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Old 10-09-2015, 15:40   #1
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Johnson 120 HP

Hi Guys Hoping someone can help with my problem.
I have a 120 HP Johnson 1985 I have just striped the motor and replaced 1 piston honed all 4 cylinders new rings gaskets seals thermostats and seals there is no corrosion in the water jackets at all . I also replaced the water pump with a new pump kit. Everything is in as new condition water is circulating but no water from the tell tale . I have removed and checked the water pump and replaced the O rings and gasket although they were new anyway and getting the same result, I have been trying to find the problem for 4 months and coming up empty. The guy at the boat shop says it will cost me around $2000.00 Pluss IFF they find the problem or not ,I can replace mine with another which is 140hp the same exact motor just a different manifold that is working for $2900.00, I don't have a lot to spend now and am hoping someone may come up with something I haven't tried . Heres hoping anyway Cheers and thanks in advance for any help Bernie
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Old 10-09-2015, 16:54   #2
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Re: Johnson 120 HP

Are you sure the little tube isn't blocked by something?


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Old 10-09-2015, 17:04   #3
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Re: Johnson 120 HP

Hi Thanks for your reply But No all waterways tubes etc are Clear Cheers Bernie
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Old 10-09-2015, 17:32   #4
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Re: Johnson 120 HP

I seem to recall some sort of bypass valve to allow cooling water to circulate throughout the engine when running slowly, ie not enough volume from the pump to keep the block full. I don't even know if that engine has this feature or if it appeared more recently.
Just bouncing ideas around


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Old 10-09-2015, 17:54   #5
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Re: Johnson 120 HP

Thanks for your input I have no Idea if there is a bypass valve in this motor ,Its what they call a Looper But I will look into that Imediately Cheers Bernie
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Old 10-09-2015, 18:00   #6
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Re: Johnson 120 HP

I must admit that My suspicions are that the water pressure in the block is to low but Every thing was working before the piston ceased and every thing I can replace has been replaced I wonder if there could be a problem in the adapter under the engine but I don't believe I have the knowledge to remove that part. Bernie
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Old 10-09-2015, 18:24   #7
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Re: Johnson 120 HP

Post the model and serial number and I'll check with a friend who is a pro.

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Old 10-09-2015, 20:48   #8
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Re: Johnson 120 HP

Hi Marc The Model No is J120TLCS
Serial No is 7028440 Thanks for any help Cheers Bernie
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Old 11-09-2015, 08:23   #9
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Re: Johnson 120 HP

Something wrong with model number. Can't find it in OMC database.

Marc
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Sur le même bateau, l'homme de terre et l'homme de mer ont deux buts différents. Le but du premier est d'arriver, le but du deuxième est de repartir.
La terre nous tire vers le passé, la mer les pousse vers le futur.- Albert Londres, 1927
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Old 11-09-2015, 08:36   #10
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Re: Johnson 120 HP

Check thermostats and diaphragms in the top of the cylinder head. that's what controls the water pressure
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Old 11-09-2015, 12:58   #11
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Re: Johnson 120 HP

Bernie,

Your model number is J120TLCOS (if 1985 is the right year).

On this particular engine, the telltale waterflow is known to be weak. Any dirt in the tubing will stop it. On normal operation, almost all cooling water is coming out thru the impeller.

Something important: Is your engine overheating?? You haven't mentionned that so I will assume it works well. So try to clean the little hole where water is usually coming out.

If engine is overheating, check impeller operation (turning with driveshaft??) The little plastic piece driving the impeller can be misplaced...

There is one thermostat in each head. Rarely these will cause any problem.

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Sur le même bateau, l'homme de terre et l'homme de mer ont deux buts différents. Le but du premier est d'arriver, le but du deuxième est de repartir.
La terre nous tire vers le passé, la mer les pousse vers le futur.- Albert Londres, 1927
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Old 11-09-2015, 14:11   #12
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Re: Johnson 120 HP

Hi Marc The Number may end with TLCS OR TLC8 Cant read it clearly on plate or plug Do they mark export engines differently? I'm in NZ . Numbers havnt been changed as its a one owner from new. Cheers Bernie
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Old 11-09-2015, 14:16   #13
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Re: Johnson 120 HP

Hi Marc yes it is A 1985 engine supposedly exactly the same as 1985 86 and 87 also the same as 140hp other than intake manifold and jetting this model also comes new with the 6 cylinder gearbox. Cheers Bernie
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Old 11-09-2015, 14:22   #14
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Re: Johnson 120 HP

Hi Motion30 Thanks for the Sugestion all hoses etc are clean with no blockages I have installed new thermostats and seats. Cheers Bernie.
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Old 17-09-2015, 21:37   #15
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Re: Johnson 120 HP

I sent it into a shop they found that when reassembling the Power head to the Leg I had put two hoses on the incorrect way so they swapped them and now is pumping water. only cost me NZ $95.00 but there seems to be a fair amount of air in the flow which is showing when run on earmuffs so my thought is its sucking air at the pump when the shop ran it in there drum the pump would have been submerged. any thoughts?. Cheers and thanks for all the help to now Bernie
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