Quote:

Street driving,,over rich condition,,,= deposits on spark plugs that will stay hot/red hot/glow and ignite excessive fuel and cause 'run-on'. Maybe time for new/clean plugs.


It just happens I was looking at some old notes from the ex-Innovate forum and there had been a discussion about the opposite situation - lean run on. I'll share here in case anyone is interested. My own experience has been no run on with carbon fouled plugs. Just my experience, not saying it couldn't be.

Q. "Hi Shrinker. I find it interesting you note running a lean idle as a cause of deiseling. Can you elaborate please?"
A. "When its lean the flame speed is slow and not completed by EVO time. Once the valve opens the air volume that has to be exhausted at idle is very small and the pressure quickly drops to near zero so the reaction rate drops considerably. Once the reaction is slowed some molecules cease combustion part way through their cycle, that is they dont combust to completion but they remain in the cylinder trapped in the various places etc. They are then hot molecules and they are of a different chemistry. They are called free radicals. They are very detonation prone and they are hot from the previous burn so they detonate erratically, thats why the engines combust erratically when they Run on (or diesel) when you switch the ignition off. Their combustion on the next time around in the cylinder sets off the main charge.

Richen up the engine and set the timing to 18 for hot cams and running on goes away. BUT you have to enrich the engine correctly not just whack a big jet it, you have to have enough T-slot exposure to exhaust the volume of fuel with the air volume thats coming through the carby circuits as well, otherwise the fuel just blobs into the engine. Blobs makes some cylinders get the fuel and others miss out and then you also get lean spots in the chamber even though you have it set rich. Its about the homogenization not just the total volume of fuel."
...
"When large cams are used the time period of Overlap where both intake and exhaust valves are open is significant, shut down the blades at the carby and the engine draws air across the overlap phase into the inlet manifold, especially when its lean because then there is oxygen left over on the exhausting cylinder anyway. So now the engine draws in exhaust gases, hot oxygen, free radicals, and blobs of fuel from the carby. Great recipe that one. Richen it up [enough] so there is no oxygen and create a burn that blows down the cylinder rather than relying on piston pump and you stop the muck."
posted by Shrinker 11-01-2010, 08:00 PM Innovate Motorsports ex-forum