OMC 20hp or 25hp to 35hp conversion?

[headdamage]

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?

Thanks.

[9D280]

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!

[joakimhansson]

I owned first the 20 and later the 35 in early 1982-84?

521cc

The 35 hp have the exhaust in the prop center. Gear ratio ?

[headdamage]

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

[TheMadMariner]

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.