Mercury Outboard Slows Down at High Speed

[Dan Salvin]

I have a 1971, 40 HP Mercury on a 16 foot Starcraft. The engine has run fine all summer (I winterize it every year and keep it in a barn over the winter) until yesterday. The engine started up fine and ran great at low and moderate speed but when I opened it up to high speed, it ran normal for 15 to 30 seconds and then would slow down by itself like I had pulled back on the throttle. It did this every time I tried to run it at high speed (it only stalled out once but started right back up). I was able to drive back to the dock at half speed with no problems. Any help would be greatly appreciated! Thanks - Dan

 

[Dave1027]

Re: Mercury Outboard Slows Down at High Speed

First thing to verify is fuel delivery. When it does it, what is the condition of the primer bulb? Is it collasped? If so then try taking off the gas cap. The fuel tank needs to be vented.

If not then try pumping the primer bulb when it does it. Does it pick up? If so then suspect the fuel pump.

 

[Dan Salvin]

Re: Mercury Outboard Slows Down at High Speed

I was finally able to get back to the boat. The primer bulb did not collapse when the engine slowed down. I tried pumping the bulb but the engine did not seem to pick up. I seems to run fine otherwise. Starts and idles normally but after 30 seconds or so when at full throttle, it loses rpms and slows down on its own. Is it worth trying to replace the fuel pump? It looks like a simple job as it is a separate unit from the carb on this model. Is it even possible to find fuel pumps for this old of an engine (1971)? Thanks for your help.

 

[Dave1027]

Re: Mercury Outboard Slows Down at High Speed

No need to the replace the fuel pump. Only need to replace the diaphrams and gaskets which comes in a kit usually for about $10.

 

[Dan Salvin]

Re: Mercury Outboard Slows Down at High Speed

Thanks again. I found the kit and replaced the fuel pump gaskets and diaphram. However, the boat is still doing the same thing - will rev up briefly and then cuts back. I noticed now however that I can keep it running at high rpm if I pump the primer bulb - although the bulb does not collapse when the engine slows it does get a little softer then when I'm pumping it. Any thoughts? Thanks!

 

[Dan Salvin]

Re: Mercury Outboard Slows Down at High Speed

So I took the boat into a nearby marina today but they will not work on it b/c it's too old. No other marina's in the area will work on it either. The mechanic I spoke with did think it was something ignition related under the flywheel. Does anyone have any thoughts about what that could be. My only choice at this point is to try to fix it myself or junk it which I'd rather not do. Again, the engine starts, idles and runs at medium rpms fine but loses power after a few seconds at high rpms. I replaced the gaskets and daiphram in the fuel pump but that didn't help. Thanks again.

 

[Dave1027]

Re: Mercury Outboard Slows Down at High Speed

The fact that you can keep it running at high speed if you pump the primer bulb proves it's a fuel delivery problem and not ignition. Doesn't matter what the marina's guess is. Something is restricting fuel flow. Since you refurbed the fuel pump and can safely rule that out then next would be the carbs or fuel lines/tank/primer bulb.

 

[ShowMeGuy]

Re: Mercury Outboard Slows Down at High Speed

I'd be looking at the fuel strainer/screens in the tank and in the bulb and then moving up to the strainer/screens at the carbs....