A friend of mine has a small cruiser that's been acting up lately. The latest problem is a knocking noise that only occurs in gear, under load, at higher RPM (2500+). When you try to get on plane, it knocks really bad. It doesn't sound like a rod knock. I can't tell if it's coming from the engine or the drive.

It's a 25' Rinker with a 350 mag and an alpha one. The motor is a rebuilt unit, maybe 15 hours on it.

2 trips ago, it started occasionally loosing a little bit of power. Not too much, but needing more stick to maintain RPM, and the tone of the motor changed at a given RPM. Then it would come back. That same trip, we sucked something up, overheated, and got towed in. We hauled the boat and fixed the water pump. Ran it on the hose, everything seemed fine, put it back in the water.

Next time out, the power loss / RPM thing was a little bit worse. I suspected a fuel problem. Later that same trip, the knocking noise appeared. We stopped for lunch, and on the way back, the first time we tried to get on plane, it made the noise. The knock is not present at all at idle, so we idled back home across the lake. We suspected detonation, but that would have to be some pretty bad detonation. Anyway, we checked timing, set it to 8° base (it had somehow backed itself down to 4°base, but I didn't set it so I can't attest to the accuracy of either measurement) and replaced the fuel filter. We went for a test drive and the noise was still there.

Oil pressure is good, boat comes up to temp and stays where it should, and it doesn't make any funny noises at idle or with no-load RPM.

Might be wristpin, but I kind of doubt it. Last time I heard a motor with a bad wristpin, it made noise with no-load RPM, and it wasn't as distinct of a knock, IIRC.

Anything in the drive that could cause this?

Where should we start looking?

We're pretty well stumped. I told him to pull the spark plugs and go from there. I guess the next step if we suspect motor is a compression check, and if it's bad, the motor is supposedly under warranty. Could it be outdrive, though?

I would first try to determine if its the drive or the motor. Drain and check gear lube. A broken tooth on a gear is possible. It would get worse fast. Give the guy who built it a chance to hear if drive checks out. Good luck.
Jim

Sounds like a possibility. If it were a broken tooth, would it show up at idle speeds in gear?

Problem is, it's not free to pull the boat, since he doesn't have a trailer. If we drain and fill the gear lube, and that's not the problem, we have to put it back in the water to run it under load again. It becomes a $125 gear lube change.

I'll definitely suggest that though. Probably look at the spark plugs first, and maybe do a compression check. Those tests are free, and hopefully that'll rule out engine trouble.

it is a 15 year old alpha, so I wouldn't be surprised if the drive is fubared.

Well, after this last post, the boat sat in the well for another month then idled a few miles to be hauled out for the winter. The outdrive was pulled and no problems were noted. The outdrive seems to function fine when spun by hand, no roughness, it shifts fine, etc. Not sure if he drained the lube yet, but he did pull the plug for a second to check for water / contamination and found none. Normal amount of wear metal on the magnet and no water in the gear lube, the lube looked brand new (which it was, maybe 5 hours on it).

Gimbal bearing looks okay, so does the u-joint.

We're at a loss. I can't think of anything in the engine that would do this. No oil pressure loss, no indication that anything is wrong when the boat is idling. Temperature is fine.

The problem started with an intermittant low power condition, the boat would slow down and it would need more throttle to maintain RPM. Then it would go away a few minutes later. We stopped for lunch, and on the way home it started and idled away just fine, then made a horrific knocking noise when we tried to get on plane. Idled home, tried a couple of different things, still can't figure out WTF is wrong with it.

Is it jumping in gear like there is not enough engagement in to forward. Can you rotate prop shaft while moving linkage to determine how much engagement you have. I horrific noise should show it self.

It doesn't seem to be jumping in gear. It does it in reverse, too. It didn't do it right away when we put the drive back on, it started after a few outings.

Try a compression check. Had a similar problem with one of my motors. Temp, oil pressure, no Smart Craft warnings, etc. all fine. Boat ran good not much of a change, but noticed a knocking noise and it got worse under load. Did a compression check and the #8 cylinder had a bad piston and rings. No compression.

First thing that popped into my mind while reading post #1 was "Wrist Pin".
Sometimes even a compression test won't uncover it either.

take it out and brick it. you ll find the problem, along with a couple more you didnt know you had.

That thought did cross our minds. Take it out, firewall the throttles, and see what happens.

I guess it could be a wristpin, but in my experience a wristpin is more likely to make noise under no or light load. than under heavy load. This doesn't sound like the only other sloppy wristpin that i've heard.

I suppose it could be a bad piston / rings.

I told him to pull the plugs, inspect them, and then do a compression check. I don't think he did either. Maybe i'll be revisiting this thread again come springtime.