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w114 - comically bad fuel economy

10K views 56 replies 7 participants last post by  rumb 
#1 ·
Long story short. About a month ago I bought a 1973 280. It drove nicely but idled badly. The local mechanic said the problem is the carb. I went out and bought Weber k248 kit which they installed and set the idle ok but as they never bothered attaching electric choke it takes 10 min to warm up at the beginning of the day (wich also makes me question how well they know carbs in the first place). The car drives OK but doesn't have the same nice feeling that the previous carburator did (don't laugh at my foolishness... also a Weber). In mean time i've noticed abysmally bad fuel efficiency - around 5(!!!) miles to a gallon. After the Thanksgiving i'm taking the car to a mechanic that specializes in older Merces but just wanted to get you opinion if indeed the bad fuel economy is due to poor carb setting (i didn't have the car with previous carb long enough to find out if the bad fuel economy was with previous carb too).
 
#2 ·
The fact it doesn't run as well as before, indicates something wrong. And the low gas mileage indicates a rich mixture. Are you plugs clean?

Adjusting carbs for idle is fairly easy to do. You will need either a tach, vacuum gauge, or better yet, a Wide band O2 sensor. After that, it's a little harder as you have to change jets. And most chokes are electric, so, switched power is all you need.

If you are local to me, I have a O2 sensor we can check the mixture.

-- brett
 
#4 ·
If you end up continuing to be bothered by carbs, there are fairly economical ways of switching to an injected m110 engine. I personally gave up on carbed w114 cars seeing specs around maybe 16mpg. I figured F it and bought an injected v8 w108 figuring none of these cars are likely to get much over 15 mpg.

A d-jet m110 would be period correct for a w114, but k-jet m110 setups are pretty damn simple.

Feel free to PM as I'm not to far.


Edit: but a good mechanic should be able to get your webers working right.

There are some articles on Weber conversions. Perhaps some of them would have some tuning info.
 
#6 ·
If you are getting 6mpg, I would almost expect gas dripping out of the tailpipe, really! That would be very rich. I would also expect black smoke on acceleration.

If not, look for a fuel leak. That old of a car, there could be a crack in a fuel line, and it's just leaking out.

-- brett
 
#7 ·
IMHO Weber carbs are one of the most foolish "solutions" They were not designed to fit correctly on MB's and from what I have read are more work and worse than what was replaced.

I would seriously try to find a good original carb.


You probably need to put in smaller jets.

Also check that there are no vacuum leaks anywhere in the car. you should have 16-17" vacuum on the manifold.
 
#8 · (Edited)
IMHO Weber carbs are one of the most foolish "solutions" They were not designed to fit correctly on MB's and from what I have read are more work and worse than what was replaced.

I would seriously try to find a good original carb.

You probably need to put in smaller jets.

Also check that there are no vacuum leaks anywhere in the car. you should have 16-17" vacuum on the manifold.
There are any number of elitists/naysayers against non-Mercedes solutions, but speaking from experience (I replaced my Solex (on a '73 280C) with the Weber conversion), I was delighted at the car's new found ability to idle, and getting 16mpg (previously 10 mpg).

Pools of gas in the driveway aside, I'm guessing you do indeed have jets that are improper for your car. Pull one or more of the plugs and I'm sure it'll be fouled. Can you verify whether you have a two barrel, or four barrel conversion? If two, your idle circuit will send gas up to about 3000 rpm or so, with the second barrel adding more gas thereafter, and at wide open throttle.

I've got a Fiat, whose Weber carbs' design is now owned by Pierce Manifolds. I *think* Pierce bought out all the Weber carbs, jets, retooling, etc. You might contact them to see what they might suggest, or do a google search on how to tune yours. If they aren't able to help, you might look toward the restoration shops or hot rod shops in your area, and ask who they'd recommend to work on yours, or whether they could tune the thing for you. Different sized jets aren't rare or uncommon, and are awfully easy to replace. If you want to experiment, buying a small assortment of the things and swapping them out wouldn't break the bank.

Good luck - I suspect fixing your issue will be easier than finding the "right"/original carb for your car.
 
#9 ·
Two comments:

1. You don't need an old Mercedes mechanic, you need someone who tunes Webers.
2. I had a 250s with webers and they were great
3. I have a friend with a 1965 220S with webers and they are great too.

I don't expect much more than 15 mpg from a Mercedes 6 cylinder car of that era.
 
