Johnson 90 to 140 conversion.

[wired]

Ive got a 1981 vintage 90 Johnson. Great Running motor. Been on the boat ever since my dad bought it new in 1981. ~125 psi per hole compression. Great engine. 48 mph with the 4 blade 17 pitch and 53 mph with the 19 pitch 3 blade. He gives it to me and of course I have to take something sweet and make it dirty. Anyway, I want to do the 140 HP conversion. 120 Honest HP would be good. So I figure I need a few things to make this happen.

1. A 115 or 140 carb set. Bought a complete unit on Ebay for $30.

2. Possibly a reed spacer set. More than likely not needed but I can look.

3. A bubble back exhaust manifold and cover. Ive been looking. I can probably pick one up for $100 ish but cheaper is better. I'm still looking. They do not seem to be in abundance.

4. The block. a 115 or 140 block will do but I'd like to use the 90 block and punch the ports out if that's possible. I have access to a mill and if its just a question of running a cutter into the port and opening up the ports to a larger size I could do that then debur and hone the block. I do not know the offset though if there is one. Is there any port offset? What size are the 90 HP opening and what are the 115/140 port opening sizes?


Has anyone done this to theirs. I know the standard warnings but its something I'd like to do and I'd like some constructive comments or advice from those who have attempted this..

 

[Bill Gohr]

Well you have most of it correct, you need...

Carbs, which ones did you buy, no 115 ones are the same as 140's, but they are close

Exhaust, I have it if you need it

Heads, I have, you can use several

Blocks, are the same, just the ports are different, you can easily report yours or I can mod it for you if you want more than stock

 

[wired]

PM Sent Bill. Thanks

 

[wired]

OK in the last few weeks Ive picked up a 140 and 115 block. Actually I bought a 1981 block and then a complete 140 that needed a block. Both 1981. Blocks are the same. Same dimension ports. same positioning etc. Only difference is the factory milled out cut for the bolt in exhaust "stuffers" . The 115 block is tapped and has bosses for the exhaust stuffers. After cleaning up the 115 block I found that there is no need to flycut anything. The stuffers fit with no height difference as measured with a straight edge. I'll mill it out and square up the ports , get it bored and use new pistons and bearings. Then I can keep the 90 as a spare.

I didnt know a block could have such a sloppy bore until I saw the inside of that 140 block. It was around a 1/8" on either side of the piston to the bore. The block is usable but the 115 block is so much better.

 

[83PREDICTOR]

So what are your real Speed and Rpm numbers?

Over on Scream and Fly you stated:
"Right now I can hit 49 MPH GPS with the 19 pitch 3 blade SS OMC prop at 5800 RPM."
and "My 17 pitch 4 blade SS Solas goes 44 mph at 5300".

Are you using a GPS?

I personally don't see how your crankshaft rated 90, bolted to the transom can get anywhere close to 45mph. My predictor with a prop-shaft rated 115 on a 6" jackplate only ran 53 (on gps) with a 23P.

 

[wired]

So what are your real Speed and Rpm numbers?

Over on Scream and Fly you stated:
"Right now I can hit 49 MPH GPS with the 19 pitch 3 blade SS OMC prop at 5800 RPM."
and "My 17 pitch 4 blade SS Solas goes 44 mph at 5300".

Are you using a GPS?

I personally don't see how your crankshaft rated 90, bolted to the transom can get anywhere close to 45mph. My predictor with a prop-shaft rated 115 on a 6" jackplate only ran 53 (on gps) with a 23P.

Three different GPS's and they agree. My wifes Garmin, My phone GPS and a dedicated for the boat one that all show 49 MPH on the 19 pitch OMC stainless prop and that was with a full tank and a 100 lb. passenger . I'm 210 lbs. I got the 17 pitch up to 45 mph by raising the motor up to the top holes but I don't really like the way its running there so I might drop it down to the second lowest hole again. Speedo is showing 53 with the 19 pitch but I don't believe anything pitot tubes say. No kidding. 49 MPH with the 19 pitch trimmed up until its almost blowing out of the water. I'll try it again Saturday with the motor all the way up on the 19 pitch prop . It might even hit 50. Its always been a good running 90 and Ive had it since it came out the dealerships door and know its complete history. Its all original except for one power pack and a stator that I changed.

FWIW I tried a stainless 23 pitch OMC prop on it 15 or so years ago from my old 140 stern drive OMC seaswirl and lost 5 mph and couldnt get past 4500 rpm. The 90 just didnt have the power to push it into the right rpm band. It ran like crap and I took it off right away. Kind of wish I kept that prop now though.

 

[wired]

So what are your real Speed and Rpm numbers?

Tell you what . I'll take it out Saturday and take a picture of the GPS and tach and post it here. Keep in mind that those numbers are in 40 degree air and smooth 35 degree water. I don't expect that it will be running that fast in the 105 degree air of the summer.

