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Antique Marine Engines and Outboard Motors Discussion on old marine engines, naptha, gas, oil or steam and vintage outboard motors.

Antique Marine Engines and Outboard Motors

Mercury Mark 20


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  #1  
Old 08-01-2018, 07:18:28 AM
Reiter Reiter is offline
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Default Mercury Mark 20

I just bought a Mercury Mark 20 and was wondering if anyone can help with some questions? The power head is not stuck. I can turn the flywheel back and forth but the motor will not turn over. I think the water pump is bad not allowing it to turn over. I have pulled the plugs and oiled the cylinders and removed the small side covers on the block allowing me to see the sides of the pistons and they look like New. The power head still has the factory seal so it has not been apart before. Any Ideas on what to check or problems any one has seen with the Mark 20? This is my first old Mercury.
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Old 08-01-2018, 11:37:57 AM
Heller D. Davis Heller D. Davis is offline
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Default Re: Mercury Mark 20

I had one like that a few years ago that would turn over only part way and found that the coil in the magneto had swelled and would interfere with the flywheel turning 360 degrees. If I remember correctly, the mag. was a bendix and I ordered new coils from them.
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Old 08-01-2018, 01:52:10 PM
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Default Re: Mercury Mark 20

Do you have any tips on pulling the flywheel? Do I need any special tools?
Thanks
Steve
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Old 08-01-2018, 05:24:09 PM
Heller D. Davis Heller D. Davis is offline
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Default Re: Mercury Mark 20

You will probably have to use a puller. I have removed them by loosing the flywheel nut till flush with end of crank and holding up on the flywheel, tap nut and shaft with a brass hammer. You might get lucky.
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Old 08-01-2018, 09:24:10 PM
Steve A W Steve A W is offline
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Default Re: Mercury Mark 20

Here's a video of how to do it with a flywheel knocker.
He is doing it on an antique motor but it's the same.
Good luck with your motor.

Steve A W

https://www.youtube.com/watch?v=RWgE9XiIBnk&t=45s
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Old 08-04-2018, 02:23:15 PM
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Default Re: Mercury Mark 20

I could not get the flywheel nut to break loose. So I removed the lower unit and found the lower unit will only turn so far and then it stop when turning the drive shaft. The power head turns over now. I can turn the drive shaft back and forth but it will not turn all the way around. I checked the gear case and it has grease in it. Do I need special tools to take the lower unit apart?
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Old 08-07-2018, 07:37:55 PM
dkamp dkamp is offline
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Default Re: Mercury Mark 20

You'll probably need a hammer-type impact screwdriver to break a few of the screws loose if they've been tight for a long time, otherwise shouldn't be too difficult.

Does the propshaft turn? If so, drain the gearcase, see if anything harder than oil falls out. If it has a chipped gear tooth, or a broken clutch dog, there's probably a remnant stuck between the gears and the case... so don't force it!
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Old 08-07-2018, 10:45:46 PM
JoeCB JoeCB is offline
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Default Re: Mercury Mark 20

I do have several older outboard motors but I'm not a "Mercury" guy so I will not offer specific info on the gear case/ lower unit except to say that Mercs are different that many other brands... you may need some special tools or at lease specific guidance. Your best source is to post your questions at "Ask a member" on the Antique Outboard Motor Club site aomci.org. Many "Merc" guys there.


Joe B


PS, what ever you do - DO NOT use a arm type puller on the flywheel.
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Old 08-16-2018, 05:18:38 PM
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Default Re: Mercury Mark 20

Thank you.
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