Knocking noise

[jsrmacleod]

I have a Mariner 90hp on my rib which has developed a knocking noise at lowere reves, I was using it the other night to tow an inflatible for well over an hour, no problems but the knocking sound is still there,it sounds kind of like a bearing gone noise but it carried on performing as it should, no smoking or anything, my mate thinks it may be a gearbox problem, with the engine stopped and in reverse you can still turn the prop and not the engine, any ideas guys???

[uncle al]

if you think its gearbox drain the oil and see if its contaminated with metal filings , or if milky white that will be water

[jsrmacleod]

Aye that was my next step, I'll need to get her out of the marina for that though, Cheers

[clydeoutboards]

Hi

have you checked to see if the gear cable needs adjusted? If not I would check this as well.

Dave

[Peter_C]

Originally Posted by jsrmacleod

my mate thinks it may be a gearbox problem, with the engine stopped and in reverse you can still turn the prop and not the engine, any ideas guys???

Does reverse work?

In neutral, can you hear the knocking noise?

Start by removing the prop and checking the hub.To make sure it is properly going into gear it might be time to drop the lower drive unit to make manual engagement possible. Lots of possibilities, and hearing it is the first step to diagnosis.

[jsrmacleod]

I removed the cover yesterday and ran it again to try and pin point where exactly the noise is coming from I fear the worst, the 2 stroke was very low when it happened, I topped it up and it ran fine but the knocking I fear could be bearings, assuming it is, which bearings are most likely to have been affected and for a man with better than average mechanical skills how big a job is it, I've done plenty of work on cars and motorbikes but not really up to speed on marine 2 strokes

[jwalker]

Originally Posted by jsrmacleod

...... with the engine stopped and in reverse you can still turn the prop and not the engine, any ideas guys???

That shouldn't be the case so before you start dismantling the engine sort that lower unit. Even if it turns out you have an engine fault the lower unit will still need fixing. Do it first to eliminate it as a possible cause of the noise. 2strokes don't use plain bearings on the crank as a car engine does so you don't get a typical bigend knock.

[jsrmacleod]

Hmmm this just keeps getting better, Not! our dive club boat has exactly the same engin so I'll compare the feel of the prop, its a weird one, it still performs as it should, ahead and astern and still motored on full throttle?????what damage occurrs when theres a lack of 2 stroke?

[uncle al]

try running the p/h on a double oil mix and listen to see if the knock either goes away ,or is quieter, you wont harm it running on a double oil mix ,I used to when I was working for the distributer

[Peter_C]

Is a motor knock for sure? You can definitely tell whether it is in the top end or the lower unit. Doesn't everyone own a screwdriver to put in their ear? Shut the motor off and one at a time ground out a spark plug and restart it. Did the knock go away on one cylinder? If so that is the bad one. Secondly always run a compression test. Warm motor up, disconnect the spark module, and open the throttle fully, before performing the test.

You really need to volunteer more information. Also a sound bite of the noise would help.

[jwalker]

Originally Posted by jsrmacleod;

....what damage occurrs when theres a lack of 2 stroke?

Piston damage and bearing damage but the ball races and bigend rollers will survive with remarkably little lubrication. The more common problem would be overheating and a seizure or partial seizure where the engine tightens up and stops. If this has happened and the engine has cooled and freed itself, there would likely be a knock caused by piston slap. It is a kinda hollow knock and can often be louder under light load where the piston is not held hard against the cylinder walls by pressure. You sound as though you want it to be the engine. If, as you said, you can turn the prop while the engine is in gear but the engine doesn't turn, sort that first.

[PeterM]

Consider some of the smaller mercs allow the prop to free wheel..so the gearbox may be fine...

Of course it could be shot. Also that on steves 250 we were all convinced it was the engine making a grim noise but it was just the way the noise moved up the leg from the box in the end.
Just trying too be optimistic for you..

[9D280]

Get it running, pull the deadman. The freewheel talked about above will rattle merrily until you come to a halt. My 70s Merc does it. Was in a mate's rib trying to empty a few litres of last year's fuel. The Tohatsu was like hitting a wall without it when we ran out...

[jsrmacleod]

You really need to volunteer more information. Also a sound bite of the noise would help.[/QUOTE]


Sound bite! I'm no more a computer whiz than I am a mechanic, I've got my mechanic pal coming to have a look tonight, we'll do a compression check and get his bore scope in to have a look to see if there is anything obvious, the general concensus is that its small end bearings, the compression check should let us know if the rings have gone.
I've checked the gear box on the other boat, its prop will turn one way by hand when in forward, my one does the same but when in reverse???? dont think there is a problem there though, deffo an engine knock, I'll try what was suggested, removing a plug ground it out and fire it up to see if the cylinder goes quiet, whatever it is, its a strip down for sure

[jsrmacleod]

Compression check carried out, the cylinders range from 125psi to 110/115psi so not a big difference, never did the bore scope, weather was very poor

[jsrmacleod]

Just a long awaited update, it was piston slap, replaced all three pistons and honed the bores which weren't too bad, now just about run back in, going well Fresh coat of hypalon paint and she's looking like new

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