I've built these kind of wheels before and the low boiling fluids are terrible at performing any useful work. i.e. Freons, ether, dichloromethane...
Use pentane or something that boils at close to this range.
Low boiling fluids have too much pressure in the top chambers that waste energy when it is compressed by the fluid filling into them.
Compressing a gas requires work to be done.
It's like driving around with the brake on.
At just under 120F, the cooling cycle temperatures, most of the Pentane is starting to condense and so there is less vapour to compress and less energy wasted.
If the cycle is maintained at just above and just below 120F (~50C)
Reasonable power can be obtained.
Make sure the lower tank is fully submered (evenly heated) during the transfer period (not much more than 20degrees past BDC bottom dead centre) The fluid is very suceptable to small temperature variations and if part of the tank is leaving the bath before the fluid is transfered you end up having the fluid transfer it's energy out through the exposed part.
This is my small engine working.
I have a 20liter tank system that turns over (slowly)with the lower half in the sun.
With warm water it generates just over 200W continuously.