90 Johnson to 140 spec conversion

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wired247

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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. 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 thats 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..
 
kenmyfam

kenmyfam

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Re: 90 Johnson to 140 spec conversion

The only constructive thing I can say is that when and if you make all these "performance and power" changes you may end up with a lot less than you expected and a lot of money spent. If you read up on the subject here you will see what I mean.
I have "fine tuned" countless 2 stroke motors over the years with good success but never on an outboard. To me they are a different animal.
I honestly hope it all works out for you and wish you every success.
 
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wired247

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Re: 90 Johnson to 140 spec conversion

Ive spent a lot of time out on the water behind very similar boats with different engines and I can tell you from my own experience that there is a significant difference between 90HP Johnsons and 140 HP Johnsons. Night and day sort of difference. From the factory the 90HP is probably underrated and the 140 is definitely overrated but theres probably an honest 25% difference. That 25% difference is porting, intake and exhaust. 25% is a huge difference. This really isnt a fine tuning thing. Its a massive tuning thing but there just arent a lot of parts involved in making it happen. Nothing exotic and maybe $300 worth of Parts. $500 if I have to source a good 115 block. Ive spent $500 on a lot sillier stuff than that.
 
Faztbullet

Faztbullet

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Re: 90 Johnson to 140 spec conversion

Get a 140 block... a lot easier. You will need to enlarge and raise exhaust ports,raise the intakes a tad and cut the exhaust chest and add and machine filler blocks.
 
kenmyfam

kenmyfam

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Re: 90 Johnson to 140 spec conversion

wired247 said:
This really isnt a fine tuning thing.
Click to expand...

Oh yes it is !!!
The fuel delivery, porting, intake and exhaust are all part of it and that is what I was referring to. Get it right and you get what you were trying to achieve. get it wrong and you end up with multiple issues. 2 stroke, the faster the exhaust gets out the faster and more fuel/air mix you can stuff into it. Are you planning to do anything with the compression ??
I have a Johnson 140HP (see signature) and a friend of mine has the same boat with a 90HP on his. You are absolutely right, there is a night and day difference.
Hope everything works out for you I wish you all the very best with your project.
 
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wired247

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Re: 90 Johnson to 140 spec conversion

I'll change the compression if I need to. I'd prefer not to as I'd like to keep it on the low side and if I do decide to wait on the compression change that is just an external parts swap to the 140 heads. I need to find out what the compression ratio was on the factory 140's before and after the HP rating change. From what I have gathered so far the 140's up to 82 ish used the same heads as the 115's and the 90's and it wasnt until they changed the rating system that the compression got boosted up and the bubble butt exhaust system was added. Ive got 6 months more to work out the details and get the right combination of parts . Once the season drops off in later september I'll pull the power head and start working on getting the right combination of parts together. I know the combination of parts needs to be right but none of this stuff is that expensive or that difficult to get. 115's and 140's share a common port sizing from what I gather and after that its just intake and exhaust.

cimg1565l.jpg
 
Faztbullet

Faztbullet

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Re: 90 Johnson to 140 spec conversion

From what I have gathered so far the 140's up to 82 ish used the same heads as the 115's and the 90's and it wasnt until they changed the rating system that the compression got boosted up and the bubble butt exhaust system was added.
Click to expand...
The 135/140 has always had a "bubble back" from day one. The 140 runs a different high compression head and was not installed on the 90/115 motors. The OEM 90/115 heads are obsolete and supersedes to the 140 head as a replacement only and need to be replaced as a pair/set and the 140's run a larger carb.
115's and 140's share a common port sizing from what I gather and after that its just intake and exhaust.
Click to expand...
They are close but not the same.....
 
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wired247

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Re: 90 Johnson to 140 spec conversion

Well I actually picked up an '81 115 block for $50 and a complete '81 140 motor for $170. 115 and 140 porting is exactly the same . No difference whatsoever at least in 81 blocks except for the exhaust rib cutout. Same size and timing. The 115 block I cut for the bubble back and squared the ports up. Just got it back from the machinist . Had it cut for .030" Pro Marine piatons. The 140 block had water jacket damage.

Ive got a set of 1 3/8" carbs from 175 GT V6 and set them up for the V4. I'll use those on the 140. I put them on the 90 HP to get the bugs worked out and got it up to 51 MPH at 5800.

Carson reeds

I'll use either the 140 heads or some 1978 85 HP heads that have been machined for the bigger pistons.

It looks like my September timeline got moved up
 
SparkieBoat

SparkieBoat

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Re: 90 Johnson to 140 spec conversion

I assume you must enjoy tinkering with motors like that. I thought 120-140s were a totally different block than the 90-115s. once you pass a 115 you may as well go to a 150-175 because all the weights that I have seen show the 120-140 the same weight as the 150-175.
My dad has a boat with a 130 on it, and it runs out really strong, so they are great motors, I just am all about HP to weight ratio. If I am going to put a 375 LB engine on my boat I want all the HP you can get out of that weight.
 
