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Thread: Yamaha 75 to 90hp conversion

  1. #1
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    Default Yamaha 75 to 90hp conversion

    Hello all.

    im a new member here.
    looked for the new member tread but as luck would have it, i could not find it.

    in any case, im from the dark South Africa, lol
    we still keeping heads above water with our people in power.

    we are about 1600meters above mean sea level.
    i do fishing from my boat and its a travel of around 585km to the coast, depending where i want to go fishing.
    we do surf launching through waves at certain spots.

    the boat is placed down onto the ground and it is pushed into the surf via a tractor and a push pole.

    so it is a hair raising event, lots of adrenaline in such a launch.
    this means you need power to run to get onto the plane and if needed you need to turn around and run back to safe ground..

    it is common for our boats to be over powered so fitting 100hp on a 18,6 ft hull is not uncommon.
    but we all know that a overpowered boat is a danger if not handled correctly.
    the boat also does ride different

    this brings me to my post:

    i have a sea cat 18,6ft cat hull boat which is fitted with 2x 75a hp 2 stroke engines.
    upgrading to 4 strokes is not financially do able and to spend R 200 000 for 3 to 4 trips a year is not cost effective.
    i would rather use the extra money spend on fuel (2 strokes using more fuel) and use it over time.

    so i did some research regarding my motors.
    there are small differences between the 75hp, the 85 hp and the 90hp 2 stroke Yamahas.
    it is the same sub assembly, with piston size and stroke wise.
    the 85hp motor is a very common motor in south Africa, they are bullet proof and they are gas guzzler's.

    our fuel that we use is not of a concern, so there should not be any worries.

    it is common knowledge that a subassembly is used to market different hp power outputs.
    the chocking of the 25 to 30hp engines shows this as well as the carb stoppers.

    comparing the 75 to the 85 few differences exist.

    according to the manual

    75 hp compression is 4,5:1
    85 hp compression is 5,1:1
    the 90hp compression is 5,9:1 (this is what i could find on the net). not much available on the net regarding the 90hp

    the strokes and bore size are all the same between the 3x different motors.

    also according to the manual (75 and 85)

    the reeds are restricted on the 75hp (opening maximum to 3,2mm) where as
    the reeds on the 85hp is 10,1mm maximum.

    carburetor wise there are not much differences: throat size is unknown on 85hp

    75 hp main jet is 175
    85hp main jet is 165
    air is the same at 180
    pilot is 78 and 80
    pilot air jet is 100 on both.
    the 85 hp main nozzle is 3,4mm compared to the 75hp being 3,2mm.

    the fuel usage per hour is 34 liters for the 75 and 34 liters for the 85hp per hour
    so with this present it is clear that there is not such a big difference between the carbs

    how much hours are on the engines is unknown.

    so i would like to get the 10hp or 15hp per motor as reliability is paramount.
    i have read that these motor can be upgraded wildly but this is not my aim.

    could you guys please jump in and advise what is recommended for these engines.

    i am not a newbie to 2 stroke engines.
    i have done some research over the years and i do understand them.
    Jennings and G Bells books are nice, it was hard to understand them initially so a reread was needed.
    the research on these engines have also taken some time. so i would say that i am more up to date with them.

    i do not have port timing maps available for these motors.
    i also do not have a scrap motor to check and measure the port timings.

    this is where your guidance and experience comes in.
    i have also listed the same tread on another forum but there is not much info forth coming.

    my other biggest concern is getting a engineering shop who knows and understands squish.

    my thinking of the motors are:

    opening the reeds of the 75 hp to the 85 hp spec (already done)
    skim the heads in order to get compression up to 6:1 maximum.
    then we need to cut the squish back in, at what degrees 2 or 3? or what is needed?

    then work the exhaust port "wider" size is unknown, but not higher.
    if i needed to lift the exhaust port, how much do i need.
    i was thinking more of skimming the head and instead of cutting the squish into the head, to cut the piston tops off for the squish.
    then i do not need to raise the exhaust ports as the piston already raised them.
    but this might bring compression down a bit. that might just put me into the 5,9:1 compression.

    do i need to work on the transfer ports as well?

    is there any restrictors in the exhaust system which i need to know of and remove.

    i have read that the guys also cut finger ports in, but is this needed?

    the last thing that i want is to go too far and screw it all up on the cylinders.

    do you guys have any pictures of porting on these motors?

    please assist me.

    thank you.
    Likes hydrospeed77 liked this post

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    By the information you've put forth, you have a better understanding of the nuances between the 75 and 90 than most people. Did you check to see if the exhaust tuner is the same between the 3?


    -Jim

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    Hi Jim

    thank you for your reply.

    yip i have done quite a bit of research on the engines.

    according to the parts list, the heads and cylinder sections are different between the 75 hp and the 85hp.

    thus i suspect that the higher compression is due to a smaller head on the 85 or that the pistons deck height on the cylinders are different.


    what is the chances that the exhaust port open sizes is different.
    i do not have a old engine at hand in order to check the port sizes etc.

    this is info which i am looking for.

    we do the following type of launching when going to fish.

    https://www.youtube.com/watch?v=UsAxJRcLeH8

    so ja, the extra 30hp will be welcome for these launchings.

    2x guys up is one thing, being 5x up is a extra 300kg which will delay you a bit.
    Last edited by Willem Wikkel Spies; 03-11-2021 at 02:23 AM. Reason: video link corrected

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    Quote Originally Posted by hydrospeed77 View Post
    thank you
    will have a look at the tread.
    just came back from a week long fishing trip.

    on our 2nd launch we had some water through the window, so the extra 30hp would have been welcomed.

    but other then that, the boat is still running very good.

    i will still keep on doing some research.
    this is a project which i will not rush.

    stay safe!!

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    i see that the video was removed.
    so we uploaded it and listed the new link.

    this is how we do it in south Africa when no harbor is near

    https://www.youtube.com/watch?v=UsAxJRcLeH8

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    the research on these motors are still continuing.

    there is so much to take notice of especially on other treads of what the guys did and what should not be done.

    if you do decide to tinker with one side, you need to take note and possibly make changes on the other side.

    can anyone advise if these engines have rev limiters build into the cdi units.
    not that i want to cut it out, but a recommendation was to:

    i did read that one had to change the exhaust port timing to 90 deg after top dead center.
    well this would then bring it up to 180 degrees of exhaust port timing which will push power higher up in the range.
    so whats the use of doing it, if the rev limiter prevents it from running higher up in the ref range?

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