Low RPM at WOT

[Groovius]

I recently purchased a used 1999 200 HP VMAX. Upon test run, was able to determine it had a blown lower unit. On the test, I managed 5200 RPM with a 3 blade 15.5 x 17 P Mirage prop, but it had a tendency to blow out both on the old motor and this newer one. I replaced the lower unit, but can now only get 4200 RPM at WOT. I have changed to a 14 x 15 P 4 blade PowerTech, with no change in RPM; it's still way too low. I suspect the gearing on the blown lower unit was 1.81, but do not know how to confirm it, and I also suspect that gearing on this replacement is 1.86, but do not know how to confirm that either. I have been assured the lower unit I received as a replacement is an exact match to the original. How would I go about confirming my suspicions, and am I even on the right trail here?

[99yam40]

Sounds like you need to get your hands on a test wheel to make sure your motor is running properly before playing with prop changes. Local Yamaha shop should have it, but may want to test it themselves.

[Groovius]

Thanks. I did go back to the original prop, with the same 4200 RPM results. Also, when at WOT, the motor acts as if it is hitting a rev limiter, which it did not do with the original blown lower unit. I'm about out of ideas and this is the last one I could come up with.

[jb123]

you say you got 5200 rpms out of a blown lower unit ???

[Groovius]

Yeah. Once I got it apart, it looked like a prop strike or two had sheared about 9 teeth off the gear in 3 separate places, and obviously the pinion was chewed pretty bad. It made, as you can imagine, a heck of a racket. This motor is on the back of a 27 foot pontoon boat, so it took a few minutes to determine where exactly the noise was coming from. The strong RPM is what originally confused me. Now, with the replacement lower unit, while quite, has presented me with new isues. I need to get this new issue figured out as I beleive I am placing undue load on this motor, and my fuel burn is higher than I beleive it should be. Thanks in advance for any and all suggestions!

[99yam40]

have you checked this site to see if they list a boat and motor like yours and see what prop they ran?

Performance Bulletin Home

I still think you need to make sure your motor is doing what it is suppose to with a test wheel and make sure the Tach is correct.

[jb123]

a 27 pontoon is quite a boat for the motor to push but i guess propped down low MAY be ok... whats the weight ?

i wasnt aware that you could squeeze a 15.5 wheel on that motor

is it carbed or efi ?

[Groovius]

I've had my mechanic check the Tach, and he claims it is true. I like the test wheel idea, and have spoken to my prop guy today. I plan on pulling the boat next week and getting it over to his shop. I really appreciate the input!

[Groovius]

Sorry if this is a duplicate...

OEM equipemnt was a Yamaha 225, with a 15 x 17 3 blade (current spare). The boat is a 1991 Misty Harbor "Performance Ski-Toon" with square pontoons, concave on the bottom and weighs in at roughly 2600 lbs This duece pushes the boat well, and I don't need to get 5500 RPM out of the motor all the time, but I do need to ensure it is running properly. That 15.5 was, as you aptly put it, a squeeze, with torque tab removed.

[Groovius]

Oh yeah...Carbs. All squeaky clean. First place I looked...