lost2a6

I have a 1998 28ft Carolina Classic with stock vortec 7.4 volvo penta engines. I'm having water reversion problems in both engines, these engines are notorious for doing this. It's a well known issue at https://www.carolinaclassicboatowners.com/forums/. The older non vortec engines doesn't seem to have a problem, so I'm almost certain that it's a result of the different cam designs among the two. With the manifolds off, I can put my hand over the exhaust ports while I spin the engine over and it will momentary pull a strong vacuum. This would be a result of the exhaust valve still being open when the piston starts on it's travel back down. I have considered advancing the cam 4 to 6 degrees or having a custom cam made. This is a fishing boat so I don't mind loosing a few horse power to gain reliability if need be.