I recently set my timing, points, new plugs etc in my 1981 240DL. I have been running 91 octane gas and it has been running well. This weekend my wife filled the tank with 87 octane and it is running less well (no surprise). However, I noticed this morning when I shut it off that the engine wanted to run on. Could this be a result of the lower octane gas in the tank?

Thanks

Steve

What degrees BTDC is the timing set? On my 82 245 I eventually set it back to 12 degrees BTDC and never had any run on problems with 87 octane. I did briefly tried 14 BTDC and used 89 octane and did have problems with the engine running after the ignition swtich had been turned off on some occasions. I then set the timing back to 12 degress BTDC and left it there for the next 15 years.

91 RON is what your owner's manual probably says but this is misleading because 91 RON is not what's coming out of the premium pump.
I believe the equation is (RON+MON)/2=pump octane number. As I understand it the numbers for the 240 (at least for the elevation in my locale) are 91+83=174, and 174/2=87. So 87 is the correct grade. If your engine is running on with 87 octane your ignition timing is probably off.

"Could this be a result of the lower octane gas in the tank?"

Switching back to higher octane gas will almost certainly eliminate the behavior (symptom), but not necessarily the problem.

I don't know the details about the 81 240, but I suspect it is supposed to be able to run on 87 octane fuel. If it's not, then there may be some problems with the engine. Carbon deposits in the combustion chambers are often blamed for this type of behavior, and can be due to burning oil among other thing. (so fixing that problem, if it exists, may not be an option right now)
(I think the idea is that the carbon deposits get hot enough to ignite the air/fuel mixture if there isn't a spark to do the job, so the engine keeps running (int its own rough way) even with the ignition off)

When you turn the ignition off the fuel pump should stop pumping soon afterwards, and the injectors aren't supposed to continue to spray much after that. (how long is the run-on happening?)

I would be tempted to try 89 octane and see what happens, and go to 91 if necessary. Correctly fixing carbon deposit problems may be tricky and expensive.

(what are your plans for the car...?)

-Steve

Could be

my 85 240 sat for 2yrs and the gas was cheep gas so now since i bought it with this old gas it hasent ran on at all it could be carbon doing that im a detailer at a well known dealer in fl and the macanics do a type of carbon cleen they spray water in to engine with a spray bottel this turnes water to steem and since water dont compress it steem cleens the carbon out of engine im talking a little shot of water at a time nor am i suggestin you do this your self but you could talk to a macanic that you trust and see what he says i had these macanics do it to a 262c bertone that i had and it stoped running on after they did it i was told to drive the car aaa50 miles to rid it of any water and as i drove it it started running better and never did it again i had it for a year after that till i got sick of getting in and out of it big leather seats and stearing wheel thing