a little?

Thanks to all helping me think this thru!


Regards
Oreo

[psfred]

Oreo:

Correct timing spec at 4500 rpm is 43 degrees BTDC, not 15.

Check the color of the insulator on the spark plugs -- if the center electrode is black, you are running too rich (or have a serious ignition miss), if tan or light gray, fine, if white and blistered, too lean.

Not as convienent as a CO meter, but at least you can get close.

That Stromberg is a beast to keep in good shape -- Weber sells (or sold, anyway) a kit to covert to a two stage "normal" carb and usually gives better performance and milage.

Peter

[Oreo]

Hi Peter

Thanks - that what I meant. I timed at max advance 43 degrees which ended up at 15 BTDC at idle. My plug are very light grey so sounds OK but maybe I'm not used to it being so clean as I'm more used to Webers and Alfas. Personally, I prefer the Stromberg over the Webers as the CD design is so ingenious. More so that its running so smooth now too.

Oreo

[nachi11744]

Hello,
You need to look inside a Haynes 190/190E manual, plenty of pictures and drawings of the Stromberg in there. I will stick my neck out and say that the Stromberg is the least finicky carb I have come across. The downfall in this application is the automatic choke. My car's carb had huge wear at the throttle spindle but it could still start and run like normal, except the mixture could not be richened to the point of idle decay or black smoke. I fixed it with a new throttle body(not normally avaliable as a spare part). All other issues were air leaks at the intake/exhast maniflod gasket and various breather hoses and breather valve missing!
I agree that your car is using too much fuel. Is it an automatic by any chance?
Damper oil: remove the three screws and take of top cover, remove capillary tube then remove the big black or white plastic cap and the damper piston is attached to this. In the carb, there will remain the tube , fill this with oil of your choice, then gently push piston back in and tighten. Fill the reservoir and replace capillary tube and refit top cover.
Choke unit:Remove heat mass(two water hoses,outlet/inlet at thermostat housing probably blocked solid) and locate three screws that hold choke unit to carb. Snap off fast idle linkage, undo three screws and take off unit. Fit *block off* gasket and road test(not just a blast around the block, a few days of driving).
The choke needle is under the big cheesehead screw cap on the underside of the choke unit. Gasket is the inverted *J* shape.
Carb Insulator: The rubber insulator between the intake and carb does crack and cause air leaks, check it carefully.
There is always the remote chance that one or both mufflers are restricted.
Hope this helps and have a good week.

[Mark DiSilvestro]

Some suggestions -

Instead of disabling it, adjust the 'choke' (on the Stromberg it's actually a cold-starting valve) so that it cuts out earlier - within 3 - 5 minutes.

While the 'choke' is operating, set the fast-idle so it will run as slow as possible, without stalling.

Try 30 weight oil in the damper.
(I assume you're filling the plastic cup reservoir and not just filling the hole in the metal carb top.)

If it's running very smooth now, you may be able to lean out the main jet slightly - say about a quarter turn or so.

Last, see if you can advance the timing a further 2 - 3 degrees, for a slight increase in power and mileage, if this can be done without causing pinging. If so, there will also be a slight increase in idle speed, which you can adjust lower for a further mileage improvement.

It might be a good idea to drive the car for a day or two after each adjustment rather than do them all at once so if there is any problem, it will be easier to reverse any adjustment you made.

Each of these adjustments may only make a slight improvement but, if you can do most or all of them, you may get a signficant increase in gas mileage.

Finally, in the US, these cars were geared for max acceleration, not gas mileage. They are fairly heavy to drag around with only a 2 liter engine and the automatic transmissions most often found on the US gas versions don't help. I replaced my '72 W115 220 with a '72 W114 250 and, besides being smoother and more powerful, my 250-6 cylinder only got about 1 MPG less than my 220-4. After the 1973 oil crunch, Mercedes began offering 5 speeds as an option on most models but, due to marketing decisions and emissions regulations, only outside the US.
You didn't say what year your 200 is or what transmission. With a 5 speed, these cars have an overdrive gear, for maximum highway economy.

Happy Motoring, Mark

[j9fd3s]

Originally posted by Oreo
Hi All

In fact I know when my oil is gone when I start to bog when accelerating. j9fd3s is that what you are getting at?

Regards
Oreo

yep. if it opens too fast (no oil to slow it down) it gets lean and has a flat spot.

30 weight works well

mike

[Oreo]

Thanks for the tips Mark and Naachi. The car is a 1979 123 model 115 engine with 4 speed manual trans. As for ignition timing, I don't know if I should start adjusting this according to a vacuum gauge instead of the timing light? Anyone familiar with this method?

