Boat Buzz

Have KL with 60 hours on rebuild. Have Ignitor Electronic Ignition, Flamethrower coil and Accell ignition wires. Motor runs great at all speeds. Engine temp 115 to 120 on digital guage.
Problem is after about 2 hours of cruising, the idle is very rough and won' t run under 1000 rpm no load. Carb has been rebuilt with no change.
My intake manifold has coolant water piped from front of exhaust manifold to front of carb mount and out the rear side then to the exhaust elbow. All other KL's I've seen have coolant piped directly from front of exhaust manifold to outlet elbow.
I'm thinking of bypassing the line to carb mount and run directly to elbow. Thinking after motor runs for extended period, some coolant water is leaking into intake manifold, causing rough idle.
Anyone experienced similar problem?

Manafold leaks can effect all sorts of stuff. Drives me nuts because the way to tell was water in your oil, but not the case. I just replaced the exhaust manifold on my W and it changed everything, smooth running, less smoke, no rough idle. I am not a mechanic, nor an engine expert, or even an engine idiot? In fact I am an artist by trade. Have yoou considered that the color on the KL is wrong? HA. Sorry. The KL is an amazing engine. The after 2 hrs of running thing kinda has me stumped though.

The plumbing looks right. There is a chance the heat opens up a crack and water gets in. Pull the copper tubes from the manifold end cap and the elbow. Plug the holes and run it with no water in the intake and see what happens.