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Old 15 September 2011, 16:23   #1
Country: Canada
Town: Yellowknife CANADA
Make: Zodiac MkI & H440
Length: 4m +
Engine: OMC 2hp/15hp/60hp
Join Date: Aug 2008
Posts: 133
OMC 20hp or 25hp to 35hp conversion?

I like the 1980's vintage OMC/Evinrude/Johnson 2 cylinder engines and I've just bought a 1979 Zodiac MkII so I've been looking for a 35hp OMC for it. However, here in Calgary the 35hp version seems to be hard to find. I have however seen some 20hp and 25hp versions.

I've seen in some threads that you can fit the 35hp carb and intake to the 20 and 25 hp engines to get something close to a 35hp engine. I think this works best on late 1980's OMC engines. Does anyone have any experience with this conversion and what year engines are best to do this conversion to?


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Old 19 September 2011, 04:47   #2
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,538
I donlt know these machines, so this is a bit "high level". For what it;s worth the old (pre thru' prop exhaust) OMCs the 20 & 25 shared a powerhead, and I think when the thru' prop appeard may be when you are alking about.

Anyhow, the first thing to do is grab the briochuires for your chosen machines, and do a hioghlevel comparison of Bore, stroke, CC & weight. SOme brochires tell you the max RPM as well, depends o nthe vintage & OEM. The good thing is you are panning on using OMC, so most of what you'll need is on the old OMC site

So, if your two chosen engines are the same CC, weight, bore & stroke, then you are halfway there. If you are lucky & the brochure shows RPM as well, the chances are one wil lbe 1000 or so higher than the other.

So, having got your warm feeling that it might be feasible, the next stop is the microfiche for your two chosen engines. Go through it with a fine tooth comb. In order I'd check:
- Powerhead for any restrictor plates, differences in exhaust / inlet etc
- carb(s) At that vintage you may find it only has one, so itl;s either going to be a smaller bore or have a different wot stop
- Throttle linkage - this is another place you could restrict opening, and if it's on a stop there may be a different follower cam to give a more linear response.

It may of course just be down to timing, but pop back if you can find no differences in the parts lists. Also remember to compare like with like - the paint job is usually enough to change part numbers on outboards!

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Old 19 September 2011, 05:35   #3
Country: Sweden
Town: Stockholm
Make: KR7
Length: 7m +
Engine: Evinrude 150 H.O
Join Date: Aug 2005
Posts: 735
I owned first the 20 and later the 35 in early 1982-84?


The 35 hp have the exhaust in the prop center. Gear ratio ?
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Old 24 September 2011, 14:49   #4
Country: Canada
Town: Yellowknife CANADA
Make: Zodiac MkI & H440
Length: 4m +
Engine: OMC 2hp/15hp/60hp
Join Date: Aug 2008
Posts: 133
There is a good thread on the subject here:

Converting a 20 HP Johnson to 35 HP

However I got lucky and found a good 1992 Johnson 30hp short shaft so I don't need to consider this option now
headdamage is offline   Reply With Quote
Old 25 September 2011, 09:39   #5
TheMadMariner's Avatar
Country: UK - England
Boat name: SeaRider SR4
Make: Avon
Length: 4m +
Engine: 55HP Yam
Join Date: Mar 2011
Posts: 28
I know it doesn't add much at this point, but just for anyone to know; the OMC 521cc series was rebatched after 1984, since then OMC start measured HP at the prop, and the 35hp was rebatched to a 30 hp and so on.
Pretty neat engines though, i've owned a '85 30hp myself and it went great. However noisy and raw at idle, they are excellent in power-to-weight ratio. Parts are not that expensive and hard to get either, and OMC did produce this platform up until 2000 I believe.
"Only the dead have seen the end of boating.."
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