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Powering the Nancy G, a 15' classic custom, I have a low hour 2002 Yamaha 70 HP TLRB 2 stroke slinging a Stiletto D811315 stainless prop that runs very rough through a range of about 600 RPMs during, what mechanics have referred to as, transition. The engine starts quickly, has good power and runs smoothly at all other RPMs. New correctly gapped spark plugs have been installed. The engine height has been optimized for correct WOT; though I don't recall exactly what that RPM is.

When I first brought this to my mechanics attention it was, to him, a slam dunk carburetor issue. A complete rebuild of all 3 carbs resulted in no change. I took it back and this time the shop owner personally rebuilt the carbs. Once again, no change. As an aside and kudos to Gwinnett Marine in Buford Georgia, they issued me a full refund for all parts and labor. "If we didn't fix the problem, we don't charge for what we did". Tremendous customer service.

The fuel line and tank were replaced during the re-fit in 2010

The helpful folks at Yamaha have mentioned the possibility of an incorrect prop. This I doubt to be the case as Whaler Centrals prop guru prescribed the above described prop. They also haven't received other such complaints with this model engine and insist it's an adjustment or some other post-manufacturing problem.

Also mentioned by knowledgable folks has been:

- the possibility of a faulty or worn "carburetor linkage cam" The one on the engine looks to be in unworn and undamaged condition..
-a possible cracked or otherwise damaged reed valve(s)
-improper final adjustment on the carburetors (Gwinnett Marine lake tested it after the rebuilds)

I'm not being anal about a barely perceptible hesitation or skip at a small RPM range. This is a pronounced rough-running condition at just the sweet-spot speed range preferred by the boats namesake.

Any suggestions or ideas?

Zap

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600 RPM is barely above idle, if at all. Is that a typo?

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The roughness continues through a range of about 600 RPMs. It starts at the beginning of transition and continues through the end of transition. I seem to remember the RPMS being 1600-2200 but just don't remember the exact numbers. It's whatever would be considered the transition from low speed to high speed in so much as the settings of the engine are concerned. Hope this helps. Sorry for the confusion.

Zap

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Tom, I believe he means it runs rough within a 600 RPM range, but he didn't specify which range that is. Zap, can you be more specific as to the RPM at which the roughness starts, and when it stops, and also give the approximate corresponding speeds?

Tim

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I have a 15 Sport GLS with a 2001 Yamaha 60 hp 2 stroke with 134 hours that does the same thing between 800-1100 rpm, I have a factory prop installed. I bought the boat used last June and when I first ran it I thought it was just dirty plugs but changing the plugs made no difference. I did run a can of Seafoam through the motor and it made it a little smoother but it still is noticbly rough in that range.

This past winter the guide I was duck hunting with had the same motor only a couple of years newer and it did the same thing as we talked about what caused it he said that his has always done it since it was new.

I have not tried to get it looked at I just figured it was a caracterstic of the motor.

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Zap, are you planing at the high end of that roughness range?

My first take is that you're just plowing - that is, running the boat at a high sub-planing speed, creating tons of drag at a point where the motor isn't making much power. If you really need to run there for an extended period, I don't know what you can do other than intentionally underpropping the boat to get higher up on the power curve at that speed.

Tim

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tedious,

Don't recall whether or not I was beginning to plane. I'll pay attention to that aspect the next time I take her out.

Zap

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Underpropping.... does that mean I'd have less pitch or more pitch?

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Less pitch.

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Hello Zap, try to narrow down (rpm wise) where the hesitation starts and deminishes. There may be a voltage issue at the stator which occurs at a certain rpm band.

Some older Merc's and Johnson (looper) O/B's went through stator issues that affected rpm.

Have you had the prop's hub checked, just a curiousity...good luck.

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I have an aluminum Michigan Wheel prop laying around that has the following numbers cast into it: 992103 13.75 X 15. I'm guessing the first set of number is a parts number and the last set is the props performance info with 13.75 being the most important to my situation. The question I have is this: Doe's this prop have less pitch than the Stiletto prop described in my original post?

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We need to know the lowest rpm's where this occurs.

Is the boat in displacement mode?
Does the rough running end when on top of the wave and barely on plane?
Has the poppet valve/thermostat been changed?

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PhilT,

Don't recall the precise RPM the hesitation starts.

Boat is in displacement mode.

Seems to get better as boat is closer to planning.

T'stat's been changed.

I'll pay particular attention to the answers to these questions when I get her out again.

Do you think the prop described in post 11 would help narrow down the problem and help lead to a possible solution?

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Zap, your stainless prop appears to be a Stiletto Advantage 1, 13.25 x 15. Your aluminum prop will have less effective pitch, because it has less cupping. But it has a greater diameter, so the net effect will probably be a wash.

