Runs better with top cover off

[Mattg8414]

I got a 1989 8 hp Johnson that is not running right. At WOT it feels like I am pulling something (not planning out) and will not get to high rps unless I take the top cover off the engine (planes out). I have cleaned and rebuilt the carb, compression test stays around 120-115 after 5 minutes of sitting, plenty of spark, plugs gapped appropriately, and the air box is clean of any debris. I have adjusted the carb so that it runs great but once I put the cover on it, WOT is sluggish (5 mph). I am running it on a 12ft v-hull with nothing in it (bare hull) except for me (170 lbs) a 6 gal tank and a milk crate to sit on.
I'm no expert on boat engines but this seems like it is starving for air, right? so I checked the vent holes in the bottom of the engine case and they are all open. No exhaust leak either.
I am officially out of ideas.
any help out there?

 

[F_R]

Classic exhaust leak symptoms, but you say not. Can't argue with that.

 

[Mattg8414]

Where would the exhaust leak come from? Could it be getting sucked back in through the bottom of the cover?

 

[racerone]

Replace sparkplug wires ?-----Perhaps one is shorting to the cowling latch.

 

[F_R]

I'm not real familiar with that model, but looking at the parts breakdown I would suspect a leaking gasket between the powerhead and midsection housing, or there are some seals in that area where the drive shaft goes through. That's where I would head, but could be bum advice too. But advice worth every penny you paid for it

 

[Mattg8414]

thanks for the advice, guess I better get to tearing it down

 

[JoshKeller84]

i had an issue like this one a previous motor, and it was solved by replacing the high speed jets in the carb - they had wallowed out and was flooding the motor - with the cowl off, more air was getting in and making a decent fuel/air mixture.