Diagnosing Low RPM Cause.

[garymace]

Hello,

I have a 2001 Yamaha 90hp 2-stroke on a 22 ft pontoon. I bought this used back in 2010.

When I first got it, it would run at WOT about 4400 RPM's and max GPS speed was 22 mph with clean pontoons and 19 mph when they were cruddy.

I read that 4500 to 5000 RPM is what I should expect from this engine at WOT.

Now, the max I can get it to rev in gear is 3400 RPMS's and max GPS speed is about 16 mph.

Also, it now stutters a few seconds when I apply quick throttle. It used to be immediately responsive.

I have tried a few other prop pitches, but now am using the size suggested in manual and the same size it had when I bought it.

I added a water filter and took the carbs apart and cleaned them thinking maybe it was bad gas that clogged something up. This did not help.

So, I am looking for any ideas on what could cause this loss in RPM and power and also a how-to on the steps to confirm or eliminate that idea as the cause.

Could a failing lower unit cause this? Engine starts quickly and idles well and revs to 5000+ RPM out of gear.

Thanks!

[dngharvey]

It could be as simple as a dirty bottom, check for growth and feel it. If it feels slick, it can hold you back.

[rodbolt17]

there is no "factory" prop.
we typically find pontoons and deck boats way over propped at delivery.
find a dealership with test wheel YB1620.
see if it will spin it about 5000 RPM.
if it will fix the hull, if it wont fix the motor.
its the only way now.
that motor should have spun at least 5300 RPM with 4 people and a full fuel load.
now you MUST separate the engine from the hull to find the correct prop for your application.

you cannot prop it with you and a half a tank then load 9 people and 6 coolers and a TV.