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Old 04 August 2011, 13:02   #1
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old 20hp engine

I have a mariner 20hp which is about 25 years old. It says 20hp on the side but is it really likely to push out 20hp for a old engine, Im thinking its probs pushing 15hp at the most, are outboards just like cars and lose power over years.
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Old 04 August 2011, 13:09   #2
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If it's the Yam based Mariner, then it's probably a genuine 20 at the prop. If it's Merc based, it's probably about 17-18hp.

However, set it up properly and it'll be pretty close to its original HP unless it's had it.
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Old 05 August 2011, 08:55   #3
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I'm sure i may be corrected however I was always told that older engines were rated as H.P at the prop were as newer engines were rated at the head. Therfore older engines gave a truer figure without tranmission losses.
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Old 05 August 2011, 09:08   #4
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Quote:
Originally Posted by scallywag View Post
I'm sure i may be corrected however I was always told that older engines were rated as H.P at the prop were as newer engines were rated at the head. Therfore older engines gave a truer figure without tranmission losses.
in the mid 80's Merc's changed from HP at flywheel to HP at prop,

if you check thro the old engines you will see

50hp became 45hp
80hp became 75hp
100hp became 90hp ( i think )
and so on

I could be wrong but think OMC were always at prop ??


my 1976/7 Merc 500 50hp became 45 hp in 1986
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Old 05 August 2011, 10:50   #5
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Definitions of naming policy aside, I think the OP is asking if the grunt is likely to fade with time.

As Nos Says, unless it's utterly gubbed it's going to be close to 20. A couple of things to check would be spark & compression. If t is really only behaving like a 15 the other two things that could have become maladjusted are the spark timing & the fuel mix.

The other possibility of course is that if you got it secondhand, at that vintage was the 15 & the 20 the same basic block? Has it had a replacement hood? (My Merc was a similar example of that - the transfers on the cowl made it look like an Early 90s machine, but when I rebuilt it I found components inside (like the pistons) that had been superceeded in the late 70s!
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Old 05 August 2011, 11:25   #6
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I'm a great supporter of older engines if they've been cared for... no reason why they should be much under as-new performance. Here is our 20hp Mariner easily pushing a 13ft GRP boat on the plane and up to 26mph (GPS speed) with 2 adults + 2 teens. Engine was 24yrs old when this was filmed.

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