Engine starts sputtering/shaking when applying throttle

daring said:

I can apply throttle just fine at low speed, but when I push it down even further, the engine sounds and feels really rumbly/shaky and the boat will not go any faster. If I press the button in on the throttle so that outdrive is not engaged, the engine sounds fine even if I rev the engine really high.

All fluids are ok.

Other notable behaviors:

- Throttle lever seems to get stuck sometimes like the cabling has been stretched. For instance, sometimes hard to get it into neutral
- When I checked the oil, it is overfilled as it reads really high on the dip stick.
- When I turned the ignition off after the rumbly/shaky behavior, I heard a sputtering sound, almost like a muffled backfire.

Mercruiser (small v8), 150 hours

Click to expand...

Hard to get "into" Neutral from For/Rev in the Water, sounds like your Shift Interrupt isn't working ( stumbles the engines and reduces torque/pressure on the prop and gears to allow it to come out of gear ) - there's a sticky thread here somewhere..

Oil high on the dip stick can mean Water in the Oil .. but if' you've run it then you'd have a milky oil mess..
Drain the Oil and Change the filter, there are adapters to drain out of the pan using a hose through the boat/garboard drain plug..
or you can use the Oil Suckers ( manual pump works best when warm )

Rumbly/Shaky.. could be a couple things, water in fuel contributing to running rough, no power, miss firing, deiseling..
Need to run on FRESH Gas from a Clean Source.. and grab a sample of Fuel (2+ cups) in a Mason Jar, let sit 24 hours..
Check for water bubbles at the bottom..

Could also have a Spark/Ignition Timing issue.

ps.. when advancing the throttle, if the shift cable is sticky it could activate your "shift interrupt" giving you that issue .. but then you should be able to come out of gear easy.. interesting.

good luck..

Check the Sticky Posts for where to start checking..