revs great when in neutral but bucks and doesn't rev past 1800 when in gear

[RoundHead]

K, we've just inherited this cute little Banshee boat with a Mariner 90HP inline 6 cylinder engine.
It apparently was running excellent or amazing last year
I changed the spark plugs only so far... don't know how deep I want to get in to this motor.
When I have it on the muffs in my driveway, it runs super great
I can put it in gear and it revs up great.

When I have it in the water, still strapped to the trailer, it runs super good in neutral. It revs up super great, no hesitation at all. Picks right up instantly to 5000 rpm
When I put it in gear, it hits 1800 rpm and it feels like your in a paint shaker at your local hardware store
Every once in a while it will give me about 5 seconds of high clean revs, but so far, in a tank of gas sitting at the boat launch, I have felt only about 15 seconds of good clean power.

I don't know what a cdi it is or a stator or a trigger is.

I know the engine is getting fuel and I know that the butterflies in the three carbs are opening.
I've tried fussing with the air fuel mixture screw on each of the carbs (there is only one screw on each carb) but I don't get good crisp clean acceleration from the motor no matter what.

I hope this is something simple as I really don't want to get into the motor too much

Thanks

 

[Chris1956]

I will presume that motor has six coils and no distributor? If so, it will have separate stator windings for low speed and high speed operation. You need to test those. In addition, the carbs could be dirty. The idle mixture adjustment needs to be set correctly, on clean and tuned carbs.

Not sure how you can "stay out of the motor".

 

[racerone]

It only " sounds good to you " on the trailer in nuetral or in gear.---In gear on the trailer on the hose is still the same as nuetral.-----Start with a compression test and check for spark on each lead that must jump a gap of 7/16" or more.

 

[mr 88]

The year of the motor would help.Having a load on the engine vs no load will give you different results and I suggest you do not red line rev it as your now doing. The transistion circuit is poor on the in line 6 which has a distributor. Most of the time you set your idle screws out 1 1/2 turns as a base point. After doing that in order to get it past the 1800 rpms you will have to open each idle screw a blade width or so then keep adjusting them out a bit until you get it to accelerate crisply.If that does not work you may have a electrical issue but I would start with the carb.

 

[racerone]

A MARINER 6 cylinder inline likely is an ADI motor and no belt driven distributor.