solid truth (90 to 115 conversion (1983)

[nac983]

ok,
so i have read all of the posts and replies on the issue of converting a 90 h.p. johnson to a 115.
i don't think i am confident that there is any solid proof of what needs to be done here.
so i want to start a new post with replies from only 100% confident mechanics.

so lets settle this for at least a 1983 90 hp
1. my 90 has a flat back exhaust cover and so do the two 83 115s that have witnessed. could i take those carbs off the 115s and put them on my 90 and have a 115? or if not will i see any improvement with no other mods or am i looking at trouble?
2. does the bubble back 115 have any (deflectors) or any other parts that my local salesmanistic mechanic suggested they had. or was he just trying to get me to buy a motor off him? and why do the two 115s that i've seen have flat back exhaust covers? what else could fit in there that the 90 doesnt have?i assume the bubble is somewhat like the expansion chamber on a 2-stoke dirtbike?
3. and finally,
can i just throw the 115 carbs on there and get some kind of gain without changing the heads and without doing any porting......as far as i know from my manuals the bore, stroke, timing, crank, pistons, and every single other part that i could posibly compare between the two are identacle with an exception to the block.

i begg of someone to read this and clear things up for e for i think it would benifit many, and clear up any confusion.

thank you
nick.

 

[R.Johnson]

Re: solid truth (90 to 115 conversion (1983)

You could gain more from proper boat set-up, and proping, than what could be an exspensive change over. As for if it could be done, I would have to go over both parts manuals. Doing that, and comparing part number's,and looking for part number changes would take a lot of time.

 

[tashasdaddy]

Re: solid truth (90 to 115 conversion (1983)

imho, it is a good way to screw up a good motor.

 

[Lion hunter]

Re: solid truth (90 to 115 conversion (1983)

Why would you want to?

For one think I would guess that different block and exhaust design allows for a higher compression and release of gasses. There was probably some brilliant engineer at Evinrude that figures out how to use the same tooling for the 90hp to make a 115. So that is why they are very similar. But if the 90hp has no provision for that excess combustion and compression(Bigger carbs = more fuel and air= more compression) then you are likely to have o detune the carbs or risk blowing a head gasket. I know that if a motor manufacturer wants to use the same motor and get less HP out of it that they just put a restrictor plate behind the carbs and in the exhuast. Motorcycle manfacturers have been doing that for years.

My 2nd point has always been (this goes for the 9 to 15 conversion as well), that if you figure how much this conversion will cost you -vs- if you sell the smaller motor and put the money toward the other you would probably be money and time ahead.

Oh yeah, anything can be done just apply a little american ingenuity. Rube Goldberg proved that for us.

 

[daselbee]

Re: solid truth (90 to 115 conversion (1983)

I was part of this discussion a few weeks ago, and I was convinced by a private e-mailer that the intake ports cut in the cylinder sleeves were smaller on the 90.
That follows, as the two blocks have different part numbers. I have not seen or measured the ports myself.