Motor slows down then speeds up

[George159]

I have a 1997 Mercury Mariner 75 hp outboard motor mounted on a pontoon boat. At moderate to high speeds the motor will slow down for a few seconds and then speed back up. It does this repeatedly at random times. What causes this and how do I repair it?

 

[Nismospeed]

Re: Motor slows down then speeds up

I had an '85 Mercury 50hp that did something very similar. I was told to make sure the carbs were clean and to make sure the water was properly circulating from the pump/impellor. The carb(s) are generally more likely to create this, as gummed up or dirty carbs may cause an irregularity in fuel flow. If youre control arm is set at WOT with a dirty carb, the lever might be up but the carb didnt open all the way, causing the fuel system to be somewhat starved with an improper fuel intake. Does that make sense? Think of it like this... Run a household fan on its highest setting, but line the back of the blade guard with plastic grocery bags. You set the dial, the motor is responding, but there is a restriction coming from somewhere. Thats how I understood it.

Another possibility would be the fuel line itself. Slim chance, less likely to be the cause, but what about a slight crack or pinhole in the airline? Check them out to make sure the lines are solid and the connections are good. If the line is sucking air in, you might have less gas going in than what you think. Hope this helps, and if not....well, I hope someone else can give you some other options. Good luck!

 

[westexasrepublic]

Re: Motor slows down then speeds up

completely disagree, anytime I found these outboard motors to run like you describe it is electrical. drop in voltage (bad stator) or bad sync due to (bad rectifier/switchbox) I high doubt it is a fuel issue when it runs fine for a minute and back again.

 

[reynold460]

Re: Motor slows down then speeds up

also look for water entering the motor run the motor pull the plugs out and look for water droplets on the plugs i have a 84 115 hp in line mariner that did this and after replacing every electrical part found water to be getting into the cylinders

 

[CharlieB]

Re: Motor slows down then speeds up

A poor O-ring seal in the fuel fitting can allow air to suck into the system, especially when the hose is flexed in turns. This WILL reduce fuel flow, and if allowed to continue can lean the upper carb(s) and burn a piston.

A very simple test is to TEMPORARILY replace a section of hose between the fuel pump and carbs with a clear hose, run the motor with the cover removed. When the power loss occurs have someone watch the hose to see if any air bubbles are passing thru it.

Remember to remove the clear hose when finished as it is NOT rated or recommended for fuel use.

Another possible cause is water in the fuel tank, sloshing about once underway a drop gets picked up and sent on to a carb, that cyl staggeres a few moments until the water passes thru then all is well again. This one is harder to check as you need to pull a fuel sample from the bottom corner of the fuel tank to see if there is water present.

Install a water separating fuel filter. It is always good to have a filter.