declining rpms at WOT

[Searun1]

I have a 2005 90 HP Yamaha two stroke. I purchased the engine this past summer and have put 43 hours on the engine since August. It is pretty new and I have been trying to keep everything perfect and follow up with any required maintenance.<br /><br />My problem has been with getting the engine to WOT. When I picked up the boat they had it propped with a 13x19 aluminum prop. I was unable to get it past 5000 rpm with four passengers at WOT. The shop exchanged the propeller with a new 13.25x17 propeller. With this prop I was able to get to 5400 and occasionally 5500 rpm which is where the engine is supposed to be.<br /><br />Lately the engine is not getting past 5100 rpm with just me and another guy in the boat. I realize it is still in the 5000 to 5500 range but what has changed?<br /><br />Is this something to be concerned with? I feel like the 13.25 x 17 is the right prop as it achieved 5400/5500 previously.<br /><br />SeaRun1

 

[jim dozier]

Re: declining rpms at WOT

Check for some simple things first.<br />1. Is the boat carrying excess water in the bilge or any other weight it didn't use to have?<br />2. Is the prop damaged, does it have nicks or bent blades?<br />3. Do you have marine growths (barnacles, algae, molluscs, etc.) on the boat bottom?<br />4. Is the boat still trimmed properly (the angle of the motor to the boat)?<br /><br />Now might be a good time to do a compression check. All cylinder pressures should be within 10% of each other.

 

[Searun1]

Re: declining rpms at WOT

jim,<br /><br />Thanks for your help. No extra weight is being carried. I drain the boat each time and not much water comes out as usual (unless its raining...) Prop is in excellent shape with no dings. No marine growth whatsoever as the boat is not moored and I clean it thoroughly after each trip. The engine is mounted properly and I tried the range of trim levels to get her rpms up. <br /><br />This boat was doing perfectly and hitting 5400 sometimes 5500 up to the last few trips. This is a new issue from the last 2-3 engine hours.<br /><br />Anyone else got any suggestions?<br /><br />SeaRun1

 

[jim dozier]

Re: declining rpms at WOT

Check the compression. Decarbon the engine. Check compression again. Install a new set of the proper spark plugs.

 

[shep70057]

Re: declining rpms at WOT

Check your plugs....

 

[dmonwai]

Re: declining rpms at WOT

Have you tried making 2 runs, the 2nd run turning around 180 degrees and do the run again?<br /><br />In my boat I've noticed I can pickup or lose a couple hundred RPM at WOT depending on which way the current is going. We can have some really strong currents in Puget Sound.

 

[jim dozier]

Re: declining rpms at WOT

Current should not affect engine rpms just speed over the bottom. Headwind will reduce engine rpm and speed over the ground. But his point is well taken.

 

[Searun1]

Re: declining rpms at WOT

I will try the new plugs this weekend. Damon I dont think it was current or headwind. I first noticed it with out a breath of wind coming back from Gibson Point. It was pretty windy at Manchester Saturday though but I went into and away from the wind and had the same issue.<br /><br />SeaRun1

 

[Searun1]

Re: declining rpms at WOT

Changed the plugs and ran the boat out on the sound for a fishing trip. Got up to 5200 RPM's which was an increase but not what I was looking for.<br /><br />Do I need to be concerned about this? Do I really need to decarb at 43 hours? What else can i do before I resort to taking the boat into the Yamaha dealer.<br /><br />SeaRun1

 

[shep70057]

Re: declining rpms at WOT

I would bring it to the dealer. It is still under warranty.

 

[BillP]

Re: declining rpms at WOT

Best advice has already been given. Don't hesitate and take it to the dealer.<br /><br />It could be the tach. My 50 mariner (mech/yamaha)does the same thing and it is the tach. I used a portable handheld tach on the flywheel to check it. Must be a short or something with the tach.<br /><br />Hexx no, you normally don't need to decarb at 43 hrs. MAYBE at 430 hrs.

 

[ob]

Re: declining rpms at WOT

Check the link and synch of the carburetor/timing linkage.Flaps may not be opening fully horiontal.Also make sure you're throttle lever isn't creeping back from wot position on you, as in a loose tensioner.