One of the reasons Ive been hesitant to go with a jackplate is that given that Ive seen little improvement raising the motor up a few inches and that by the prop calculators I'm just about as fast as is possible with my setup as is, I have a hard time seeing how raising my motor up another 5 inches is going to do much for me.

 

[teamhart2]

of course I have to take something sweet and make it dirty.

First two things that popped into my head reading this
1. Thats a hell of a life motto

2. I'd like to meet your wife

kidding aside, those are pretty good numbers for a 90, my 85 would run 40 with a 19p and a tail wind, though it was tired, the prop was a not to nice aluminum, and that tail wind was never as strong as it could have been

 

[teamhart2]

oh, also, you gonna put a jackplate on it once you have the extra HP???

 

[wired]

oh, also, you gonna put a jackplate on it once you have the extra HP???

If I cant do it with just prop I might.


I'm going to have to pull one of the side covers off the 90 . Its got a 90 HP engine tag on it but OMC was kind of flaky back in the day and it wouldnt surprise me if they put a 90 tag on a 115 engine if they ran out of 90's that day but is got the factory 1 5/16" carbs on it per the specs and its jetted per factory 90 specs that year.


I often ask my wife where she learned to talk like a sailor and she always give me funny looks.

 

[wired]

I started thinking about it and realized I have a good set of 140 heads sitting here and a new set of head gaskets. What the hell. So I'm in the middle of pulling the heads off the 90 and swapping the 140 heads on there. It is a 90 BTW. Tiny ports. I just cranked it over and checked the compression. Cold it looks like an additional 7-9 lbs on the two Ive checked so far. I'll get it buttoned up today and take her out this Saturday and see how it runs with the 17 pitch. Best Ive been able to do is 45 on that prop and I havent hit the 5800 rev limiter yet on that one. It is an 81 engine but one of the CD packs is a later limited RPM unit so I stop at 5800. Best Ive been able to do with the 17 pitch so far is 45 mph at 5400 or so. Without propping it up I'm not going to get past 50 mph even with the 19 pitch unless I change the CD pack. with that spare engine and few $5 ones I got off ebay I'm set for CD packs.

 

[wired]

I understand this is 90 is a 31 year old engine and I know this things got a LOT of hours on it but I can still see the crosshatch pattern on the cylinder walls. The pistons seem sloppy to me but I havent seen one with new pistons in it to compare it with. Rings are all good or there at least. Heres the heads from the 140 compared with the 90 heads. I'll leave the 140 heads on there until I get the 140 short block built and ready to install. A few horses here and there wont hurt.

 

[teamhart2]

Looking good, that 90 might just be a real strong runner, TOPenticer on here has/had his enticer running a whole lot faster with a 115 than most of us would have guessed

 

[wired]

Looking good, that 90 might just be a real strong runner, TOPenticer on here has/had his enticer running a whole lot faster with a 115 than most of us would have guessed

Yeah, here I was stressing over a couple of PSI light in cylinder #4. I opened it up yesterday and it looked great with no scoring. Should be a little quicker now anyway with the slightly higher compression.

I'm sure the boat being a few hundred pounds lighter since I rebuilt the interior and took all the water logged foam out isnt hurting things. That boat used to go 53-54 ( on the speedometer ) in the mid 80's so nothing much has changed considering that the cranking compression is still around 125 ish in most of the holes even before I changed to the 140 heads.

 

[wired]

I do have a whole extra complete bubble back exhaust setup. Does anyone think I would gain anything by installing it on the 90? Just the outer shell and exhaust manifold. I wouldnt install the filler blocks. I suppose it wouldnt hurt anything but is it going to help? Ive got a few more months to live with this 90 and my hands don't like being idle.

 

[wired]

I managed to make it out today before the sun went down. With the 140 heads I managed to get the 17 pitch prop up to 46 mph at 5700 ish trimmed all the way up on the gauge . Ambient air was 42 degrees . I'll get the 19 pitch on before the weekend and take her out for a spin Saturday. I'm thinking 51 might be just about right but I think I might run out of rpm before I can get her that fast. .

 

[Bill Gohr]

Well get yourself a 21 if you're going to run the 90 with those heads, squeeze it all you want, the powerband is done at 5200 with those small ports, the bubble back woul be a waste on a 90

 

[wired]

Well get yourself a 21 if you're going to run the 90 with those heads, squeeze it all you want, the powerband is done at 5200 with those small ports, the bubble back woul be a waste on a 90

Good point. Those are some pretty small ports. Its probably as squeezed as it will squeeze. The 140 is in the works.

 

[wired]

So what are your real Speed and Rpm numbers?

.

I need a new speedometer

 

[TheNerd]

Pretty interesthing trying to squeeze the ponies out of those motors, but wouldnt it be easier to pick up a 150 or 200 merc. Could sell what you got and pick up a motor ready to go, just bolt on. An old guy once told me if I wanted something faster, sell what I had and buy something faster, trying to make something faster will cost more in the end. He was right. I have learned the hard way a few times. Just my 2 cents.