Newbie repowering a 175 Osprey

[RGT]

Hello all, newbie here with a 175 that I would like to repower. I have been looking around and reading up on what I can find and it looks like a 2 stroke Yamaha 90 is a good option, its light weight puts it to the top of my list for now.  The boat gets used for tubing but the Nissan 90 on there will not allow for skiing and that is something we would like to do with it, otherwise I might be tempted to put a F70 Yamaha back there. We use the boat in rivers, bays, and out 15 miles or so( CT, MA, NH, ME) The other motor option I might consider if the right deal were found and I can't find a cheaper Yamaha 90 (I have only found a new one $7200, I have seen some cheaper prices but when I contact them they are sold out) is the Evinrude E-Tec 90, I would trade the extra weight of this motor for its newer technology. I would welcome any input from you all on your experience with various engine options. I would particularly like hear from someone who has swapped in an older 2 stroke with an E-Tec to see if they noticed huge savings in fuel and oil over an older 2 stroke. If anyone knows where I might get a good deal that would be great too....
Thanks,
Roger

[fitz73222]

Before you plunk down some serious coin on a new engine how is the Nissan propped? What pitch propeller are you running and what is your full throttle rpm's compared to what the factory calls out? Those are excellent engines which I believe are re badged Tohatsu's and a very bullet proof simple design. You may just have to repitch for tubing and water skiing and the engine will do you just fine. My $.02...

[seabob4]

Before you plunk down some serious coin on a new engine how is the Nissan propped? What pitch propeller are you running and what is your full throttle rpm's compared to what the factory calls out? Those are excellent engines which I believe are re badged Tohatsu's and a very bullet proof simple design. You may just have to repitch for tubing and water skiing and the engine will do you just fine. My $.02...

X2.  That 90 should do great on 175.  That was pretty much the standard package (the HP, not the Nissan) when we were building them...

[GoneFission]

Ditto on what Fitz said.  You should be able to ski behind a 175 easily with a 90, and even a 75 should get a skier up under 200 lbs if propped right.  It's all about the prop - you could put a 200 on there with a 27 pitch prop and it probably would not pull up someone on a ski.  

What diameter, pitch, and blade prop are you running now?  What RPMs are you turning at wide open?  

[RGT]

I don't know what the prop specs were, it had no markings and was on the engine when I bought it,  I know the boat got up on plane slow and would only run about 26kts, which was very different than the spl 88 that I had got the boat with, it would run high 30's and be up on plane very quick. after I left last season my brother inlaw brought the prop to the Tohatsu dealer and they sold him a lesser pitched prop, he said it may get up on plane a little faster and still runs the same top speed. I had to screw around most of last season(I was only home for a couple of weeks) with getting the crank re-indexed as it turned out it had been spun when I got it so the top piston was running 20 deg. out of phase, I had hoped upon getting that fixed the motor would run like it should. Compression is good and equal, I have spent litteral hours with Tohatsu tech service on the phone, replaced the Power box, gone through all the settings, had two shops go through all settings, carbs are good and clean and adjusted properly...exhaust passages are clean, reed valves in good shape. It runs smooth and always starts just gutless....I only get home for a few weeks each summer and am tired of screwing around, last year I ended up renting a boat for most of my time home..

[gran398]

You have prtty much just answered the question. She should be running right there with the 88SPL...upper thirties and taching out around 5500 WOT.

If she's mechanically sound as you've indicated...the shipmates are correct...its the prop.

[GoneFission]

You have prtty much just answered the question. She should be running right there with the 88SPL...upper thirties and taching out around 5500 WOT.
If she's mechanically sound as you've indicated...the shipmates are correct...its the prop.

You can twist these 3 cylinder motors pretty tight - OEM specs are 5500-5850 RPM, and you can take them to 6000 without much worry.  But you NEED to be running at least 5000 - if not, she's overpropped and you need less pitch.  Look on the prop and usually you will find the specs stamped on the hub - something like 13x17 would be my guess.

[seabob4]

You have prtty much just answered the question. She should be running right there with the 88SPL...upper thirties and taching out around 5500 WOT.
If she's mechanically sound as you've indicated...the shipmates are correct...its the prop.

You can twist these 3 cylinder motors pretty tight - OEM specs are 5500-5850 RPM, and you can take them to 6000 without much worry.  But you NEED to be running at least 5000 - if not, she's overpropped and you need less pitch.  Look on the prop and usually you will find the specs stamped on the hub - something like 13x17 would be my guess.

CJ, not always.  The prop on the RH motor on the Cape Horn is simply marked 15M.  Don't know the manufacturer, could be Yam (200 HPDI), Michigan Wheel, Solus, who knows.  Obviously 15" diameter, but M?  "Medium" pitch?  What exactly would that be?  17 or 19?

One of those little mysteries...

[fitz73222]

I don't know what the prop specs were, it had no markings and was on the engine when I bought it,  I know the boat got up on plane slow and would only run about 26kts, which was very different than the spl 88 that I had got the boat with, it would run high 30's and be up on plane very quick. after I left last season my brother inlaw brought the prop to the Tohatsu dealer and they sold him a lesser pitched prop, he said it may get up on plane a little faster and still runs the same top speed. I had to screw around most of last season(I was only home for a couple of weeks) with getting the crank re-indexed as it turned out it had been spun when I got it so the top piston was running 20 deg. out of phase, I had hoped upon getting that fixed the motor would run like it should. Compression is good and equal, I have spent litteral hours with Tohatsu tech service on the phone, replaced the Power box, gone through all the settings, had two shops go through all settings, carbs are good and clean and adjusted properly...exhaust passages are clean, reed valves in good shape. It runs smooth and always starts just gutless....I only get home for a few weeks each summer and am tired of screwing around, last year I ended up renting a boat for most of my time home..

