Of any temp measurement it's head temperature that tells you most directly if the engine is in trouble or not, measured with a thermocouple on the washer under the spark plug usually. I'm sure there's a relationship between oil temp and head temp but it would be hard to know what it is without having both measured on the same bike.
The lean=hot thing is a bit of a myth, or at least not completely understood by most. IF your bike was already running on the rich side of stoichiometric and you make it a step less rich then the temperature of the head and exhaust gases will increase. This is telling you that you are still on the rich side of stoichiometric NOT the lean side. As you keep making it less rich the exhaust gases will keep getting hotter until you reach a peak temperature at exactly a stoichiometric mix. If you continue past that point into the lean side of stoichiometric the exhaust gas temperatures will actually start to go down again and they go down faster on this side than the rich side so lean running can actually be cool running. If you think about it this makes sense: a stoichiometric mix has the perfect ratio so all of the air and all of the fuel burns. If you make it lean you have less gas in the cylinder to burn and less of the oxygen gets burned, so less heat is generated by a truly lean burn than a stoich one. If you make it richer than stoich you can burn all the air but not all the gas and the extra gas spraying around is evaporating and cooling things again reducing temp vs. a true stoichiometric burn.
I think the confusion comes from the fact that we are probably always running quite rich relative to stoichiometic. Peak horsepower occurs well on the rich side of the scale and detonation resistance is much better when rich as well. Even modern fuel injected cars that are computer controlled run rich when you stomp on the gas as a protective measure. Very high cylinder pressures occur when the throttle blade is wide open and running quite rich during this condition keeps you away from the range where detonation sets in. The difference is the modern car can drop back to stoichiometric when you lift off the gas and cruise there. With our primitive carbs this is a tough trick to do and I think we always end up compromising well on the rich side of stoich. From that point of view where we are always well to the rich side of the curve, yeah, going less rich = hotter.
If you want to know more about head & exhaust temps and how it relates to fuel/air mix and detonation there was a great series of articles by a pilot a few years back on the subject. He was testing in a fully instrumented engine with cylinder by cylinder control over air/fuel mix and head temp sensors on every spark plug. Sadly the author had a stroke and was unable to keep flying or writing but the articles he did complete are really good. This is a chart of the basic relationship and a link to his writing:
EGT = exhaust gas temp
CHT = cylinder head temp
ICP = internal cylinder pressure
http://www.avweb.com/news/pelican/list.html
The lean=hot thing is a bit of a myth, or at least not completely understood by most. IF your bike was already running on the rich side of stoichiometric and you make it a step less rich then the temperature of the head and exhaust gases will increase. This is telling you that you are still on the rich side of stoichiometric NOT the lean side. As you keep making it less rich the exhaust gases will keep getting hotter until you reach a peak temperature at exactly a stoichiometric mix. If you continue past that point into the lean side of stoichiometric the exhaust gas temperatures will actually start to go down again and they go down faster on this side than the rich side so lean running can actually be cool running. If you think about it this makes sense: a stoichiometric mix has the perfect ratio so all of the air and all of the fuel burns. If you make it lean you have less gas in the cylinder to burn and less of the oxygen gets burned, so less heat is generated by a truly lean burn than a stoich one. If you make it richer than stoich you can burn all the air but not all the gas and the extra gas spraying around is evaporating and cooling things again reducing temp vs. a true stoichiometric burn.
I think the confusion comes from the fact that we are probably always running quite rich relative to stoichiometic. Peak horsepower occurs well on the rich side of the scale and detonation resistance is much better when rich as well. Even modern fuel injected cars that are computer controlled run rich when you stomp on the gas as a protective measure. Very high cylinder pressures occur when the throttle blade is wide open and running quite rich during this condition keeps you away from the range where detonation sets in. The difference is the modern car can drop back to stoichiometric when you lift off the gas and cruise there. With our primitive carbs this is a tough trick to do and I think we always end up compromising well on the rich side of stoich. From that point of view where we are always well to the rich side of the curve, yeah, going less rich = hotter.
If you want to know more about head & exhaust temps and how it relates to fuel/air mix and detonation there was a great series of articles by a pilot a few years back on the subject. He was testing in a fully instrumented engine with cylinder by cylinder control over air/fuel mix and head temp sensors on every spark plug. Sadly the author had a stroke and was unable to keep flying or writing but the articles he did complete are really good. This is a chart of the basic relationship and a link to his writing:
EGT = exhaust gas temp
CHT = cylinder head temp
ICP = internal cylinder pressure
http://www.avweb.com/news/pelican/list.html
