New Owner with questions

[Destroyer]

Unless my mind is playing tricks on me I seem to recall somewhere in my years of boating that the waterpump impellers on outboards and I/O's do very little work while on plane.

I think I recall reading somewhere that the force of the boat moving thru the water causes water to be injected through the intake holes in the lower unit and the force is so great that it actually partially bends the impeller out of the way and goes up into the motor by itself. In fact, if you look carefully at the intake holes you will see that they are chamfered in such a way so as to channel the water into the intake at speed. If the intake plate was put on backwards during a cleaning (a common mistake) then the water will not be forced into the motor during high speed running. It will work fine at low speed when the impeller is doing all the work, but not at high speed. In fact, that's the reason the makers of outboards don't use a solid impeller like you would see in a sump pump. The soft rubber can flex and get out of the way at high pressure, thereby making more efficient cooling.

I don't know if this is your problem, but it's a quick easy check. Just look at the intake plate(s) on your lower unit and make sure that the holes are facing forward so that water is scooped up as you travel through it. If they are facing backwards then you just found your high speed cooling problem.

[DBG]

Thanks for all the tips. I think it must have been the fresh water cooling system. This morning I drained it, flushed it, and replaced the coolant. I put her on the hose, warmed it up, and let it run at high RPMs for about five minutes. Then revved it a coupla times. The needle stayed in place right around 160. On to the next issue...

When coming down to idle after running her at high RPMs the engine dies. Any ideas? Thanks!

[macojoe]

Try coming down slow to burn off the gas in the carbs, when just coming down fast the un-used gas will flood and stall the motor.
Also might need some carb work but that should help, it did for me back in the day on my 75

[DBG]

What do y'all think about installing a 4" heat exchanger? Is it necessary?

[cfelton]

I would definately change to a 4" heat exchanger if possible. After I rebuilt my 170 it ran a little warm, about 170-180 while up on plane. I bored it .040 over, so the cylinder walls are a little thinner and it has a little more compression which will cause it ta run a little warmer. I replaced the entire water pump and it cooled down a little. After I got it broke in I changed the oil with 40wt. motor oil and added a quart of LUCUS OIL STABILIZER and my oil pressure rose 20 psi and it really cooled on down. Now it rarely gets above 165 degrees. The engine temp depends on the raw water temperature also! Try the Lucus oil stabilizer, its really great stuff!