1974 Mercury 850 Thunderbolt

[Thistledew]

Hello! I have a Mercury 850 that just started giving me some problems. I recently put on a new fuel primer bulb and a complete wiring harness. The motor has been running good this year so far until I decided to pull my kids behind in a tube. They like to go slow(young kids) so the boat never gets on plane and plows a lot of water. When doing this the boat will bog down and lose all power and stall like its starving for fuel after a few minutes. If I pump the primer bulb it will start back up and be good for a bit longer. The weird part is that if I disconnect the tow tube it will go wide open all around the lake no problem. Does anybody have an idea what might be going on? I am thinking maybe it's sucking air around the new primer bulb. My other thought and worry though is that it may be overheating. It pees water but maybe not all the blades are intact. Would overheat cause this issue too? Im mechanical but new to outboard motors! Just looking for some ideas to go along with mine. Thanks for your input!!

 

[racerone]

If you do not know the condition / age of the impeller then hurry and install a new one !-------Note----I do not believe that motor has any kind of warning system / overheat horn on it.

 

[Thistledew]

It's getting a new one on Friday before I go back to the lake for the 4th of July!! Already ordered and on its way!!

 

[racerone]

Might as well order fuel pump repair kit/s for this motor.

 

[Thistledew]

That was another thought but then it ran like a champ with no real load on it. 5000rpm purring like a kitten. I'll get a pump repair kit as well. Thanks!

 

[clueless75]

I had the exact same problem with my 1975 merc 850 while towing two adults (about 400lbs) and when going slow the motor bogs down but I just simply accelerated (which it did very slowly) until I got on plane, then once I was on plane I slowed down to around 2500-3000rpm and it ran flawlessly. Maybe giver a shot? The kids will only be going fast until you're on plane at which time you can slow down to their speed (or at least the slowest speed you can go while still on plane). It worked for me and didn't cost anything. Also maybe you can try adjusting the tilt of the motor when tubing so you don't plow so much water? Or maybe tow one kid at a time and see how the motor acts when under less load? Just a thought, worth a shot!

 

[Thistledew]

Thanks! I will be back on the water this weekend and I'll certainly be trying it again!