JJONES

Are you certain its a nock and not a back fire threw the exhaust?Bad gas or water in carb will cause a backfire under load,drain the float bowls and see if there is water or debris.easy check.This will also hamper performance.

1BIGJIM

iTrader: (1)

Take the filter out and check it for metal particles. Cost you nothing. My bet is on a spun rod bearing.

Gizmo

Ed, you and I had the same first thought!
Does the knock change as the engine warms.
Another thought, It is possible that the knock could be a broken piston skirt. I had that happen after a rebuild, a long while back. .02 worth...

B one

who did the rebuild? do you know if the checked all the clearances properly? I have ran into this before. once was a wrist pin I knew that it was a pin problem I just didn't want it to be. The other time I did not check main bearing clearences, I was the only owner of the car sence new so when I rebuilt the motor I put all std. parts. It turns out the the motor from the factory had 20 over mains, and I did not check them. Take the motor out and start over. It's not that hard to get it in and out. I't could save you thousands. TOP FUELERS do a rebuild in 45 min. I'll start the clock GO.

sleeper_dave

It was a longblock bought from a company in florida and shipped. 1 year warranty, but by the time it's shipped, it's almost not worth it. No way to tell how they handled clearances. It was the cheapest motor he could find, and it was really all he could afford. ($4500 boat, $1500 motor, DIY motor swap)

I doubt it's a backfire. Too loud, too sharp, too consistent. I can't really say if it changes as the engine warms up, it doesn't seem to, but we haven't really run it hard enough to know. Once it starts knocking, he backs off the throttle. Hasn't gotten it on plane since it started. Generally warms it up at the dock before trying to get on plane, but I don't think it would go away if we ran the motor cold.

It's not a rod knock. I've heard enough rod knocks by now to recognize one of those. It almost sounds like it, but it purrs like a kitten at idle, and there's no oil pressure issues. Any knock i've ever heard is audible at idle, and sounds a bit more painful. We haven't checked the oil filter, though, that's a good idea. I suppose it could be a spun main bearing, I have never experienced one of those so I don't know if this noise is consistent with that type of failure. It does sound reasonable, since under light or no load the crank should hold itself in the center of the journal without a good bearing, but under load it would deflect and hit the edge of the journal, but like i've said, i've never heard a spun crank bearing to be able to diagnose. If the oil pressure is still good, though, who knows. I suppose a plugged or partially blocked oil journal could cause a spun main bearing without a loss in oil pressure..

pulling the motor out of the boat is very difficult. He doesn't have a trailer, but come spring, I can put his boat on my trailer. The first time we swapped it, we took it up to a friends shop with a 4 post lift. The lift only went up about 6 feet, so we couldn't get the boat under the lift. The best we could do was take a cherry picker and strap it to the lift... with the legs bent back. This gave us enough height to lift the motor up, but not enough reach. So while we were raising or lowering the engine, we had to push the engine forward (in relation to the boat) to clear the transom and drop into place. Luckily the cherry picker got us high enough that we could put a long chain on it and get enough room to push. All in all, it was a very shaky procedure. Took all weekend, and then some, by the time the drive was back on. We really, really don't want to have to do this again. We were really hoping for an outdrive problem.



once it's out, the work on the motor is no biggie. there was no core on the new engine, so he has another motor with a presumably good bottom end. We can go through it and do it ourselves reasonably easily. It's just getting the damn thing in and out of the boat that's the problem.

Edward R. Cozzi

I love that lifting rig you guys made up. I bet there's dead OSHA guys spinning in their graves! (Please be careful.)

I've seen a bad wrist pin make no noise at idle, but start thumping upon acceleration in neutral. It was on a "rebuilt" engine too. I isolated what cylinder it was by pulling a spark plug wire one at a time. The bad cylinder usually gets much quieter with no spark.

sleeper_dave

That's exactly what the (only) wristpin failure I heard sounded like. My buddy sold that car to some kid who put many more thousands of miles on it without failure, beating the piss out of it.

But this is different... makes noise under load but not in neutral. Which kind of has me stumped.

I think the crankshaft main bearing sounds pretty likely though. That could explain everything. Next step is to get back in the boat, check the oil filter, plugs, and compression, then go from there. I have a feeling we'll be pulling it and tearing it down.

pachanga

My old school teaching always taught that a knock under load was a mainbearing....but unless it has a high volumne oil pump(or maybe a spun bearing) there should be quite a noticable drop in oil pressure(especially at idle). I will also add that on the few vehicles I have had with sloppy mains when 1st started always rattled pretty good or had a deep thlock,thlock,thlock sound until the oil filled the mains back up!

senzaspectre

I had a simular problem, I thought what sounded like a rod knock was actually back fire. It was hard to tell with underwater exaust.

Would only happen some of the time, while trying to get on plane. The sound was rapid.

satisfactionII

sleeper-dave, I don't know the answer to your engine knock but you should check this web site. In reference to your engine pulling thing, this site shows you why women live longer than most men.

http://www.gophergas.com/funstuff/womenlivelonger2.htm

It is very informative. Also, when you re-installed the engine di you guys make sure it was aligned properly so that you didn't damage the coupler?