Newbie repowering a 175 Osprey

[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.

One indicator of internal block leakage is if your are constantly having to adjust your low speed mixture screws in order to maintain a decent idle or the engine keeps stalling at idle for no apparent reason assuming the idle speed and mixture is correct. Leaky crankshaft seals can also contribute to power loss. I'll assume your engine builder replaced all the seals and lightly honed the cylinder walls to break the glaze and get the rings to seat correctly, another power loss contributor. Look for fuel seepage between the block halves or running down the block from under the flywheel as external leakage indicators.

[RGT]

fitz' that would explain what I am seeing. looking at the parts blowup of the engine there are seals that are part of the crankshaft assy. that they don't detail as they seem to only available with a complete crank assy. I guess if I had put in  a new crank kit instead of just having the old one reindexed I would have new seals in there....this motor is over 30 years old now and I know it sat in some guys garage unused for a lot of those years so these seals drying up and failing would not be unexpected.....I guess I will have to look into this some more....

New prop is 13.5 diameter by 15.7 pitch.
The old prop was a 17, don't know dia. does not seem like it was over propped with 17, and the 15.7 made very little if any difference.

They did replace external crank seals and run the hone through quickly. The seals on the crank to seperate the crankcases did not get replaced. I did not know to request it....no external signs of leakage, idle speed does vary, though it is set high enough that stalling is not usually an issue.
Thanks