1975 Johnson 115 Won't Top End as Fast

[74arrowglass]

Hello, I bought a 1975 Johnson 115 to put my 1974 Arrowglass 15' tri-hull. My first outing was on July 4th weekend when the boat was in the water for 3 days. At first, the boat would top out at about 44 mph, But it now will only top out at about 33. I do not know what could cause this because the motor is running perfect, except for this. I was thinking maybe a spark plug was bad? I already cleaned the fuel filter. Any ideas? Thanks alot!!!

 

[ezeke]

Re: 1975 Johnson 115 Won't Top End as Fast

Compression?

Spark test at 7/16 gap?

Water separating fuel filter?

Gas fresh, mixed with good TC-W3 oil at 50:1?

Gear oil new?

Champion QL77JC4 plugs at .030?

 

[rolander]

Re: 1975 Johnson 115 Won't Top End as Fast

Water in the boat??!!!??

If the boat has been sitting in the water for all that time have you checked for water in the bottom of the boat? If you have a bunch of water in there from slow leak somehwere that will big time affect the way your boat performs.

 

[74arrowglass]

Re: 1975 Johnson 115 Won't Top End as Fast

The compression was checked a few months ago and was good. And I'm 99% sure the boat isn't holding water. It's actually on the trailer now and a little water came out but not much. So i guess I will start with plugs. Thanks for the replies!

 

[rolander]

Re: 1975 Johnson 115 Won't Top End as Fast

Well my boat was bogging and not getting the RPM it was supposed to be. turned out to be a fuel problem.

 

[ezeke]

Re: 1975 Johnson 115 Won't Top End as Fast

If you have lost a third of your power the first thing to check is compression. If you have a piston problem you can destroy the block if you keep running the engine.

Whatever was at 44 MPH is not there now.