Dealing with dieseling-run on

[Kern Dog]

I got advised by some guy on another forum...he suggested that I try running race fuel.
Fuck that.....$15 a gallon gas to fix this? He also suggested backing off the timing or putting in thicker head gaskets to drop the compression ratio. When I politely declined, he got pissy.
It was shit advice but I was polite in how I disagreed with it.
First, this engine does not knock, at all. I dealt with detonation before with this very engine so I know what it sounds like, what it feels like. In 2022 I went through this engine and put in quench/dish pistons. This put the ratio to 9.8 to 1 and with that, it hasn't knocked once, even in hot weather.
I used to run 91 octane gas but tried 89 octane about 18 months ago and it ran great on that too, still no knocking.
If an engine isn't knocking, using higher octane fuel is a waste of money and possibly even reducing power since higher octane fuel does not burn as easily.
I'm not going to use expensive race gas to stop "run-on" in a street car. I'm not stupid enough to pull the heads to lower compression in a car that isn't knocking. I'm not backing off the timing for an engine that isn't knocking either.

[Kern Dog]

Since it annoys people to read a thread where the OP never comes back to update the progress, I hereby step back in here to post what I have figured out.....during the time that this forum was off line and impossible to view.
A member of the FBBO forum read of my issues and given his extensive experience with race engines and nitrous oxide, he had some good advice. His years of dealing with detonation and other ignition issues taught him some things and he was kind enough to share his wisdom. In fact, he even shipped a set of spark plugs to me FREE of charge.
See, my NGK spark plugs that I've used for many years have extended electrodes. The NGKs he sent do not. the heat range was the same but the "recessed" electrodes and the idle speed of 700 RPM seems to have STOPPED the run-on. I drove around in mid 90s degree weather and shut it down here at home to....silence !
S C O R E !!

[Kern Dog]

Compare the differences:

Old plugs:



New plug above the old plug.



Yeah, it made all the difference.

[Charger-Bodie]

are the plugs the same or different number?

[Kern Dog]

Different NGK plugs.