Have you removed your Rad Fan?

[Zac]

Dawn, you are on the right track.  A 4-cylinder will have more combustion chamber surface area and will produce more friction heat than a twin of equivalent displacement.  Just look at the radiators of a twin and a 4-cyl sitting next to each other, the twin's will be smaller.

As far as thermostat/no thermostat, there will be an optimal coolant flow rate, and it will be different for every engine, radiator size, radiator construction, etc.  If you wanted to put a 2 temp gauges on the bike, one at the inlet and one at the outlet of the radiator, you could run experiments with different size orifices replacing the thermostat and determine the optimal case.  As an example, I have the factory race prep manual for my '64 Triumph TR-4 (the four wheeled kind) and it says to replace the thermostat with a restrictor plate and gives the proper orifice dimension.  That car also uses a special thermostat stock that is more restrictive that a standard thermostat.  A standard one will fit in the housing, but the car will run hotter than with the more restrictive special (read: expensive) thermostat.

-z.

(for the record my R6 has a thermostat and fan, my wife's MZ has a thermostat and no fans, and her TZ250 obviously has neither)

[MZGirl]

My fan wasn't all that rad.  Rather heavy.  But you can't have it.  It's a spare for my streetbike (MZ Replica).  That bike's got a lot of spares now.   ::)

[Super Dave]

If a thermostat fails in the closed position, you may smoke an engine.  

For our racing applications, you don't need a thermostat.  Tape will be great if you need more heat in the engine...tape.

I don't have my fan on.  Usually, you've moving faster than the fan goes, so it doesn't have much use.

But, like Greg said, it can be used to cool down the coolant in the radiator after stopping.