Small running issues - 1971 250 / M130 engine

[Popeye]

1971 W114, 250, M130 engine in stock configuration (Zenith 35/40 carburetors, stock ignition). New to me this past winter. It always has been well serviced – just not driven the past few years. I've driven 600 miles in the past four months.

Fundamentally the car runs wonderfully; it pulls strong, sounds good, shifts well. I have disassembled, cleaned, and reassembled the carburetors (with fresh gaskets/o-rings). I did not change any settings on the carbs - assuming the mechanic who serviced the car is smarter than me (I did a quick synchronization check - not perfect, but pretty close). I replaced the rubber vacuum lines in the carburetor and distributor. Fresh fuel filter. The feed fuel line from filter to carburetor appears original – I have a replacement on order from Mercedes Classic center. Other fuel lines on the “engine side” are new. I plan to replace the lines at the fuel tank, replace or clean the strainer, and clean the fuel level sensor.

I have two niggling issues, and curious where to start looking:

1. Starting the car is difficult. It takes two or three turns of the key to get it to start. Each turn of the key only a few seconds of cranking - in total 5-10 seconds of cranking to start. Then it runs a little rough/lumpy for the first minute or so: I have to give a little throttle to keep it running (1000-1500 rpm). Not violent. Then the motor settles into the “proper” high idle after 1-2 minutes, smooth as can be.

2. Initial tip-in of the accelerator results in a slight hesitation. When the engine is not fully warmed up – say after less than 10 minutes of running – the engine can stall if I floor it from a red light. Once the engine is properly warmed up, if I floor it from a stop it will jump a bit, hesitate, then accelerate really well. At higher RPM it drives and accelerates beautifully. The accelerator pumps on both carburetors provide a nice stream of fuel - but I have not measured the quantity, nor checked if the nozzles are pointed correctly.

Thoughts? My mechanical ability is decent, but I am clueless in the diagnosis. (I’m enjoying the learning process – and loving the car!)

[meltedpanda]

is timing correct , dwell ?

Vacuum advance working correctly?

if these are good then I would look at the carbs, the 35/40 are tricky to sync, about a 7-8 step process if I recall ( I have it here somewhere) and the mixture regulating adjustment is best done with a CO meter

[Todd Miller]

Sync the carbs for real. It's not hard.

During that process, adjust the mixture.

Make sure that the choke plates both close fully, and that when closed, whatever triggers the throttle plates to crack open for an increased idle, is working.

Make sure your coil wire (the big diameter one) from the coil to the center of the dizzy cap is a solid core wire. If it's carbon core, it'll have too much resistance for easy cold starts. You can make your own by purchasing bulk solid core wire, off the roll, at NAPA, and they'll sell the metal ends, and rubber boots. 10min job.

Make sure you don't have too much resistance in the spark plug wires, spark plug boots, and spark plugs. Too much resistance = hard cold starts.

If you're running anything other than a standard Bosch or NGK spark plug, especially a Bosch Platinum, replace the plugs, and make sure the plug gaps are only .028". Wiz-bang spark plugs cause hard cold starting. Wide spark plug gaps cause hard cold starting.

Once this is all sorted, try starting it by pumping the accelerator pedal a few times, in order to blast some fuel into the intake. It's a big intake. Then crank her over and it should light right up.

That stumble you're getting is a lean stumble. You're cracking open the throttle plates, which causes a huge dump of air, and the carburetors aren't able to supply fuel that fast. Adjusting the mixture, and making sure the accelerator pumps are spraying into a location that creates a good spatter of fuel that the air will blow through, will make this go away.

[cth350]

From memory, since it's been close to 30 years since I've driven a 250/8, when you want to do a cold start, you were supposed to tap the accelerator pedal twice before turning the key. But you had to think about it and not do that for a warm start because it might flood. If you got it wrong, you would just floor it w/o pumping until it started.

And then take Tony's advice for getting it better. Regarding the wiring double checking he suggests, there's a spec for "overall resistance" that says the total resistance of the wire plus plug must be below 5k ohms.

With the traditional setup, the plugs and the wire had no resistance to speak of, but the wire ends had resistance. The current parts books follow the bosch recommendations, which are not actually consistent with the car. Like Tony said, go with a copper plug.

-CTH

[rumb]

"Once this is all sorted, try starting it by pumping the accelerator pedal a few times, in order to blast some fuel into the intake."

My own experience if different. If the car has sat more than 2-3 weeks all the gas has evaporated out of the carb, so I have to crank in many times - in short bursts - to prime everything.

If I have driven it lately I press gas pedal down half way to set choke. I never pump it as that usually floods it.

Hot start I hold pedal all the way down and crank till it starts. again no pumping.


As mentioned above follow the 7-8steps in order to set carbs up.

[t walgamuth]

My routine in starting any carbureted car is to pump it a couple times. Also before anything else push it all the way down to set the automatic choke. Warm starts just a small pump or two should do the trick.

[Popeye]

Thanks everyone for the input. Much appreciated, and I have a nice list for next Saturday.

Starting with timing, then carburetor adjustment (as found on JamieKop's website). I've been hesitant to adjust the carburetor, as the previous owner is the long-time mechanic of the car (since the late 1970's) and is much more competent than I. Fundamentally the carburetors seem to work correctly: choke plates close upon cold startup, and gradually open as the car warms up. Cold and hot idle are set correctly. The vacuum actuated throttle control functions. (15" of vacuum at warm idle.) However, it's that "last mile" that probably makes all the difference - they are not perfectly in synch, for example.

