So we ran it quite a bit on the muffs tonight, now confident that the cooling system was working properly (thanks Brian and Jeff)! Thermostat never went above 145F where it's supposed to open; but, we noticed that the upper manifold on the right (drivers side, where the water enters the system) stays cold while the upper manifold on the left (passenger side) is getting hotter. When we shut the engine off, the two equalize rapidly (i.e. I think the water is flowing equally when not running); and again, the thermostat never read above 145F after 10+ minutes. Is this normal? It makes sense to me that the further upper manifold woule take the longest to cool down compared to the other one. We've run Sea Foam through the gastank (2 cans, as per Jeff's reccommendation) and a 1/2 bottle through a spray can right into the carb. Lowest it will run is ~800 rpm, not quite in idle. Is this something we can adjust? Any other ideas other than spraying the carb with Sea Foam? Thanks again for the help everyone!
