Greetings,
Could use some general advice regarding engine performance.
After a drive and the motor is warmed up, I notice that when I turn off the motor it may continue to turn over once or twice before quitting. What is this a symptom of? Slightly advanced timing?
Thanks in advance,
Marshall
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Here's an article that talks about all of the issues with dieseling or run-on after shutting off the ignition. Timing shouldn't be the cause because there's no power to the coil so no spark to send to the plugs:
Recently after I built new wiring harnesses, I noticed a change in the engine turnoff behavior. It didn't chug, but it turned another revolution or so more than before.
I chalked it up to several capacitor-diode pairs I added near various electrical noise generating parts (ignition, fans, etc). I suspect the capacitors discharge back into the B+ and keep everything running for a second or so.
I also suspect that almost no one else would have the same situation unless they had a honking big stereo unit with a lot of power filtering.
Pickett must be running some big, honkin' electrolytic caps in his Speedster.... 
Two questions and a comment:
What RPM is your engine idling at? If it idles above 800 rpm, try turning the idle down (on both carburetors if you have two) to 800 or slightly lower. Most of the time, that can cure it from "dieseling".
What Octane gasoline are you running? If it's 87, try running your tank down close to the reserve and then put in a couple of gallons of 89 or 93. That might cure it, too.
You could also try installing new spark plugs that are one heat range lower than what is in there now. I know there is no ignition so no spark after the engine is turned off, but a hotter plug may be just hot enough to keep the combustion cycle going for a second or two before the plug tips cool off. This is, admittedly, a longer shot than the first two, so try those first.
Mine will diesel a little now and again, usually after a long hard run on a hot day. I've gotten in the habit of easing the clutch out with it in gear with my foot on the brake as I turn the key off. And before anyone asks, my timing is adjusted correctly and the engine never knocks when running. But the car doesn't like prolonged operation at highway speeds, oil temperatures can get as high as 220 degrees on a hot day if I run for several miles over 60 MPH. I suspect the rear grille pulls a vacuum at high speeds which fights against the fan. If I stay at 55 or less there's no problem. I prefer back roads anyhow, so I haven't really thought about a fix.
