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Old 12 August 2012, 10:47   #1
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Knocking noise

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???
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Old 12 August 2012, 11:12   #2
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if you think its gearbox drain the oil and see if its contaminated with metal filings , or if milky white that will be water
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Old 12 August 2012, 15:38   #3
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Aye that was my next step, I'll need to get her out of the marina for that though, Cheers
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Old 14 August 2012, 02:04   #4
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Hi

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

Dave
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Old 14 August 2012, 03:10   #5
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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.
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Old 14 August 2012, 06:07   #6
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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
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Old 14 August 2012, 09:31   #7
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...... 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.
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Old 14 August 2012, 10:28   #8
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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?
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Old 14 August 2012, 11:00   #9
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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
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Old 14 August 2012, 12:47   #10
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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.
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