1968 Merc 110 9.8hp - Sputters or stuck at mid rpm with passengers/load

Picked up an old '68 Merc 110 9.8hp that runs OK as long as the 10' zodiac is empty. It does sputter a bit a mid rpm on full throttle but eventually revs up and planes the boat after a few seconds. The sputtering is just before the boat planes.

If I have another person aboard or there is load in the front, the engine will sputter at mid RPM and won't plane the boat.

Any ideas of what could be causing the motor to sputter under load and fail to get up on the RPMs?

I have a video of the sound here (click sound icon in bottom right if it doesn't play):

https://imgur.com/acFhnoo

Thanks for any suggestions!

I think it is only running on one cylinder.-----Do a compression test.-------Then check for spark on both leads that needs to jump a gap of at least 1/4".

racerone said:

I think it is only running on one cylinder.-----Do a compression test.-------Then check for spark on both leads that needs to jump a gap of at least 1/4".

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Compression is 120 in both cylinders, cold. Quite amazed by that actually given how old the engine is.

Spark from both leads, but haven't got a inline spark tester on hand, will get one and get back to you. Here are the plugs I pulled (put in clean 2 trips ago):

Top cylinder: (had drop of gas between elements)
Bottom cylinder:

Note that these are new plugs I put in before the second trip, it already had original plugs in there and had the same issue with those. Will get a picture of the original plugs.

Doesn't that motor use Perma-gap plugs Champion L76V? What does that decal on the coil housing in pic. 1 say?

Quicksilver said:

Doesn't that motor use Perma-gap plugs Champion L76V? What does that decal on the coil housing in pic. 1 say?

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Point taken. The marina gave me the gaped plugs when they looked at the engine and looked up the "correct" plugs. Regardless, the problem occurred even before I changed the plugs. No difference with the new, gaped plugs.

Previous owner worked on the point ignition. Here are the plugs that were in the engine before:

That's not a 1968 motor--it had a magneto. Can't see switch box in the photos, which would make it a Phase Maker. If so, the points need cleaning and/ or the plastic blocks on the points need changing--they crack over time and allow the 300 or so volts to go to ground.

Jeff

This video seems to be the issue mine is having:

https://www.youtube.com/watch?v=wWmFDt1tvFk

It does that vibration and rough rpm when trying to get over a certain rpm range.

Could it be the same issue - bad stator or stator trigger (if these old 110's even that those?)

Post the serial Nr. to identify the year. Could also be bad plug wires, fuel pump, dirt in carb filter or tank, bad fuel line from tank to engine. You can check the spark with a timing lite.