How did you validate that the crankshaft was re phased to the correct firing angle? If it was #1 cylinder it would be easy to check with a screw in dial indicator in the spark plug hole and see where TDC is relative to the timing mark. Understand the firing order and check TDC to the timing mark on the remaining cylinders. How was the spun journal replaced? It didn't wreck the crank? I've never delt with a pressed crankpin issue just seen them on smaller Yamahas. I guess they aren't keyed?

[RGT]

I measured tdc of the 3 pistons and marked the flywheel, number one was about 15-20 deg. advanced, which would be what you might expect if something stopped the prop and the inertia of the flywheel continued to twist the crank, something has to give and in this case it was not a key or shear pin. The crank has since been remomved, realligned, and reinstalled. I understand from Tohatsu tech support that this is a common problem.

If I am running at full throttle with an overpropped engine such that the engine can't acheive full rpm, wouldn't trimming the motor up until the prop was breaking the surface and slipping in foam cause the rpm to go up? on this engine there is no percepable change in engine rpm when I have tried this. I don't have a tach but based on measured boat speed it seems to be only running about 4k rpm. I will see if my brother inlaw can tell me what he put on for a prop. and if they were able to determine what the prop on the engine was...  when they opened up the motor I was hoping they were going to find clogged exhaust passages, as that is how the motor acts,  but they assured me it was all clean, not sure if there can be blockage in the lower unit that they would not have seen?

[seabob4]

I measured tdc of the 3 pistons and marked the flywheel, number one was about 15-20 deg. advanced, which would be what you might expect if something stopped the prop and the inertia of the flywheel continued to twist the crank, something has to give and in this case it was not a key or shear pin. The crank has since been remomved, realligned, and reinstalled. I understand from Tohatsu tech support that this is a common problem.

If I am running at full throttle with an overpropped engine such that the engine can't acheive full rpm, wouldn't trimming the motor up until the prop was breaking the surface and slipping in foam cause the rpm to go up? on this engine there is no percepable change in engine rpm when I have tried this. I don't have a tach but based on measured boat speed it seems to be only running about 4k rpm. I will see if my brother inlaw can tell me what he put on for a prop. and if they were able to determine what the prop on the engine was...  when they opened up the motor I was hoping they were going to find clogged exhaust passages, as that is how the motor acts,  but they assured me it was all clean, not sure if there can be blockage in the lower unit that they would not have seen?

Chit, I have a universal (works with all O/Bs) tach that you can have for shipping...

[slvrlng]

One thing I don't think anyone has mentioned is motor height. Where is the ventilation plate when you are running? Skoot has the same motor on his 19-6. He just repropped last year, I will see if I can find that thread about what he ended up with on his motor. I do know that he was having the same problem and once he raised the motor and different prop it acted like a different boat.

[RGT]

I believe I set the motor in the lowest mounting holes, which made the ventilation plate just about even with the boat bottom with the trim all the way down. I suppose it would be an easy enough excercise to try it higher up.....but unfortunately i won't be around the boat until June, and I don't think my brother inlaw would be willing to go out in the winter to try it.

Thanks,
Roger

[Circle Hooked]

I think that boat is rate for up to a 120 hp, a 115 Merc opti max would make it fly.

[fitz73222]

I measured tdc of the 3 pistons and marked the flywheel, number one was about 15-20 deg. advanced, which would be what you might expect if something stopped the prop and the inertia of the flywheel continued to twist the crank, something has to give and in this case it was not a key or shear pin. The crank has since been remomved, realligned, and reinstalled. I understand from Tohatsu tech support that this is a common problem.

If I am running at full throttle with an overpropped engine such that the engine can't acheive full rpm, wouldn't trimming the motor up until the prop was breaking the surface and slipping in foam cause the rpm to go up? on this engine there is no percepable change in engine rpm when I have tried this. I don't have a tach but based on measured boat speed it seems to be only running about 4k rpm. I will see if my brother inlaw can tell me what he put on for a prop. and if they were able to determine what the prop on the engine was...  when they opened up the motor I was hoping they were going to find clogged exhaust passages, as that is how the motor acts,  but they assured me it was all clean, not sure if there can be blockage in the lower unit that they would not have seen?

Well this will be a stretch not being familiar with the engine design but this used to be a problem on high hour in line 6 Mercs and 3 cylinder OMC's where there was no apparent reason for loss of rpm's with perfect compression and that was internal leakage of the fuel charge between adjacent cylinders. Manufacturers use different means to keep the fuel charge, vacuum and pressure separate from one another buy using sealing rings on the crankshaft or a lambert mechanical seal design machined into the block to stop fuel charge bleed through into the other cylinders. When these start to leak with upward and downward piston movement creates positive and negative pressures in the block which are part of how two strokes deliver fuel. If the negative and positive pressures begin to bleed over and interfere with normal scavenging the fuel charge gets interrupted or confused so to speak and it wont pull enough fuel to make power. If your engine uses the lambert seal design, there is no repair and the block is scrap. The root cause can be internal engine vibration that causes crankshaft whip ( the inability to rotate concentrically or wobble) and wears away the sealing surface in the block. OMC recognized this in the late 70's on V4's and mid 80's on 3 cylinders by dramatically increasing the center main bearing and crankshaft diameter to minimize whip. Mercury just stopped production of the I6 in '88 and created a whole new design of I4's. This rambling may not help but it could be a reason for the issue. Talk to a gearhead that knows your design and see if he agrees.