787s overheating in '97 challenger 1800

[switt]

Seems I need a top end on at least 1 motor every season. Running at 7K for 10 min or more consistently requires me to limp back in on 1 motor. Pisser works but don't think am getting enough flow through the cyls at high speed. In reading the flow diagram, below, I see that the water flows into the 3 holes in impeller, through head, pipe, cyls, and finally out through the "T" tube and calibrated outlet fitting in resonator outlet. Am looking to increase water flow through the cyl jackets at high speed. The 3 impeller holes look good and the flow on the hose is more than adequate. Anyone found a way to increase the flow through the cyls or have any ideas? Perhaps recalibrating/enlarging the outlet fitting in the resonator?

 

[mikidymac]

How do you know it is overheating?
Do not modify the cooling system.

 

[switt]

Water sizzles on bottom end and need wait to cool before can restart. Am putting in ele temp sensors in bottom. I think the head sensors beep at 160, not exactly sure what a bottom end temp is considered hot.

 

[switt]

Placed temp sensors between base of cylinders at case. One engine kept temp below 160, or 71C as the ele. temp gauges report. The other crept up to 73C and backed off throttle to try and keep below 70C. I'll be checking the water flow/lines on that engine. These temp sensors may be the best investment I've made to the boat as it provides real time data for little more than $10.
MIXFEER Round Led Digital Car Truck Water Temp Gauge Temperature Sensor Temperatura Moto + Voltmeter Volt Voltage Meter 2 in 1 12.0V 24V

 

[switt]

PS, those temp sensors also report voltage. Little over 12V not running, little over 13V when running. Magnetos doing their job!