The 350 Mags on my 32' Wellcraft will overheat when in gear at low RPM - between 600 and 800 RPM in gear. When the throttle is bumped over 800 RPM in gear, the engines cool right down to thermostat temp and stay there. The raw impellers have been changed, the raw water drive belt has been changed, the recirculating impeller has been changed and the thermostats have been changed. At all speeds over 800 RPM - whether in gear or in neutral - engines are cool and this includes speeds up to 3800 RPM - boat is limited to this upper RPM as the props have been detuned and balanced to allow slower trolling speeds.Any idea on what is creating the temp issues at slow speeds? I have looked into having smaller diameter pulleys made to speed up the raw water pumps at low speeds but the cost is prohibative for just 2 pulleys.HELP!!!! please
Have you checked the raw water intakes? You may have a grate (external) which can become partially clogged, often with bottom paint, or other debris. Or, you may have internal sea strainers; the kind with baskets. Check to make sure those are clear. While your at this, check the intake hose for firmness and maintaining its "round"--old hoses sometimes partially collapse, which could also restrict water flow.
Have you ever had the risers and manifolds replaced? Five years or so is about the service interval. It may be time.....
Now, to your props. The 350 MAG should turn a MINIMUM 4400 RPM at WOT. You are way over-wheeled. The engine is lugging --working harder than it has to. If you need to slow troll you can use just one engine, bump in and out of gear, drag a bucket or sea anchor ("drogue" in that application), etc.
I would not consider changing pulley sizes. As you know, your engine has a single serpentine belt. Changing the pulley size would alter the speed, and thus the characteristics, of all your belt-driven accessories.
Could be a cooler problem if you have them on your engine. I see you replaced the raw water impeller,did you take the hoses off the pump to do so? If you did you may have installed them backwards. I have seen more then one pump and impeller replace again and again for that reason. Its rumored that it dosent make a diffrence but thats a bad rumor the suction and discharge hoses cannot be reversed. If they are you will still get some cooling and some water out of the exhaust but not the amount your supposed to.