From your latest post, I believe most if not all of your problem is that you're plowing / bogging - as described above, you're operating in that transitional range where drag is maximized and the motor is not producing a lot of power. It's the equivalent of driving a standard transmission car in too high a gear up a hill. No planing boat is going to operate well in that mode, and I think the 15 in particular performs poorly when in between displacement and planing. I suspect you're also running very nose up, throwing a huge wake, and the boat changes speed and attitude if you turn, so you're always adjusting the throttle to hold speed? All your power is going into generating that wave, but you're not going fast enough to get on top of it - I'd imagine your mileage is terrible too. Even more of concern, you're running with the throttle butterflies relatively open so you're pulling a lot of fuel, but you're not running fast enough to be pumping a lot of air - this could lead to overheating.

The bottom line is that you really, really don't want to operate in that transitional range for any length of time. Sometimes conditions force you to, but it's best avoided. If you really, really need to do so, I'd drop down to a lower pitch. I think you're already a bit underpropped with that 15 though - I think people usually use 17s or more with the 70 two-strokes, at least I did when I ran one.

The other potential solution is really simple - just slow down. If you drop down a bit, maybe to about 1500, you'll give up a little speed, but not much. And you'll throw much less wake and improve your mileage and handling.

I run an F70 on my 15, and the effect is even more pronounced as it develops power even higher up in the RPM range. You can go plenty fast, or go at displacement speeds, just not in the middle!

Tim

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I'm having almost the exact same symptoms on my 1985 90 horse Yamaha, which is a very very similar engine. I am rapidly narrowing in down to a stator or other timing issue (if its not carbs there's not a whole lot else it can be). I will be very curious to see if you get this issue resolved.

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Very good Tedious. From your description it sounds as if you've been boating with me. With the Nancy G being a bit stern heavy it sounds as if the problem might be more prop related rather than engine performance related. Just damn. Maybe I can find a loaner prop to see if things can be improved. Thanks to everyone for sharing their knowledge. Would a 90 HP engine help the situation?

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Zap, I have a prop you can borrow if you can't find one local. It's a 13 x 13 power tech! guaranteed to bring the rpms up, but be careful, this prop will redline your setup. If you are interested, email me your info.

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If its really just bogging (and it sure sounds like it) then a bigger,heavier motor is likely to make it worse. You could try a smaller motor but you'd lose top end, of course.

Next time out, just try slowing down to just below where the motor starts to feel like its straining. Honest, you won't give up a ton of speed and your motor will be happier.

Tim

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Huck, Thanks for the prop offer. However, I'll try to find one local.

cwk6, I'll look into the stator possibility.

Tedious, As this condition rattles every loose item on the boat and anoys the ever-livin'-crap out of me I've avoided that certain RPM range like the plague. There's a fine line between plowing and planing. The Nancy G falls off plane very easily. I'd considered that the weight trade-off would likely negate any positive effects a more powerful engine would provide.

Thanks again for all of the input.

Zap

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I agree, the 15's fall off plane very easily - it's one of the few things I don't like about the hull.

One other thought is that you could consider one of those foils that install on the AV plate - supposedly they allow you to hold plane at lower RPMs.

Tim

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Didn't realize that falling off plane is common to the 15s. Makes feel a bit better about my set-up. As much as I dislike to look of the foils I did install one. It didn't seem to help much at all.

Zap

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I don't think my 15' falls of plane easily. I can't remember what the lowest speed is but i know that my boat will stay on plane at 20 mph, probably less because it didn't seem to work the motor that much. I also have a prop with less cup, a 13.125x 17 Turbo and a hydrafoil. I don't like the look of them either but what could I do. I tried that same brand/ size prop that you have and didn't experience any "bogging" that you describe.

I understand that stiletto puts holes in the back of the prop so it will vent exhaust and slip, keeping the rpms up through the transition phase. Is not the transition phase the time where fuel consumption will be the greatest? Could it be a fuel pump or filter going bad? Could the spark not be hot enough? Owning a 15' with a 70 Yamaha myself, I'm very curious of the outcome of this.

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I've never recommended a foil..... however "Smart tabs" might be your ticket....

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Huck, since you have the same setup, yours would be a great comparison. What happens if you try to run in the range Zap has trouble with (1600-2200 RPM)? I mean stay at those speeds, not just pass through them on your way to planing.


Tim

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I sure will. So long as I can do some fishing between tests;-)

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Zap, I went fishing today and tried to replicate the conditions you say makes your motor run rough. I slowly throttled up and plowed until the boat planed off. Through it all it ran smooth. Even when I had the same prop you have, I don't remember it doing what you describe.

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Most of the roughness during transition was due to improper settings on the carburetors. Upon visual inspection of the linkage the mechanic who diagnosed and corrected the problem said "They (the settings) look close. But close don't get it".