Help! I need more power from my Mariner 28HP

[mches106]

Just took out this 1978 Mariner 28HP for the first time and it barley got a 14ft Aluminum boat moving (maybe 15MPH) Where do you suggest I look first.
This is what I have done so far.
1) new plugs
2) new gas tank with fresh gas
3) drained and cleaned fuel filter
4) New impeller and thermostat (they where bad when I bought it)
It has a good idle and the prop is fine.
Thanks for any help.

Mark

 

[WillyBWright]

Re: Help! I need more power from my Mariner 28HP

Make sure both cylinders are firing. If one sparkplug is burning clean, look at that one first. Considering that overheat was a possibility in the past, look for water droplets on a sparkplug indicating a leaking head gasket.

 

[Silvertip]

Re: Help! I need more power from my Mariner 28HP

A 28 HP engine should push a 14 footer on plane with absolutely no trouble unless the boat is full of water logged foam or the engine has one dead cylinder. Compression and spark tests are in order.