Mariner 30hp 10mph loss of top speed

rkuz

Cadet
Joined
Jun 10, 2012
Messages
15
Hello, my outboard is experiencing a 10 or 12 mph loss of top speed. Its a 1989 Mariner 30hp on a 16' starcraft seafarer. I got this used motor early this year, and by my self in the boat I took it up to 30mph once (a little too fast) and would normally run it around 25 mph on the gps. The motor still starts, and idles fine, but is hitting a max top speed of 18mph. Acceleration may be diminished a little, but pretty close to what it was. It hits 17mph with 1 or two people in the boat. Nothing has changed in the boat from spring when it was hitting near 30mph. Any suggestions on where the issue lies?

I have tried:
opened fuel tank up and checked pickup tube in tank. No blockage there.
Scratched markings on prop to check for prop loosing grip (spun prop) no movement there.
Now running 90 octane ethanol free gas (was running regular pump gas with stabile previous.)
new plugs (per manual NGK B7HS 0.024 gap.)
Checked spark, both jump from plug to motor at about a 1/4" gap maybe a little better.
 

rkuz

Cadet
Joined
Jun 10, 2012
Messages
15
No tach so not 100 percent certain, but I would say lower RPM's.
 
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