The spark plugs are new (replaced when I bought the car) - however I'm not sure what type, and will confirm. (Compression is decent / uniform across all cylinders.)

Thanks again!!!

[aluminum]

You don't drive / Start >the car often and those Cars Carbs drain down > evaporate fuel
making for a Low fuel situation for cold starting after sitting and that's most likely the main reason for the rough start /Run after " Sitting " You can install an electric Fuel Pump at the rear by the tank a low pressure pump and no more problem .
You turn on the key and wait until the fuel pumps up "about 1 or 2 mins " then start it.
Also check your heat risers in the exhaust Manifolds making sure they are operating or your going to suffer slow warm up issues / Best fix for the Carbs adjustment is to have someone familiar with them do the adjustment or if you want to learn bring tools necessary and drive and set the Carbs until good this may take several settings before satisfaction as they are tricky and require first that all the linkage is in good aliment and not sticky at all < without this the Carbs > will not return to the same exact place upon releasing the Throttle loosing *** " synchronization " *** a must with dual Carb's IE : Without the linkage being correct the the Carbs and not correct. As a further Example if your linkage is not perfect then you Carb synchronization is also than not perfect < Lubricate all the parts of the linkage and you will discover binding area's .

[Popeye]

Thank you.

I won’t get to the motor this weekend, as I bought a Craftsman timing light and a Sears dwell meter (actually engine analyzer) on eBay. These won’t arrive until next week.

In the meanwhile, I hope to fix a broken diaphragm in the trunk (new diaphragms arrived today). And maybe polish some paint - having fun in my garage!

[Todd Miller]

Instead of using dwell to set the points gap, if you set the gap to the narrow end of the adjustment range (.012") you'll get the strongest spark. The coil charges when the points are closed, and a narrow gap allows longer charging. That, plus a solid core coil wire, correct resistance in the rotor, plug wires, and plugs, and a plug gap on the narrow side, will give the fastest cold starts (along with correctly functioning carbs, of course).

Once you have it dialed in, starting nicely cold, taking throttle, and running correctly, you can go back and widen the plug gaps in increments, testing for a few days between each change, and see how it starts cold with a wider gap. A wider gap gives better performance and fuel economy, so you gap out the plugs, until you reach that point where it still starts good cold, and more gap makes it crank longer when cold.

[Popeye]

Hi all,

Just getting back to the engine tune. (I was distracted by noise/vibration from under the car. I think it was the exhaust rattling against the rear motor (transmission) mount cross plate - and hopefully fixed.)

Two questions as I roll up my sleeves on the ignition:

1. My distributor vacuum advance has two ports: I assume one for retard, one for advance. The port furthest from the distributor will not hold a vacuum and does not move the adjustment rod. The port closest to the distributor holds a vacuum, and rotates the internals clockwise (I assume advance).

If I am right - I have advance but not retard - how critical is the retard function? Searching the forum suggests it is only needed for emissions at idle - and not needed for general operation. (If anyone has Bosch 1237122611 to sell, let me know!)

2. The timing guidelines seem to be vastly different between the Haynes manual and my Mercedes Service manual. With no vacuum connected, at (1) idle, (2) 1500 rpm, and (3) 3000 RPM, the Mercedes manual calls out: 1-9, 31-39, and 41-49 degrees. The Haynes manual calls out 0, 19-28, and 29-35 degrees. The diference is stark... what am I overlooking? (I have not yet checked to see where the motor currently is timed; like I said it is fundamentally running well, and I assume I need to "tweak" vs. "adjust" things.)

Thanks,
Mike

[subforry]

I would disconnect the vacuum retard. Consider this, loosen the distributor enough to move it but so it wont rotate when running. Warm up the car, with the vacuum advance connected, advance the dist until it runs the fastest and smoothest. Retard the timing a bit. Adjust the carbs/idle speed if needed, lock the dist down. Drive it. If it pings when you floor it, runs too hot at idle, or is hard to start when heat soaked, then retard the timing. Use the highest octane fuel you can get.

I guess the main point I am making is the timing numbers are old when fuel was different and based on emissions standards of the time. Your engine could have different compression from a rebuild, different cam, etc. So set timing to what the car says and enjoy.

As a side note the hotrod community is converting me to use manifold vacuum for the advance instead of ported. This makes for lots advance at idle and high speed cruise, retards the timing when the throttle is opened thus reducing pinging.

[Popeye]

Thanks!

[meltedpanda]

I go by "ear" on these ol gals. As suforry stated, listen for that sweet spot , drive it and then adjust based on symptoms if any.. good luck

[aluminum]

Dump that distributor for an advance Only distributor at salvage ( you want the Good advance Curve ) >
Run as much advance as you can depending elevation just set and drive it until you get
Pinging or Top end miss he back it off 5 degrees or so They like a lot of advance .
Get a solid copper (inside ) coil wire to dist for a good Hot spark > NGK Plugs work best
Make sure your heat risers are opening > 16 tho. is best Point Gap & use Point Lube also consider to convert to electronic ING
Unless You run the motor every 2 or 3 Days the flue drains down often in those carbs making it hard to start quite often>some install
an electric pump at the rear and turn it on and wait a Min or so before starting >it will run much better with more advance and a distributor swap ( Most any advance Only type will work and fit ) That Retard Returd was a smog deal they are junk