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Old 12-03-2009, 06:25 AM   #1
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City: Port Charlotte, Florida
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Vessel Model: 1983, 34 ft., Marine Trader
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Generator Clunking

I am looking at a 1978 Mainship with a 7.5 KW Onan generator.* When the broker started the generator (He said it would sound clunky before he started it) it sounded like it was grinding up a bunch of rocks.* It sounded like this for a couple of minutes....... then it was running with the normal diesel sounds.* The broker said they call it a "Rock Crusher" because of the noise it makes when started.* Has anyone out there heard if this or any other generator making this type of noise upon starting?* Is this normal operation of a Onan generator or is this guy blowing smoke?** I know diesel engines can sound pretty loud but how loud is too loud?

Thanks for your input.


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Old 12-03-2009, 07:27 AM   #2
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RE: Generator Clunking

First rule of buying a used boat is to assume the broker is going to forget to tell you about problems or flat out lie. Contact the local Onan service shop and without disclosing you are buying a boat, ask what might cause this noise. You will probably get the straight scoop. Chuck

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Old 12-03-2009, 10:47 AM   #3
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Generator Clunking

If the generator in question is an Onan MDJE 7.5kw, what you are hearing is abnormal. We have one of these generators on our boat---- it's been there forever, possibly since he boat was new---- and it starts smoothly and runs smoothly.

I should qualify "smoothly." It uses a two-cylinder engine that produces a lot of vibration. This is normal.

It can start hard and run rough for awhile IF insufficient time is given for the glow plugs and air heater coil to do their jobs. I was told when we bought the boat to hold the spring-loaded pre-heat switch on for about 5 seconds. This is what we did and the engine always started very rough and took a five or ten seconds to smooth out. Then I read the manual and found that the pre-heat switch should be held on for about 45 seconds under normal temperature conditions and a minute (but no more) in colder conditions. We started doing that and the generator starts quickly and idles smoothly (relatively speaking) as soon as the start switch is engaged.

The other thing that can make one of these generators run rough and idle poorly at startup is a failed glow-plug. One of ours failed and the generator became very rough starting no matter how long the pre-heat switch was held, which makes sense since one cylinder was not getting any heat at all. Changed the glow-plug and we were back to normal starting.

But rough running at startup and metallic or grinding sounds are two different things.* Even when the generator was starting rough, it didn't make any more noise than when it was running smoothly, it just ran rough.* Even before we knew the proper pre-heat time, or when the glow-plug failed, the engine did not sound like it was crushing or grinding anything, it just ran very rough until the cylinders heated up enough for proper combustion.

If the generator in question is not an MDJE than none of what I've written may apply. The MDJE and a Northern Lights are the only two marine generators I've had any experience with. The Northern Lights, by the way, is a whole different ballgame.

-- Edited by Marin on Thursday 3rd of December 2009 12:51:06 PM
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Old 12-04-2009, 08:26 AM   #4
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RE: Generator Clunking

I would look for a missing bolt on some component that, when loose, will not affect the operation of the unit. Had that on a power washer, once I replaced the bolt, no more rock grinding sounds.
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Old 12-05-2009, 07:59 AM   #5
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Generator Clunking

When boat shopping, particularly in this buyer's market, obvious issues like you have noted are a reason to run away. What else has the owner*not maintained properly? 80% of the brokers (or 90% - pick a number) are weasels and cannot be trusted. Sadly, the current economic situation has given rise to a very high % of poorly maintained "toys." So buyer beware is even more applicable

-- Edited by sunchaser on Saturday 5th of December 2009 10:01:28 AM
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