TOPIC:

I have seen bikes for sale where the owner has painted the entire engine. Is reduced cooling something to worry about?

No!
I paint all my engines black.
No problems in 30 odd years of thrashing them.

The simple answer is yes, slightly.  But the next question is by how much?  Is it enough to cause problems with air cooled engines?  The answer to that question is probably "not likely".

It seems like I read somewhere that Kawasaki determined that black engines radiated heat better.

wrote:

It seems like I read somewhere that Kawasaki determined that black engines radiated heat better.

I had that in my mind  too

Don't remember where I heard  or read it ,  But  I  did have it in the back of my mind

 

Water heater tanks are painted black to retain heat. 
Steve

What kind of water heater are you talking about & do you mean the internal surface of the tank or the exterior?
My water heater is grey.  I've never seen a black water heater:  every one I've seen is either grey or white.
If black worked better, you'd think the mfgs would be all over it for the better energy star rating.
It has been my understanding that black objects in general radiate heat better than lighter objects. 
For example I think a black roof radiates heat better/faster than a grey roof after the Sun goes down.
During the heat of the day the black roof absorbs more heat from the Sun.

Found this. kzrider.com/forum/6-paint-a-bodywork/377...es-it-affect-cooling

Aslo the internet claims black radiates heat best. That’s why all car radiators are painted black.

OK....digging back into my heat transfer class from 1980...

There are three mechanisms for heat transfer....conduction, convection and thermal radiation. 

Conduction is the primary method for moving heat through the engine cylinder walls.  It depends on the material type and surface area as well as the thickness of the material.  Metals conduct heat more readily than most materials.  Paint isn't particularly good at transferring heat so if you paint your engine you will slow the heat transfer to the exterior fins.

When the heat reaches the exterior it is whisked away by convection, caused by air flow over the engine. 

Thermal radiation is heat that is given off from a hot body to a cooler one.  A good example is the sun.  When you are in direct sunlight you feel the solar radiation (It will actually cause sunburn if you stay exposed for too long.)  but when you move into the shade the radiative heat source is no longer active.  (For anyone who is interested in this phenomenon...google black body radiation.)  With respect to a motorcycle engine....because it is hotter than its surroundings it will thermally radiate a small amount of heat to the surroundings.  

If you paint the engine you are forcing the internal heat to take a slightly longer path to pass through the painted cylinder walls to be removed at the cooling fins.  The engine will run slightly hotter and the exterior surface will be slightly hotter.  How much?  Not very much...maybe a degree or two.  The Kawasaki engineers would have likely calculated the amount before they decided to paint Z1 engines black.


 

The tank of a water heater is painted black electric or gas. There was a time my place didn't have power to run a water heater or anything else. I was using a 12V system at the time. I would charge the battery with my car after work. I brought an old gas one home, took the skin off and put the tank on the roof over a vent for the sink. Worked great and used it even when I had the power restored. Every water heater I've scrapped was painted black to aid in heat retention. 
I still have the lights in my kitchen and front room. Ready to live off the grid at any time. I hope not but if needed it's still there. 
"It was the best of times and it was the worst of times."
Steve 

Not sure, but I thought that the color black absorbs all wavelengths of light and reflects none. Therefore, black colored objects will absorb more heat, but dissipate that heat faster than other colors ? 

Hair splitting discussion.  I've participated in a few so not criticizing.  

I'm not an expert on Kawasaki silver engines but I do know that many OEM silver engines are painted silver.  Without some protection the aluminum will oxidize and turn ugly.  Some paint has valuable upsides and minimal if any downsides.