Is that good running it too rich though? Isn't that just as bad as running to lean?
Too Rich or Too Lean are relative terms. It depends on How Much too lean or too rich you are.
For power, cylinder temperature, throttle response and even fuel mileage, a bit on the rich side is much better than too lean. A slighly rich mix will effectively raise the octane level of the fuel.
The problem with the ideal fuel mix is that emissions go up some. That is why all of the above advantages are abandonded and our modern engines are purposfully run leaner than optimum.
Add to this a 10% Ethanol content which reduces fuel mileage about 10% (along with an overall reduction in performance) and you can see where richening up the mix and running 100% gasoline can really make some worthwhile improvements in all but the emissions category.
Though now it is pretty universally understood that the reduced emisions benefits of ethanol fuel is more than offset by the additional polution created in making ethanol, and the fact that you have to burn 10% more of it to go the same distance.
A more lean mix than what is necessary for lowered emissions can be quite detrimental in a number of ways including extremely high cylinder/exhaust temps, detonation or knocking, not to mention performance issues. A lean mix will effectively lower octane.
A much too rich mix will foul plugs, leave harmful deposits on valves and piston rings and can wash the protective oil film off the cylinder walls greatly increasing upper cylinder wear.
And, if rich enough, you will begin to get raw fuel mixing with your oil. That is Very Not Good!
Still, you're generally better off with a too rich rather than a too lean mix as negative effects come on much more quickly as you lean the mix than they do as you richen the mix.
