Knocking

[Patrick123]

I bought a boat a while back, almost ready to take it out. It is a 1977 Fiberform with a 350 and a recently rebuilt Mercruiser drive (please forgive my ignorance for not remembering the model #.) Anyway I am pretty new to I/O boats but I like to think I know my SB Chevs. The engine sounds like it's knocking when it is running in my driveway with the garden hose hooked up. It sounds like a cold diesel. Except for the knock the engine sounds and seems to run great (at least in my driveway) and oil pressure is strong. I have gone all over the engine with a mechanic's stethescope while it's running and it doesn't really seem to be a rod like I would suspect. I bought the boat not running (wiring issues) but was told the engine was good. Is there any chance that a knocking noise is normal? I don't really know what sounds to expect with the engine bolted rigidly to the hull of a boat sitting on a trailer, I'm pretty new to this. If it isn't normal where should I start in my tracking of the noise? The drive supposedly was rebuilt recently. When I got the boat, the outdrive was off and the u-joints and bellows and stuff all look brand new, so I believe it. Thanks in advance for your help.

 

[Boatist]

Re: Knocking

Patrick<br />Welcome to the board.<br />Should make no more niose the a auto 350 chev.<br /><br />Does it sound like all cylinder knocking or just one?? Things that can make Knocking noise. Timing too far advanced, more of a ping sound.<br />Valve lifter out of adjustment.<br />Bad rod bearings.<br />It is a diesel.<br />Flapper valve in exhaust.<br />NO oil pressure or very light oil like 5 weight.<br /><br />Out of the water you will be able to here exhaust noise.<br /><br />Good luck and come back and tell us what you find. I am sure others will have more info.

 

[Patrick123]

Re: Knocking

Thanks for the help. I have rolled the timing back to 4BTDC just so I can rule that out. While I'm not positive, it doesn't sound like a lifter or valve. It does sound like a rod. It is gas. In the past with a small block in a car I can take a mechanics stethescope and hear a bad rod bearing and make a pretty good guess as to which one it is (maybe my hearing was better.) With this boat, I can't hear anything abnormal with the stethescope. The engine is quiet enough that with the stethescope I can hear the oil pump, but no knock. I seem to hear the noise most with the stethescope on the bellhousing or on the block back by where the casting number is. That's why I think it's strange. I would think that if something in back outside the engine was rubbing or banging around, I would see some shavings of something by now. <br /><br />That sucks the sound isn't "normal" or "common." Anyway, thanks for your help, and thanks for the welcome

 

[2770]

Re: Knocking

Try your steth. on the metal part of the exhaust tubes behind the engine.There are flappers inside each one.Very often a metalic sound will come out of there when the shutter closes and opens.this is pretty normal and can be corrected easily.You can also try to raise and lower the outdrive while running the motor and see if the noise changes.Ujoints can make noise at certain angles.Usually the more you raise the drive the more chance to hear noises. CC

 

[Patrick123]

Re: Knocking

Thanks again for all your help. I had read about the exaust flappers after doing a search, but this boat has exaust through the transom so it isn't the flappers. Even though the gimble bearing is new, I pulled off the outdrive and started it with the drive off just so I could rule out the bearing, it still made the noise. Today I took the boat out to putt putt around and see what happened. I didn't get on it too hard, but the noise slowly got worse. It has progressed into a very unplesant metal to metal scraping kind of sound, though it may be quieter at idle now. I am still pretty sure it isn't the engine. Raising or lowering the motor doesn't seem to have much if any effect, but steering might. It seems that while I am steering the noise quiets but if I hold the wheel steady regardless of angle the noise continues. I felt around behind the engine and I can feel a gap between the coupler and that bellhousing engine mount thing, so I don't think they are rubbing. That's all there is that can rub right? I am getting ready to go out there and check every bolt I can get to back there to make sure they are tight. Anyone else have any ideas?<br /><br />Thanks again for everyone's help.

 

[Patrick123]

Re: Knocking

Oh and the guy I bought it from thinks he remembers that the drive is a mercrusier 228, if that sounds right, he wasn't 100% positive. Thanks again.

 

[B_Eager]

Re: Knocking

Was the motor pulled and not aligned properly??

 

[Patrick123]

Re: Knocking

The last time the motor was put in was at Northwest Jet Boats where the drive was rebuilt and they aligned it. I have had the outdrive off and back on and not had it aligned, do I need align it when I do that? From what I understood I need to have it aligned if I pull the engine, but I guess I'm not sure about when I pull the outdrive. The outdrive comes off and goes back on very easily, it is easy to get the driveshaft through the gimble bearing into that female splined coupler. Maybe that's the problem? Thanks again for your help.

 

[bernieb]

Re: Knocking

Patrick ,you may consider taking the boat on the water with a buddy and move the distributor while watching rpm.Be careful working around the interrupter swich to the left while searching.You will need the offset dist.wrench.I have heard that sound you talk about in my 350 chev for the last five years and lately I'm getting some lifter noise around the # 8 cylinder.good luck.

 

[magster65]

Re: Knocking

Maybe it's one of the u-joints?

 

[Patrick123]

Re: Knocking

I think I found the problem. I can't really see the port side without pulling the gas tank but on the starboard side behind the engine on that exhaust pipe where it runs behind the engine, it looks like there is an allen head screw in horizontally, parallel to the flywheel, screwed into that exhaust pipe that is rubbing on the flywheel. I don't use that exhaust pipe because the exhaust is through the transom. Looks like the engine will have to come out and I will have to look at the problem more closly. Now I have a couple more questions.<br /><br />The guy I bought the boat from has an automotive short block in there with all the marine stuff on it. I am guessing that the flywheel was automotive. Is that why there isn't any clearance there? The previous owner never had it running, I am the first to get it running. Is there a special marine flywheel that I need that is thinner than an automotive flywheel? Or should I just pull out all the old exhaust system somehow since I am not using it? Any other thoughts regarding clearance between the flywheel and that exhaust pipe? Any thoughts are GREATLY appreciated. Thanks in advance.

 

[Patrick123]

Re: Knocking

Is there a way I can take out that exhaust piece without pulling out the engine? If so, is there something I need to do to "plug the hole" like a gasket without the cutout for the exhause bellows ot something?<br /><br />Thanks again.

 

[DJ]

Re: Knocking

Possibly a flex plate? Starter ring gear cracked.

 

[Patrick123]

Re: Knocking

It is actually pressing on the flywheel itself. You know, even though I can't see it real well the flywheel looks like it was out of a 4-speed car or truck, because the surface is there for the clutch to ride on. I think I have a few automatic transmission small block flywheels sitting out in the shop. I am going to try to pull the engine tonight and see what I can see. Maybe I will try a different flywheel, the flywheels for automatic transmission are much thinner. <br /><br />Thanks again everyone.