If he is running OK at speed, but heating at idle, it is highly unlikely that he is low on water flow or radiator capacity. Chasing higher water flow does not seem to do much for idle cooling, based on the tests I did. It is all about airflow at idle. That is why they go to the 1.3 pulley ratio for AC compared to the .95 for non-AC, to run the fan faster. Griffin radiator is now recommending the 1.3 ratio on ALL applications. They say that with the newer fuels, more efficient ingitions, carbs, and heads, etc, the engines are harder to keep cool. I have found the same thing in my tests on the TT340, which is very tough to keep cool at idle. Runnning the fan faster (with a clutch so it doesn't hurt power) and getting it in optimum position in the shroud are the biggest improvements you can make to idle cooling.