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wired247

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Re: 90 Johnson to 140 spec conversion

Before they changed the HP rating standards the 140 was the same block and crank/pistons as the 115. They all weigh in at at 290 lbs for the short shaft. Then in the mid 80's the newer looper 140 was introduced and the old 140 was called the 115. A lot of people say the difference was the crank rating VS prop rating of the HP and that is true to an extent but the crossflow 140 was also detuned with smaller carbs, lower compression etc. when they started calling it a 115. OMC HP numbers were always to be taken with a grain of salt anyway. They made major changes and never updated HP ratings . What I'm building is a 1979 spec 140 with higher compression and enlarged ports and better reeds. It will more than keep up with the later heavier 140 and beyond motors.

I'm hitting 53 MPH on the water with the old 90 using a 23 pitch prop. I don't think an extra 50 HP or so is going to hurt me much.
 
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boobie

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Re: 90 Johnson to 140 spec conversion

I guess I'm just old and lazy. For all that work, I'd just buy a bigger mtr. LOL
 
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dazk14

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Re: 90 Johnson to 140 spec conversion

I guess I'm really interested in the bigger carbs you used with the 90hp and the performance differences... and MOST importantly the jetting specs - which is no small task - not that it's running on the ragged edge (which makes the task easier).

Speaking of ragged edge...We've built more than our fair share of ported, modified "Jetski" motors and the jetting was always a ton of work...while using a VERY expensive knock sensor (a must) that didn't like to get wet...
 
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wired247

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Re: 90 Johnson to 140 spec conversion

dazk14 said:
I guess I'm really interested in the bigger carbs you used with the 90hp and the performance differences... and MOST importantly the jetting specs - which is no small task - not that it's running on the ragged edge (which makes the task easier).

Speaking of ragged edge...We've built more than our fair share of ported, modified "Jetski" motors and the jetting was always a ton of work...while using a VERY expensive knock sensor (a must) that didn't like to get wet...
Click to expand...

To be honest I'm really not using a "bigger" carbs on the 90. Right now I have 1 5/16 carbs on there from the 1981 140 HP engine. Only difference from the 90 is they are a few sizes richer on the mains and idles jets. The 90 in that year used 1 5/16 carbs from the factory. I put the 140 carbs on there and it runs the same top end but the idle is smoother with the boyesen reeds and throttle response is nothing short of amazing. I did put the 1 3/8" carbs on there but it ran about the same . Those 90's have so much low end torque its hard to feel a ot of difference if you overcarb it. It just goes a little faster on the top end. 1-2 mph. Best Ive had it was 53 mph and thats with the motor just bolted to the transom . Nothing fancy. Biggest change Ive done to the 90 was bolting on the 140 heads. Gave me 7 or so psi cranking pressure and woke it up pretty well.
 
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wired247

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Re: 90 Johnson to 140 spec conversion

boobie said:
I guess I'm just old and lazy. For all that work, I'd just buy a bigger mtr. LOL
Click to expand...


Work? What else am I going to do? Sit and watch TV? I completely gutted that boat to the bare hull and rebuilt it doing all the work including sewing the interior in 3 months. A little work here and there and I have a new 140 for a grand or so. Ive built so many high performance and race engines over the years that I hardly even think about it as work. Just a pleasant pastime.

cimg1567k.jpg
 
SparkieBoat

SparkieBoat

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Re: 90 Johnson to 140 spec conversion

yea 50+ MPH on that boat would be expected. nice hobby.
 
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wired247

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Re: 90 Johnson to 140 spec conversion

dazk14 said:
Speaking of ragged edge...We've built more than our fair share of ported, modified "Jetski" motors and the jetting was always a ton of work...while using a VERY expensive knock sensor (a must) that didn't like to get wet...
Click to expand...

Its been my experience with carburetors over the years that the larger the venturi, the lower the signal to the jets. Big carbs are much easier to jet and tend not to wander much from the original jetting when mods are done to the rest of the engine. They are also much less responsive on the bottom end. A small venturi jet ski carburetor is going to have a lot of jetting issues that an oversized carb boat or car engine won't.
 
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wired247

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Re: 90 Johnson to 140 spec conversion

Started it up for a 10 minutes or so today. It was getting late and I didnt want to **** the neighbors off too much. I was expecting a lower tone from the exhaust with the opened up 140 ports . Nope. Smooth as silk at idle and sounds like a chainsaw. It was leaking a bit from the cylinder head covers so I'll have to pull those off and seal all that up with goop. Right now Ive got the 1 5/16" carbs on there but I think I'll jet up the 1 3/8" carbs a bit and install them . They ran a bit lean on the 90 and by lean I also mean really strong on the top end.
 
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