Long story but I'll try to hit the highlights with all the pertinent info.
Bought a boat last fall. It's got a 1998 Yamaha 115 V4 2 stroke on it. The boat is an Xpress H56 with Hyper lift hull. I took a boat mechanic with me to check out the motor before buying. He noticed that the prop shaft was slightly bent. Ended up buying the boat, the mechanic had a machine shop cold straighten the shaft. I put a 20P Laser II prop on this spring to help with bow lift as I added on to the front deck this winter before recarpetting. Boat ran great, then about 2-3 weeks ago the lower started ticking. Got worse and got loud. I removed the bottom plug and there was metal all over the plug. The oil looked like metallic paint. As luck would have it, the mechanic who I paid nicely, in cash last fall to check and go through my lower, dismissed having anything to do with the lower unit failure, without so much as the courtesy of looking at it. He diagnosed his lack of responsibilty for the problem over a text message.
So I order an SEI lower and install. I also put on a 6" jack plate. I did the 5 hour break in exactly as described. During this period of running, I worked up to 2500 RPM's and while running at that speed the motor started spitting and sputtering. I thought I had sucked something into a carb, or thought that maybe water got through my water seperating filter. After a few minutes of spitting and sputtering, everything levels back out. I do the 6-10 hour break in exactly as described, the motor runs fine. When the time comes to finally give her full throttle I did and the motor was VERY sluggish. As in it took upwards of 45 seconds - 1 minute to get on plane and the top end was turning around 4200 RPM's.
Since the motor spit and sputtered, I assume the loss of power is a carb issue. I removed both carbs, tore them down and cleaned them. I removed every jet that had a screw slot across it, cleaned them and reinstalled. The floats looked good as did the needles. I decided since the motor was 20 years old I would replace all the fuel lines from the motor to the carbs, including a new OEM primer bulb and also installed a new OEM fuel pump.
I took her back out and it did the same exact thing. I ran the jack plate up to the max and it did the exact same thing. I removed the jack plate, put the motor back on in exactly the same hole position as it was in prior to the lower replacement. The motor did the exact same thing. A buddy of mine suggested that I try a 16 pitch prop to get my RPM's back up so I did.
The motor jumped up on plane and topped out at 5400 RPM's, but I lost 8 mph. Dropped from 45-37. The motor sounds great.
I decided that the new SEI lower must be the wrong gear ratio. I checked my old lower and it's a 2:1. SEI only lists a 2:1 for my motor so I dropped it this afternoon to check it. I checked and double checked and it's indeed a 2:1 as well.
So, that means that I've lost power, somewhere, somehow, right?
Compression on all 4 cylinders is 120-122. I tested spark with a gap tester this afternoon set on 7/16" and all 4 are jumping that gap well. The fuel system is spotless with all new lines. The idle timing is spot on. I have not checked the WOT timing yet but will. There is no fuel being spit out the front of the carbs so I think the reed valves are okay.
I am at a loss here. Granted I'm not a boat mechanic but I'm a fairly mechanical guy. I don't have all the testers and such, and I've scheduled an appointment at a reputable Yamaha mechanic shop, but that's over 2 weeks from now.
Does anyone have an idea what the cause of this could be or what else I can check? This sounds like one of those $1000 "we can't find an issue" problems on the horizon to me.
But, a new but same gear ratio lower unit on the same motor, with the same prop, should turn the same RPM's and run the same speed, correct? What am I overlooking?