85hp to 115hp conversion

wojciecj

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Apr 14, 2013
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Okay guys, Ive got a good running 1975 85hp Evinrude that I've got on the back of my ski boat. I have been getting into barefoot skiing lately and the old 85 needs to be at WOT to keep me up. I have a 1973 115hp Johnson lying around that I've been using as a parts engine (there is a bad cylinder head and crankshaft bearing). Can I just swap the carbs to get more top end power? I know the throat size goes up but do I need to do anything with the exhaust end? Ultimately, I just want an extra 5mph out of it.
 

racerone

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Dec 28, 2013
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But you may find that the 85 hp is a smaller displacement engine.-----The engines look the same but they are not the same.
 

Chinewalker

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In a word, no. If you need more power, rebuild the 115, or find a bigger motor...
 

wojciecj

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I'm aware the 85hp engine is 92 cubic inches and the 115 is 99 cubic inches. What is the rational for not swapping carbs if any? My thinking is that with the current set up there is a certain amount of air/fuel mix that goes into the cylinder with each stroke. If I increase the volume of mixture going into the cylinders won't I get more power? Also what repercussions (if any) could there be to the engine if I do this? Is the worst that could happen extra carbon build up and bogging down at idle?
 

jakedaawg

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My thinking is that with the current set up there is a certain amount of air/fuel mix that goes into the cylinder with each stroke. If I increase the volume of mixture going into the cylinders won't I get more power?

It's not that simple. Too many things to explain in a post but ... Port timing is a big one. Just because you put a bigger carb on doesn't mean you will get a larger charge into the cylinder.
 

F_R

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Jul 7, 2006
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Carburetors only feed in the amount of fuel/air mix that the powerhead draws in. Much confusion arises from the fact that on SOME motors, power is REDUCED by a smaller carburetor that restricts the flow. For instance, a 35 might be restricted down to a 30. But that is NOT the case with your 85 to 115hp proposal. The 115 is a bigger engine. End of story.
 

jakedaawg

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btw, I used to do a lot of footing...to the point that we developed calluses on our feet...not because they hadn't invented the ski yet, but close...we used inboards.

With that said we did use a 115 yami on the back of a 16' senza one season but you had to go off a knee board or a ski. The 115 on a super lite little Senza could not even come close to allowing a water start. Just my humble opinion. Heck, we even had to do a half circle with boat to get it on plane while keeping the line tight and then straightening out to pull up a guy on a comp slalom ski.
 

emdsapmgr

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I'd go ahead with the carb swap. I put a set of 125 carbs on a small bore 100 hp from 1972. Of course the 125 carbs overfueled the block at the start. I was able to rejet the 125 carbs down somewhat and wound up with a net rpm gain over the stock 85. Wound up with about 250 rpm's increase from the new carbs. That was enough to let me run a prop that was one size larger. Ran if for years and was happy with the new holeshot and the top end. Likely the 115 carbs will run on your 85, but you may find it needs to be rejetted somewhat (smaller main jets) in order to get the performance you desire. You could also consider putting 4 rubber intake stuffer blocks in your engine. From a 79 115/140 crossflow. They are NLA, but will net 3-5 hp on your block. p/n 322684.
 
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