#10 ·
15-18 MPG is about normal for the M110 no matter how it is fueled. The E model W114's might get a wee bit higher mileage but not by much.

6 MPG sounds a bit drastic though. I'd take Brett's advice and look for a leak. Inspect the fuel pump as it is not rare for them to leak internally or externally.

How does the oil look? Thin, clean and over-full?
 
#11 ·
This is the mileage i get:
city: 6mpg (most of my driving)
highway: 11.5

Mike, i would LOVE to be getting 15mpg! The oil is clean.

Grubeguy, it's Weber Redline K 248#4 kit (38 DGES) using whatever jets it came with.

So assuming that the jets are fine (It's stock that came with the Redline kit) the other suggestions is a fuel leak or vacuum leak, yes?
 
#12 ·
The Redline was the kit I bought - LOVED it. I liked especially how it was usable right out of the box - fit like a glove, so to speak. The adapter had obviously been engineered for the specific manifold too and worked right out of the box with the stock air cleaner. AND, if you've read anything about the Solex carbs Mercedes used at the time, you know what a step up in reliability and simplicity your Weber is.

Try looking at WEBER Carburetors distributed by Redline : BMW or email redline@redlineweber.com for info. They might be able to assist with tech questions.

Did you find any leaks or obvious issues? How did your plugs look?
 
#18 · (Edited)
The "semi-hemi" design of the M110's combustion chamber results in a lower than normal vacuum reading at idle. This is why in the early years of its production the engine was equipped with the auxiliary vacuum pump mounted on the front of the cylinder head.

14" does sound low but not drastically low.

Did you pull the plugs to check the burn?

Have you checked the timing? Should be 10 degrees BTDC with the vacuum disconnected and apx. 4 degrees ATDC with the line connected.
 
#19 ·
Since i started poking around the car it seems there is very little vacuum to speak of that does anything.
I see one line from intake manifold going to vacuum pump in the front (from which it leads to break booster).
Another line going from intake manifold to transmission and another going to the vacuum reservoir.
I also see one line going from vacuum advance to the Weber carb.

Lines for 2, 3 and 4 in the pics below are cut. Only 1 has some vacuum going to the EGR valve and the vacuum reservoir.

Can anyone enligten me what 1, 2, 3 and 4 do?
http://shoulderthelion.com/pics/IMG_0062_s.jpg

Also on the Weber carb it has fuel evap vent that's left open (should it be?) and closed off vacuum retard vent (should it be?)
 
#20 ·
Those are emission related solenoids. They are connected to another relay which is triggered by the temperature switch mounted in the cylinder head. They allow the timing to be advanced when the engine temperature is below optimal operating temperature (70 degrees C I believe). When this temperature is reached the solenoid opens and vacuum is sent to the retard mechanism for emissions reduction. One of them operates off the A/C compressor and it cuts the vacuum retard when the A/C compressor is engaged.

Germany's attempt to comply with the U.S emission standards. Not exactly a rousing success but they did enable the vehicles to be imported.
 
#23 · (Edited)
EGR valve on a '73? Never seen one, doesn't mean there never has been one, just that I have never seen one equipped with one on a '73 model.

Are you referring to the PCV (Positive Crankcase Ventilation) ventilation tubing leading from the top of the cylinder head into the air cleaner?

Not meaning to step on your toes Rumb but there is no comparison between the 300E and the M110's emission system. No computer controls or sensors on the M110.
 
#25 ·
#26 ·
See if this helps.

Mercedes-Benz Model 114/115

Go to the "Engine manual" section for the 1973 model. Sections 01 and 07.2. That should put you on the right track. Of course you'll have to make adjustments for what was removed for the Weber conversion.

I believe there is also a vacuum line which was connected to the fuel regulator/by-pass valve on the return line. It's been quite a while since I had a carb mounted on my M110 so I'm working from old memories.
 
#33 ·
I'll check 2 or 2 other sparks tomorrow and will report.
Last time i checked the pulsating was lesser (probably because this time the engine was "almost"warm while last time it was on for over 1 h).

I found this note regarding Weber carbs fuel pressure:
"Fuel pressure is critical when you change from a stock carburetor to either a Weber or Holly carburetor. The Weber carburetors are very sensitive to fuel pressure. The needle and seat will not take more than 2.5 - 3.5lbs of fuel pressure. If higher fuel pressure is attempted, the carburetor will flood or poor running condition and poor fuel mileage."

I wonder if rich mixture wasn't the compensation for "poor running" resulting in "poor fuel mileage"?
 
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