Car starts with the choke at about 1400 rpms for 5 mins or so but I will adjust this to 1000 or so. Probably I over rotated the choke hot water housing in the "rich" direction.

I agree that all these changes will make small improvements but there must be one big overlooked item that caused the high mileage. My thoughts narrowed this down to:

1) High float level
2) Excess fuel pressure

Well, the float level adjustment seemed to do a lot and I put in a new fuel pump due to an oil leak (not fuel leak). I believe my oil level seem to go up sometimes indicating fuel dilution but I thought it was my imagination. All this is very contradictory to my plugs readings that do not indicate a rich mixture.

Naachi: The choke vacuum pull down on the top. How does this work? Is there supposed to be a vacuum line connected directly to this?

Despite all, I still prefer the Stromberg over the Weber.

Oreo

[nachi11744]

Hello,
Ignition timing spec is 40-45 degrees BTDC at 4500rpm WITHOUT vacuum connected, vacuum adds about 8-12 degrees on top of that, so TOTAL advance can be as much as 58-60 degrees above 4500rpm. You can be as high as 18 degrees BTDC at idle(800rpm) and still not have pinking on acceleration.
The W123 M115 Stromberg is a different animal from the W115 before and the later W124 200 cars. It has a small diameter rubber hose that carries the idle air/fuel mixture from the bottom and ahead of the almost closed carb throttle plate to the top of the intake manifold and there is a small idle speed adjuster screw on the intake manifold. There is a set up procedure if the factory setting of the throttle plate is lost due to *unauthorised adjustments*. On this carb, the throttle stop screw is locked at the factory and all idle adjustments are done at the adjuster screw on the manifold. Strictly speaking, the CO% should NOT be altered, only idle speed adjustments. I will look up the factory adjustment procedure if you think you need to do this adjustment.
All carbs are completely free from masochistic tendencies, so there must *other factors* for the high fuel consumption you are suffering from, the PO may have made several *unauthorised adjustments*.
Fuel pump oil leak: Very few M115 fuel pumps leak oil, the bakelite spacer is the usual culprit. It warps or cracks, then oil leaks like it is from the pump. There was an updated fuel pump supplied by Pierburg(OE brand) to cure oil weeping from the pump but I think that part made it into production before your car was manufactured. The pump bolts need to be sealed as well as they are thru the block and the timing chain whizzes by the bolt holes, splashing lots of oil around and then the bolts weep from the threads and oil is blown around by the cooling fan, giving the impression of a huge oil leak along the left side of the engine.
Choke pulldown:There were many modifications to this, indicating that it has been a problematic area. Initially, there was an internal vacuum passage. Later, that was superceded by a pulldown cover with a vacuum hose connection to the intake manifold, IIRC on the W123 it is connected to the vacuum operated hot air intake on the airfilter housing intake spout. Later still, replacement carbs were supplied and it was suggested to connect the pulldown to the vacuum governer connection thru a plastic T-piece. It needs to be connected to a constant manifold vacuum source, so that WOT after a cold start will not cause the pulldown vacuum to disappear, reengaging full choke.
Fast idle: It is adjusted to 1700rpm and CO7% with the engine at operating temp and the fast idle cam on the second(middle)step. The fast idle link is lenghtened or shortened to speed up or slow down the fast idle, later carbs have an adjuster screw next to the throttle stop screw to alter the fast idle. Seems to me that it is already too fast on your car. There were probably a few hundred updates on the choke unit, pulldown and adjustment procedures and specs over the 20-22 years Mercedes used the Stromberg on 4 cylinder gas engines, so you know why I do not bother with the choke anymore!!!
High fuel pump pressure and a high float level can be check out with simple tools, an inexpensive pressure/vacuum gauge can test the pump and a steel rule can check the float level.
Do you have a manual for the M115? That would be the way to go and there are several excellent German websites that have a lot of good DIY stuff for the W123 4 cylinder gasoline cars. Do a search and let the forum know how you make out.
One last thing, are you sure it is the correct carb for your application, there is a metal tag with some numbers on it affixed to one of the airvalve housing screws, perhaps you could look for it and let us know what is on there. I'd hate for it to be a W124 carb on a W123 or something like that. I have seen the Volvo 175CDUS manual choke carb fitted to some Mercedes M115 engine cars around here, others junkyard carbs off W124 or W201 M102 engines. Just a heads up for you.
Have a good week.