[Bradleydel]

I have an 03 Merc 9.9 that will turn 4900 RPMs when the throttle is wide open. But, when the throttle is slightly backed off it will turn up to 5100-5200 which makes a significant difference in speed. I have played with the mixture screw with no luck thinking it may be running a little rich/lean. The motor is on a new skiff so I can't say it "just started doing this" and the boat it was previously on I didn't have a tach. Anyone got any idea what the issue may be, or if it is an issue at all? I would definitely prefer WOT to be WOT. Thanks for any input.

Brad

 

[Dillusion]

Check the fuel line, fuel filter, fuel connectors, etc.

I had a fuel problem where my connector at the tank and at the outboard would become loose by a few degrees causing air to get into the mixture/not enough gas for the engine and it would stall out and 'jump' RPM's at different speeds.

Also check the fuel line for any hairline cracks or pin holes.

 

[Bradleydel]

Everything from the fuel line to the motor is brand new including a new filter. But, I have had problems in the past with the disconnect fitting at the motor so I will check that out. It never will jump, the motor runs absolutely great, except this one thing.

 

[DuckNut]

You state that you get more RPM's if you back off WOT just a little and then you get full rpm's. This is telling me that at WOT your motor is being pushed beyond the efficiency range.

What is happening is that you are trying to cram too much fuel through the system and it is actually flooding ever so slightly but not enough to stall. Many things contribute to the problem - drag on the lower unit, prop size, amount of stuff in boat, overall weight of boat, hull shape, is the hull warped, etc.

My boats up north runs great at WOT when I, a passenger and dog is in it. But it will do the same thing as yours when I put 1,000 lbs of decoys in it.

Since you know the issue - live with it; or if it is bothersome then the easiest thing to do is lighten the load or change the prop. Personally I do not care and just live with the know limitations.

 

[Bradleydel]

You state that you get more RPM's if you back off WOT just a little and then you get full rpm's. This is telling me that at WOT your motor is being pushed beyond the efficiency range.

What is happening is that you are trying to cram too much fuel through the system and it is actually flooding ever so slightly but not enough to stall. Many things contribute to the problem - drag on the lower unit, prop size, amount of stuff in boat, overall weight of boat, hull shape, is the hull warped, etc.

My boats up north runs great at WOT when I, a passenger and dog is in it. But it will do the same thing as yours when I put 1,000 lbs of decoys in it.

Since you know the issue - live with it; or if it is bothersome then the easiest thing to do is lighten the load or change the prop. Personally I do not care and just live with the know limitations.

I could be wrong but I don't feel like the load is too great. The problem I'm describing is with a total load of around 600lbs (boat, motor, me, gear, etc.). The motor is mounted as high as I can get it without it blowing out which has the antiventilation plate approximately 3/4" above the bottom of the boat. This has been my first four stroke small motor and maybe it doesn't have the "umph" of a two stroke that I'm used to. I could live with it, but if it could be corrected properly then that would be even better.