Thread: Johnson "70mod"

  1. #301
    Team Member hupiveneilija's Avatar
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    If its rich you need to go larger jet on top and smaller bottom jet. Upper is airjet, bigger, the leaner idle&middle range. bottom is fuel jet, bigger and fullthrottle goes richer.
    Thats situation with blacktops.

    Older uses topjet as fuel jet, larger, the richer idle&middle range BUT ONLY IF ITS BEHIND SCREW, if not then its air jet as above. Bottom does the same as above.

    Thing is its rich from the factory so it lasts longer with different users, read as dont seize pistons with too much of a heat because poor cooling.
    If you want it to go little crispier you have to lean a little not make it richer and give 2to3deg adv in ignition timing but that nessecary dont make wonders, also have to be carefull with heat on that thing!

    These are my thoughts, feel free to correct if needed.




    BTW has anyone ran CCMS reeds without stops like Boyesen tell you to do with theirs? IMO spitback is more less with Carson reeds than Boyesens.



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  2. #302
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    no chance of altering timing , that is a very bad idea.. omc altered it and then put it right. it will be staying as it is.


    what I want to do is go bigger on bottom and top,
    if I go to 60D on top this is 5 up and 52D at the bottom it will be just 2 up.. therefore will be very slightly leaner.
    none of the oval ports had the old style carbs, they all come with the updated black top carbs.

    Cooling of the motor is fine as its got all the OMC cooling mods done at the factory. different exhaust cover, different tell tale placing, different
    thermostat and spring and also different water pump impeller and plate. this gives very food cooling .

    i would prefer to go to the 1995 jets for the 70hp, this then makes the carbs right for the 1995 onwards 70hp motor...
    .
    it is just going to be a bit difficult to find the jets.
    i will keep watching on ebay, they are far too expensive to order from brp ,especially if they dont work.
    might just buy a set of proper jet drills and drill out some spare jets i have for the 60hp carbs..

  3. #303
    Team Member hupiveneilija's Avatar
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    You need to move more air through the engine then you may benefit that bigger mainjet power wise. It may accelerate little weaker when put bigger airjet because these like ro be rich side when accelerate in middle range.
    Factory did bigger main because of cooling the engine in high rpms and in the same time made air bigger so fuel consuption dont go high when cruising speeds.

    I would try smaller mainjet but leave air untouched.

    And if you get that rev close to 7k it leans just a little with stock jets to be good enough.

    I have now 52 main and 36 middle jets on whitey and it runs real nice up to 8000, gets enough fuel to run over 7500 allday long. I could run it 50 main but now its safe as said.

  4. #304
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    hang on a minute
    what carbs are you talking about with 3 jets ?

    I thought you cooked the white motor ?

  5. #305
    Team Member hupiveneilija's Avatar
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    Quote Originally Posted by hupiveneilija View Post
    BTW has anyone ran CCMS reeds without stops like Boyesen tell you to do with theirs? IMO spitback is more less with Carson reeds than Boyesens.
    Anyone?

  6. #306
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    CCMS reads and boyesen reeds are made differently .
    I used boyesen a long time ago in bikes and would never use them again, they did not last very long the ones I had on mx bikes. and I did not realy notice a power gain over oem steel to be honest.
    but nearly everyone on the boat forums prefer ccms to boyesen ?

  7. #307
    Team Member hupiveneilija's Avatar
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    Went today for a testrun with blue powerhead and short tuner. Its awake now, accelerates from 45kn to 57kn like never before and run real smooth, it feels more relax now. It still got "little carbs" with 56 jets as before when it went that 57.5kn, it takes that speed quicker now. Have ign 21* instead of 25😂😂.
    Prop might slip a little more than before its really good otherwise now for drivability.
    Take same speed @7900rpm. Short of pitch/prop.

  8. #308
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    What cyl head are you using on this blue engine one now ?
    and exactly what prop make and size did you use at 7900 rpm ?

    what is compression now on cold engine cranking with no oil bores to make false readings ?

    you mean 60hp carbs or what ? what are they from ?

  9. #309
    Team Member hupiveneilija's Avatar
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    Good morning

    Im still using that same welded one. Carbs are stock 70hp 1992 model, "little ones". Think its still a little rich with those 56jets.
    Meant to measure comp yesterday but forgot it, its there where it has been before around 130to135.
    Still using that same srx, modified a little, AGAIN!

    Next will put those bigger in place and try AGAIN! Have hopefully sorted couple problems around that.

  10. #310
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    ok lets get this straight so anyone coming in now can see what you are using.
    you have a 1992 56 cu in block 1992, block deck height is original.
    you are using small carburettors ( small carbs means 60hp carbs)...
    exhaust ports taken to sst60 spec.
    it still has the idle reliefs.
    cylinder head has been shaved, welded, milled and now has narrower but deeper combustion chamber taking spark AWAY from piston and is just 21 cc.
    you run on normal pump gas that is around 91 to 93 octane... that is the usa equivalent to Finland 95/98 E5 or E10
    you use a over hub omc silver streak modified to give more pitch so atleast 24 inches of pitch.
    you run a standard long shaft 20 inch mid still with original exhaust with the can connected to its normal club foot with 2.4 to 1 gears ratio.
    the Boat is 17ft long and without driver weighs atleast 780 pounds..
    The boat jumps on plane and roars to 65.5mph at 7900 rpm ...( with 2 people in it )
    you have now shortened the tuner and get quicker response from mid range to full throttle.

    and you think that your motor will pull a 27 inch pitch prop.

    .

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