Johnson 25 HP Outboard

[jford07]

Just picked up a 25 HP Johnson (90's model??) from my dad and it doesn't seem to be running right. Motor starts fine and runs normal at idle. For the first 5 minutes motor will run normal through all power ranges. After 5 minutes, motor slows and becomes sluggish. Won't run over 1/4 throttle and when attempting higher RPM makes high reving noise with no increase in thrust output. When released back to idle lots of white smoke. Spark plugs have been changed and it seems that one is becoming fowled faster than the other.
I don't have much experience with boat motors and was trying to rule out/in some of the major problems that can occur- head gaskets, cracked block, water pump, thermostat. Any help would be really appreciated. The motor has been sitting for a couple years, but prior to that it was running normally.

Thanks again

 

[bktheking]

Re: Johnson 25 HP Outboard

Start with a compression test , i'm thinking possibly a blown head gasket.
Any signs of discolouration of paint on the head?

 

[1946Zephyr]

Re: Johnson 25 HP Outboard

If you have a plug fouling out, I would check the fuel pump and make sure there isn't a ruptured diaphram. If you remove the fuel pump, squeeze the primer buld and see if gas comes out of where the fuel pump mounts on the motor.

 

[tashasdaddy]

Re: Johnson 25 HP Outboard

white smoke is water, head gasket possibly. water getting into cylinder, piston cannot compress water.

 

[jasper60103]

Re: Johnson 25 HP Outboard

and to check year, look for a model number on the transom clamp.