"RUN-ON" After shut down

[BSNMOE]

I own a 73 with MFI. Two weeks ago, with a remaining 1/3 tank of 10% Ethanol gas ,I filled up with MARATHON Gas. Marathon has a pump intended for "recreational vehicles" which is non alcohol 90 Octane. I did notice after fill up and about 50 miles of driving that my idle was a little higher. (almost 50 RPM higher than my usual 950 RPM) At the time I thought that the higher idle was due to a unusual hot day and left it alone. This morning after a ride with much cooler temps the car still idled a little high. I turned my idle lean 1 click thinking that this gas must be burning more efficiently.
I was down to 1/2 tank and filled up with more Marathon and drove about 20 miles and returned home, shut the car off and it continued to run briefly. Prior to this I was at a few red lights and the idle was at a steady 950RPM. I could not duplicate this run-on again but, I wondering what ,if anything, could cause the high idle & run-on even after turning down the idle mixture?

[Steve@Rennsport]

Hi,

The run-on you experienced is primarily due to insufficient octane, given the timing and CR.

JMHO, but I would not use 90 octane fuel in these cars, despite their modest compression ratios. These cars were designed to run a fuel thats no longer made so the closest thing is today's premium 91, or better (even with ethanol).

You can combat the long-term effects of ethanol by using (marine) Stabil in the fuel.

[BSNMOE]

Steve,
Prior to this I would fill up using between 89 and 91 octane thinking that it would ignite better as the MFI Manual suggested. The non ethanol is 90 octane which is pretty close to what I was using.
Could it be that the non ethanol is changing my AFR and making my car more rich as with ethanol we have to run our MFI cars more rich than the 70's?

[db_cooper]

Ethanol has a very high octane number and is used to boost the numbers..its possible that the non ethanol pump indicated 90..however..it was not a 90 octane and had regular non ethanol octane fuel in the tank..like 87.

You have a 73 with MFI...what model? The "T" and "S" both had MFI (unless it is a 73 1/2 CIS "T")

The fuel octane requirements for the engines would be very different. The "S" would be very sensitive to fuel octane, timing and mixture. The "T" much less so.

10% ethanol fuel usually requires some changes in the fuel calibration from pure gasoline. Without an O2 sensor, MFI may need some tweets for smooth operation. Don't assume it needs to be richer..our modern fuels are not the same as the old days.

Its been a very long time since I worked with MFI..I recall there is a fuel cut off solenoid on the MFI pump wired into the ignition circuit...or I may be wrong. If so..is the solenoid operating?

Engine "run on" or dieseling with any 911 is very unusual..and not a good sign. Even at 950 RPM idle speed..the engine should not diesel when the ignition is off.