244 DL 1977 - running very rough
 

First, take a compression test to eliminate things like rings, valves, head gasket, etc. Then go to to electrics, fuel, etc.

CIS cars seem to be pretty sensitive to air leaks from hardened injector seals. makes them run way too lean.

I have replaced the seals on the injectors, cleaned the injectors, cleaned and gapped the new spark plugs.
Idles ok, but will not rev up above 2500rpm. Then a you release the accelerator revs pickup a little then back to idle.
Plugs are very black!

I don't have a compression tester, I did try spraying brake cleaner around the engine bay to see if she would idle up - No Change!
Next step seems to be take the intake manifold off. Seems to me like this car has been run for sometime with a poor tune.

Any suggestions.

did you get a pop test run on the injectors when you had them out? a diesel injector shop has the setup to do that, or maybe a older indie mercedes or vw or even volvo shop.... but volvo stopped using CIS in about 1982, VW used it up to about 1989, Mercedes used it on their 6 cyls through 1992 ... its becoming a lost art.

the pop test should confirm two things, no fuel dribbling if the pressure is lower than the pop value, and even consistent spray patterns with the volume sprayed by each injector within 5-10% of the rest. My 1990 Mercedes 260E that sat for a long time before I got it turned out to need 4 of 6 injectors, we replaced all 6 for consistency, and its run like a new engine since.

If your plugs are black and your coil is hot - that's an ignition problem. Here's a good basic explanation:
"Overheating ignition coils can be due to a number of causes. The symptoms are always the same though: poor or non-existent hot engine starting and rough running and misfiring whilst running.

The first is simply due to age: if it's traditional "wet" coil filled with transformer oil, then the oil can either leak out or simply break down with age. If so, the fix is simply to replace it and there are often modern compatible dry resin coils which do away with the oil filling completely. Dry resin coils can also fail with age or hard use, but it's far less common than with wet coils. A failing coil which usually works perfectly well be over-stressed if the vehicle is left idling for long periods on a hot day, such as during summer traffic jams. In such conditions, the coil will usually recover if the engine is turned off allowing the coil to cool down, but the damage caused by overheating is cumulative and the coil will become increasingly prone to overheating.

The second most common cause is a poorly tuned engine (if the vehicle is equipped with a distributor). If the ignition timing is incorrect or if the points gap has lessened due to wear then the coil can be energised for too long and will overheat. Note that the points gap and distributor timing are related: if the points are replaced or re-gapped, then the timing MUST be checked and reset both statically (engine not running, turned over by hand) and dynamically (engine running). Incorrect timing can also be down to a faulty or non-functional ignition advance and ****** mechanism. If you do not have ignition contact points, then there may be a fault with the electronic ignition module within the distributor.

If the vehicle has completely electronic ignition with no distributor then it's possible for faulty components to cause overheating coil. Most incorporate a variable dwell feature which prevents the coil from overheating when the engine is idling for extended periods. But I suspect this does not apply to your vehicle as you refer to a singular coil rather than the multiple (dry resin) coil packs mounted directly atop the spark plugs as per modern fully electronic ignitions. In any case, diagnosis of a fault with this set up requires electronic diagnostic tools."

of course, the 1977 240 has a distributor and it has distributor based spark timing, however it doesn't have points, they adopted electronic ignition pretty early on.

That's what the last sentence in the third paragraph refers to - the Hall sensor?

Even though it's frustrating when a car won't start that's the time to do your best diagnosing. Since you improved the ignition faults my next step would be to check fuel availability, right then when it's acting up.

Next time it stops spray some starting fluid in the throttle body while it's cranking and see if it responds. If it does then fuel problems. If it doesn't then back to ignition problems.

When you pulled out a spark plug was it wet or dry.

Checking the plugs again is a good idea. Although not as accurate as checking the injectors it will quickly tell you if fuel is getting to the plugs. If they're wet then it's back to checking for spark.

testing CIS injectors requires a POP tester, a diesel injector shop can do this, and flow test them to ensure all 4 spray within 5-10% of the appropriate fuel/second at an appropriate pressure. they can't really be cleaned, there's a pressure release valve inside.