Classic Australian Wooden Power Boat Association

I'm working my way through the 318 Chrslyser overheating when at idle. The temp is running at 60 degrees C at planing speeds but climbs to 90 + at idle. So a new thermostat and pump impeller are both going in but I'm now wondering if the inlet side of the Magnaflow cam driven pump is underdone. The pump has a 1 1/4" inlet but is fed by a 1/2" hose coming off dual water pick ups.

I'm thinking that at idle the cam pump is only spinning at 300 rpm and may be starved of sufficient water.

Any thoughts or suggestions?

I would prefer to see a 5/8" coming from the T of 1/2" hoses, and how strong is the bypass spring for when the thermo is closed? What temp thero?

Today I replumbed the inlet side of the water pump. The hoses from the pick up to the Y piece are 1/2" id. The T has been replaced with a Y fitting. The outlet from the Y fitting to the pump is 3/4" id. The 90 degree fitting into the pump has also been eliminated. A new thermostat is now fitted - 160 degree which is the same as the old (original) one replaced. Final job is to check the water pump impeller.

Job finished and problem solved. Replaced the impeller today and ran the engine for about 10 to 15 minutes at idle, pumping water from a bucket. All good for the Glenmaggie celebration weekend.