20hp johnson not getting enough air

[Thompsoncustom]

I have a 1990 johnson 20hp that I've upgraded with a 35hp carb and intake. Well yesterday I took it out and at WOT on my 14ft alumicraft it would do 28mph, I was out there working on a leaking gas line and when I got it fixed I ran it with the hood off to see if it was still sucking air and it did 31mph but as soon as I put the hood but on the motor it dropped down to 26mph. So it appears to me the motor is getting to much fuel and now by fixing the leak it no longer sucks air at WOT but has even more fuel.

So if there something I can/need to adjust to suck more air, I'm sure I could restrict the fuel with a 30hp carb jet but I'd rather force more air if I can. With out tearing it completely apart and opening the intake and exhaust port is there anything else I can do?

 

[Chinewalker]

Sounds more like you have a small exhaust leak. Unless you have a blockage in your air openings in the belly pan or cowling, there not really any way it could be too little air. I believe by 1990 the cowling was the air source, through the rear appliqu? cover. Possible some mud wasps made a nest in there.

 

[Thompsoncustom]

not really any way it could be too little air

The only thing I could think of was maybe to little vacuum causing not enough air to be pulled in and the fuel remains the same. Not sure where the belly pan or cowling are i've checked to see if any of the reeds are sticking shut but them seem fine. I assumed if its sucking air somewhere on the intake said it would be more lean and rich.

I'm an outboard novice so you'll have to bear with me here. Are you saying you think it's plugged/leaking in or around the exhaust cover or the exhaust leg that runs down?

 

[F_R]

Lots of people have suspected their engine isn't getting enough air. It's always something else. Since you have re-engineered your system, I'm not going to even attempt to diagnose it. But exhaust leaking under the hood certainly will cause the effect.

 

[Chinewalker]

An exhaust leak would be one of two likely places. Either along the gasket at the base of the powerhead, between the powerhead and motor leg, or i could be on the port side of the powerhead at the exhaust cover.

 

[Thompsoncustom]

Started it up on the muffs tonight looking for exhaust leaks. There was a exhaust cover bolt that was lose that could have been causing issues but couldn't tell at idle or just above. I tightened the bolts all down on the cover and than sprayed it everywhere under the hoods that might leak with soapy water but didn't see anything. I'll have to get it out on the water and see if it was just the lose bolt causing my headache.