Merc motor won’t give me top speed

[MittenMouth]

I was out on the boat today and at the end of my fishing session I was coming across the lake and **** was quick to get on plane but the motor wouldn’t give me full speed and had some trouble getting the RPMs up. I could get off plane but it would only hit 30-35 and I was doing 55-60 earlier without a problem. The motor is a 150 2 stroke Merc vr6 on a 2005 Nitro. Any ideas as to what’s up with the engine?

[iabass8]

Warm the engine up on muffs and do a compression test on all 6 cylinders. Start there.

[PECo]

Check to see whether the throttle cable has slipped.

[DINK WHISPERER]

Mine has done that before, once it ended up being the spark plugs. The other time it was my water separator needing replaced. Is the fuel that's in it old? 

[MittenMouth]

I just had the compression tests done a few weeks ago and they were all 120-125. I just bought the boat (at that time) and everything was checked out. 

 

2 hours ago, PECo said:

Check to see whether the throttle cable has slipped.

How would I check this?

 

2 hours ago, DINK WHISPERER said:

Mine has done that before, once it ended up being the spark plugs. The other time it was my water separator needing replaced. Is the fuel that's in it old? 

Hmm good to know, I’ll check the plugs tomorrow for sure. 

[iabass8]

Fresh gas, new plugs gapped correctly, fuel filter, de-carb the engine....go over the basics. If you’re planing without issues your hub is likely fine. If your boat isnt going into slow mode or throwing any alarms, lack of accleration can be a sign of dropping a couple cylinders. Worst case sinero though. Go through the easy stuff first. Throttle linkage is behind the throttle arm and goes to the motor.

[slonezp]

Do you use fuel with ethanol and if so, have you had all your fuel lines replaced yet? Ethanol eats away at the fuel lines and the "ethanol resistant" lines were introduced around 2006ish 

[PECo]

10 hours ago, iabass8 said:

Throttle linkage is behind the throttle arm and goes to the motor.

If you have a Hot Foot, check that linkage, too.

[MickD]

I agree with going with the easy stuff first, and my first look would be fuel problems likely associated with ethanol damage to the hoses, clogged filter from same cause.  You've had the compression checked recently, so not likely that unless you've experienced something pretty drastic that  could cause scoring, like running with low oil injection.  Didn't run out of oil, did you?  I think that your idle quality will go to hell if cylinders are scored.

 

If the fuel filter is accessible, change it, then run on a borrowed set of hoses and tank of known good fuel.

 

As someone has stated already, bad fuel cannot be solved by simply putting new fuel in-ethanol damage can contaminate the fuel, leave residue that clogs filters, eats hoses and puts more crap in the system from them.  To check it out you need to run from fully clean fuel storage and delivery system and a clean filter.

[WRB]

Low end torque and power is OK based on hole shot being normal. It sounds like a fuel source problem. Change both the primer bulb and fuel filter after checking the throttle cable. If the engine starts normally the plugs and compression shouldn't be the problem based on normal lower end performance is good, engine doesn't doesn't rev past 4,000 rpm.

Tom

[Fishing Rhino]

As WRB said, there doesn't appear to be any problem at lower rpms, since the motor gets the boat up on plane normally.

 

It seems to me that the computer control module, or whatever it may be called in outboards, is limiting the rpms for some reason.  Low water pressure, engine overheating or on the verge of overheating, low oil level in the reservoir, etc.  Seems to me the best way to get to the source of the problem is to take the boat in and have it diagnosed by computer.

 

 

 

 

[Way2slow]

Mercury runs something like a dual ignition system.  The stator has a high speed and low speed winding and you could have a stator giving problems or one of the trigger modules.  Either way, sound like you new to you motor has just gotten a lot more expensive because both off these problems require a competent mechanic to check them.

If the motor gets on plane fine, there is not much you can do in your driveway, other than just take the shotgun approach and start throwing parts at it.  It's going to require a way to load the engine, like a dino or test wheel in a test tank, neither of which many people keep laying around their house.