TECH Q&A

Richard Simonds

Star Technical Advisor

Questions about a 1969 280S

Q: I recently acquired a 1969 280S: I noted that you also own a ‘69 280S while reading your “Tech Tip” article about installing a modern electric fuel pump (The Star, July-August 2018). Maybe you can answer a few questions for me. First, when the car is cold, it can take five or six attempts to start it, an issue cited in the article. Do you think adding an electric fuel pump and pressure regulator would be of benefit? If so, can you offer any advice on how a mechanic would integrate an electric fuel pump into the car’s existing electrical system?

A: Congratulations on your new car. As a predecessor to the S-Class, you’ve got lots of space and elegance in what should be a satisfying vehicle to own. But the way in which it gets fuel to burn and a spark to ignite the fuel uses technologies that the automotive world now considers to be obsolete: carburetors and points-based ignition.

So, my first piece of advice is that you need to find a mechanic who has experience working on cars from the 1950s and 1960s, and preferably one who specializes in Mercedes-Benz classics. If you can find a mechanic or an experienced friend who can teach you how to do basic maintenance on your own car, up to and including tune-ups and oil changes, you’ll get even more satisfaction out of your ownership of this classic.

 

Q: You mention the ignition system. I thought starting problems in old cars usually occurred because the car was getting too little or too much fuel.

A:  There is a saying among mechanics of these older cars: “Most fuel problems are electrical.” What they mean is that before you start fiddling with the carburetors and fuel pump, you need to check spark intensity and timing. I’m assuming here that in the pre-purchase inspection or soon after purchase, you had the engine checked for compression and leak-down problems.

First, be sure all grounding connections are corrosion-free and tight. Then check connections on all electrical circuits – from battery to starter, to coil, and so forth.

Next, ascertain that the spark plugs are cleaned and gapped and that the spark-plug wires (especially wires made with carbon cores rather than copper) have no faults in the voltage getting from the coil through the distributor, and to the spark plugs.

Also be sure that the distributor points are clean and gapped correctly and that the condenser is the right capacity and discharging correctly. Related to this is the dwell angle. And finally, the workshop manuals typically recommend setting distributor advance at a running speed (e.g., 2,500-3,000 rpm), not at idle speed. Have your mechanic follow the correct factory recommendations for the distributor settings – including advance curves.

Because you’re also considering some minor upgrades when you talk about installing an electric fuel pump, you might consider replacing the points-based distributor with an electronic one, such as a Pertronix or 1-2-3.

 

Q: Assuming we get those tasks done and still have a problem, then should we consider installing an aftermarket electrical fuel pump?
 

A: Not necessarily. Your car has a Zenith two-barrel (dual-venturi) carburetor fuel system, so fuel is likely to evaporate out of the float bowls when the car is not used every few days. Gasolines today, especially those with added ethanol, are more volatile than those available when this car was new: Today’s fuel mixtures rely on tightly sealed fuel systems that keep fuel vapors from leaking. With the old fuel tank and carburetor in your car sealed only for liquid leakage, the float bowls are likely to leak. A fuel pump that fills the float bowls before you switch on the starter may be the answer.

However, there are several other possible causes for starting problems that would be worth exploring in your new car before you install a fuel pump. You will need to find a mechanic who has experience adjusting and rebuilding carburetors, especially on Mercedes-Benzes. In the case of my 280S, the linkages had gotten a bit loose and the front three cylinders weren’t getting the same amount of fuel as the back three; (i.e. the front and back carburetors were out of sync). The first step was to balance the linkages between the two carburetors.

The carburetor valves are connected by a linkage such that both valves open and close simultaneously. The process is simple: Unfasten the linkage so that one valve is free to move and set it so that the air being pulled into that carb is the same as the other carb, then retighten the linkage. There is artistry needed here that requires experience to get right; lots of gadgets have been developed over the years to assist in the process.

The choke (butterfly valve) on one or both carburetors may not be closing fully. This could be as simple as correcting the adjustment on the heater coil that regulates how much the valves are closed when cold and how quickly they respond when electricity is applied to the coil to open the valves.
Beyond that, there may be problems with the carburetors themselves, which may not have been rebuilt for a very long time, if ever. When the shafts get gummed up – mostly from too little use and not getting up to temperature, or because carburetor cleaner spray hasn’t been used on the valve shafts to keep them moving freely – that can keep the choke from closing fully. Also, there are large diaphragms adjacent to the float chambers in the Zenith carbs; if these begin to leak, they will allow raw fuel to flow into the manifold and then into the combustion chambers.

It may be worth rebuilding the carburetors, a satisfying skill to learn if you’re inclined to reading how-to books and enjoy figuring things out. Carburetor rebuild kits come with replacement diaphragms. Check with the Carburetor Refactory in Richmond, California, at 510.237.1277 and www.carbkits.com.

You’re probably better off finding a carburetor service or locating an experienced specialist to rebuild the carburetors: They will ensure the correct jets are installed – rated for sea level to 6,000-feet altitude. I’ve occasionally had the wrong needles installed.
I have also had mismatched jets installed in the low-speed circuit and the high-speed circuit – causing erratic running and vibrations, especially at highway speeds – when the two carburetors were not providing equivalent fuel mixtures due to mismatched jets. If the carbs have already been rebuilt and you’re still having problems, then you might consider the fuel pump as the source.

Depending on how you feel about originality, you might first have your mechanic rebuild the mechanical fuel pump, which may resolve the starting issue. If your mechanical pump is doing a good job, it is probably not worth the expense to purchase and install the electric pump and pressure regulator.

However, you can bypass the mechanical pump and go directly to installing the electric fuel pump, rerouting the hoses to go from the tank directly to the electric fuel pump and then to the carburetors. Just make sure that the pump you install is rated for no more than 4 psi; additionally, install an adjustable fuel-pressure regulator to ensure that the carburetor needle valves – rated at 4 psi – can close at the right float level to avoid flooding the carburetors. To power the electric fuel pump, my mechanic connected it to one of the circuits off the fuse box that are activated only when the key is on.

Have fun with your new car.