I have 2 carb'd hp500 motors. Both in perfect shape. In my springtime routine, I just changed both raw water impellors, and both 160* thermostats. I take the boat out, fire the motors at the dock and let them warm up a bit. The port motor runs great...but the starboard motor runs like its taking forever to warm up. I leave the dock on the port screw..get out past the piers and when I put the other motor in drive it wants to die. After I finally get it in gear and running, they run great. Port motor comes up to temp perfectly..stb motor is fighting to come up to temp. Port is 160*...stb is 130* (maybe). Stb water pressure is way higher than port pressure. So the stb motor basically wants to run like its not warmed up enough. Almost like its getting too much cooling water. I know your gonna say the thermostat is stuck open...but it is new, and the old one acted the same way too. Is there anything else it could be ? Any ideas ? Thanks guys.

HP500's are supposed to have 140* Tstats.
I would swap your gauges to veriify the temps first.

I highly doubt the water temp has anything to with the poor idle. It should idle fine at 130*. I would guess that the problem is in the carb.

Sounds like carb to me also. Get it running rite, then go after the temp and pressure.
I have been wanting to put a water pressure gauge in. Can you tell me where your sender is installed? And what kind of pressure #'s I can expect? Good luck with yours, and thanks for any info that you can give me