99 200hp OX66's. Max RPM ISSUES. Need Advise

[Windham14]

Hey guys I own a 99 Contender 28 with twin Yamaha 200hp Ox66's. The boat has been a one owner boat up until I bought it a few months ago, always garaged and maintained with an open checkbook. Motors have 700hrs on them and I think I have a max rpm issue. The motors run absolutely flawless. Never have given me one second of a problem. Idle and cruise speeds are perfect. I found what I think is a max rpm issue when testing props. No matter what prop I put on this boat the rpm's never go over 4800. As stated before the motors run perfect in every situation and never miss a beat, I just don't feel like 4800rpm is enough.
Last weekend I replaced the following and got the exact same result last night....

- VST filters - they looked perfect
- Spark plugs - looked good
- All low pressure pumps
- Pulled 02 sensors and had them professionally cleaned
- New Racor fuel/ water filters

What is throwing me is the motors run to the exact same rpm and do it at the exact same time. How could both motors be experiencing the exact same issue? IS there an issue at all? The reason I think there is is because there should be at least a few hundred rpm difference between the 19 and 21 pitch props, there isn't.
I know the manual says 4500-5500 is the correct max rpm but 4800 just doesn't seem right to me.

The only other thing I have noticed is I can still pump the primer bulbs with the motors running- they are not rock solid. Is this an issue? Doesn't change a thing when I do pump them. Thanks for any help or advise. Just want to make sure the motors are at their best.

[DennisG01]

My primer bulb does the exact same thing as yours - always has.

Yeah, I would expect a pitch change to show a difference in RPM, too.

-- Are you trimming the engines up?
-- Are the tabs stuck down?
-- Can you swap another tach in there to see if that's the problem? Kinda weird, though - an odd issue, for sure.

Maybe some of the more technical-savy guys on here can offer an explanation - but personally, I'm not aware of any computer programming to limit RPM at that specific RPM.

[Windham14]

I have the motors trimmed as much as possible but they can't be trimmed much before the props blow out. It's the nature of this hull. Hoping Rodbolt will see this

[Windham14]

Update on motors...
The port and starboard motors both have excellent compression numbers and most are identical or within a few psi of each other. Anyone have any other direction to go with this?

[99yam40]

see about finding a Yamaha shop with the correct test wheels for your motors to see if they can spin up to the proper RPM while in a tank or tied to trailer or dock.
That should tell you if the motors are able to do what they should.

Rodbolt17's help is requested

[DennisG01]

Does the boat hull have any growth on it? If there's even just a layer of slime, that could absolutely account for the issue. Dirty bottoms are no good! You mentioned it was always garaged, but if it's been sitting in the water since then...

[99yam40]

Does the boat hull have any growth on it? If there's even just a layer of slime, that could absolutely account for the issue. Dirty bottoms are no good! You mentioned it was always garaged, but if it's been sitting in the water since then...

Not sure slime will cause the same WOT RPM with two different pitched props

[DennisG01]

Not sure slime will cause the same WOT RPM with two different pitched props

To be honest, I wasn't quite sure if it would do that, either. Slime can definitely cause a low RPM issue, but the two different props thing is strange. Maybe the higher pitched-prop just can't generate enough torque to overcome the drage so it keeps the RPM's down? Kind of a long shot, I know, but I figured it was at least worth throwing it out there - if there IS slime, that needs to be taken care of either way.

Other than that, it really does sound like the computer is limiting RPM - but I just have never heard of that particular RPM as a limiting level. Need someone that knows that part of the equation to negate/validate.

[Windham14]

The boat looks like a brand new boat. Cleanest used boat I've ever seen. No bottom paint and has never sat in the water except when it's being fished. It looks like a new 28 Contender.

[Windham14]

I have my Yamaha tech from our local Regulator dealer coming this evening to check all the voltages, fuel pressures and things I don't know how to do. I will report back. Please keep chiming in

[99yam40]

ask if they have the test wheel for that motor to bring with him to test motors.
and maybe a shop tach to make sure your tachs are reading correctly also
Test wheels will make it easier to test other things on motors at load also without hanging off the back of a boat cruising across the water

[Windham14]

I will do that, thanks!

[zenoahphobic]

I'd be very suspicious of the tachs, but then there is two of them; as there are two engines. What are doing the same?
So what I would do is run the boat WOT and noting RPMs do a very hard turn one way, there must be some change in RPM, which would be a reduction as you have increased the load, the inside motor would see the most reduction. Do this turning the other way and note slight changes. Both turns will not produce the same reduction as the steering probably not that precisely set Port to Starboard; and the gyroscopic forces (both powerheads rotating the same) will alter how the boat turns.
If your tacho's don't change then I suspect their sensitivity is not good enough.

Maybe your engines have limiters set that low, just a guess should be greater than 6000, mine limits at exactly 6000 (too exact) with the prop I have feeling like it has more, i.e that a slightly larger pitch will still see the same 6000(more speed perhaps) (although I haven't tested that).

Do your revs ever go up as you trim the motors up to the point where they start to slip/ventilate? If they never do then it is a rev limiter issue.

[Windham14]

Throttle adjustments and TPS settings were off some. Set that, everything else looked perfect. Going to run it this evening and see what happens. Will also monitor o2's and fuel pressure

[rodbolt17]

you can hunt wabbits and chase ghosties.
or.
find a shop with test prop YB-1626.\
should spin it at 5300-5500.
if it does fix the hull/rigging issue.
if it does not fix the engines.

those test wheels have saved me many hours of wabbit and ghostie chasing.