about 126 degrees C and you have to worry about internal lubrication problems and over temping the engine. Usually if the engine doesn't like how lean your running it you will notice it falling off in power, and often flame outs at the end of a long straight. Don't get me wrong some engines like to run there but you have to figure out how many liters you want the engine to last. Unless you take it to far to the other side, its a safe bet that parts will last a bit longer on the richer side. Reading the plug is crucial. cheers