Engine "sag" at part throttle

[pops]

My 1995 Mariner 115 HP (4 cylinder) runs fine at low speed and at full speed. When I am crusing at about 2/3 throttle (around 4000 RPM), the engine will change tone and the RPMs will drop slightly for 5-10 seconds, then it picks right back up to normal operation. It will do this intermittantly until I advance the throttle slightly.

It sounds like the engine might be dropping a cylinder, but only in the part throttle range I have described. I believe this engine has the system that allows 2 or 4 cylinder motor operation depending on throttle setting, but I'm not sure exactly how this works, and the owner's manual doesn't mention this feature.

Anybody have any suggestions on where to start looking for this problem. Sparkplugs are fresh, carbs were rebuilt a couple of year ago, boat has fuel filter (changed), Stabil in the tank, motor starts easily, and full throttle operation is fine.

 

[pops]

Re: Engine "sag" at part throttle

Got absoutely no response on this post! With all the expertise on this board, someone must have opinion about possible causes (and fixes) for this problem.

 

[pops]

Re: Engine "sag" at part throttle

Thought I would move this to the top of the board one more time. Comeon folks, somebody out there must have expertise on Mariner motors. I am sure this is not the only 115 Mariner in the world with this problem.

 

[CharlieB]

Re: Engine "sag" at part throttle

Sounds almost like the boat is 'porpoising'. The bow is dropping slightly, creating more drag on the hull, the motor slows slightly from the increased load, struggles until it lifts again, then the condition repeats.

Have you changed the load in the boat any different than you used to operate the boat? Moved any weight from on spot farther forward or back? Replaced or had the prop repaired?

 

[Faztbullet]

Re: Engine "sag" at part throttle

How many rpms does it drop?

 

[pops]

Re: Engine "sag" at part throttle

Fastbullet, RPM drop is only 200-300 at the most. There is almost no loss in speed, just a change in engine sound like a cylinder might be cutting out.

CharlieB, porposing is not an issue. It is a Ranger bass boat, and when this happens, it is trimmed up and running 40-42 MPH on pad same as always.

 

[Faztbullet]

Re: Engine "sag" at part throttle

Replace them fuel pump diaphragm 1st as it may be a lean fuel supply to a carb . If this doesnt fix it rebuild carbs as you could have a swollen inlet needle lean out a carb or a sticky float. Also if you have the gray fuel line with red writing replace it as it will come apart internally causing weird fuel issues....

 

[pops]

Re: Engine "sag" at part throttle

FaztB, thanks for the info. It gives me an area to start looking into! I do not have the "red writing" hose.