since doing an oil change the other day, one of my 360's acts funny above 3000rpm. I can get both motors to 3400ish RPM but the starboard engine backs itself down to 3000 on its own. Sometimes it will jump back up but not often. I back both engines down to 2800 to cruise and it's smooth. Pressure hovers around 40psi. temp is 160ish and I used 25w40 weight oil. It could just be a coincidence that this is happening after the oil change...or not. Fuel is less than a month old. and again, it's only happening on one engine. issue presents low-level vibrations and hum when it happens...just as when engines are out of sync before I trim them both even. any thoughts?

My port motor would turn past 2500, stalling at idle. Replaced control module solved problem.

my chrysler manual (original from the late 1970's) says straight 30weight or multi viscosity such as 10w-30. But, I dont think oil weight is the issue. Other motor runs real strong on same oil weight. There is a hung valve is causing this. Weak or broken valve spring perhaps. Will inspect further and prob replace a head.

Dan, Chrysler engine manual that came with my boat, states use straight weight 30.

I was experiencing the same proble with one of my 360's vibration, wouldnt hold hight rpms. On a suggestion I replaced the spark plugs two were loose could back them out by hand. Since then motore are running better, starting easier, smoother less vibration, etc. just a thought...