Thread: Johnson "70mod"

  1. #181
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    little charger is right, its the depth of the pocket or depth of combustion chamber how ever you want to say it.
    if you had a low compression head to start with you could be fine ?


    you just need to measure the depth of the chamber on this now ?

    I will also check out what my spare head is today as well, mine is the later version head part number omc 339221.
    will be interesting to see how deep that is at the moment.

  2. #182
    Team Member hupiveneilija's Avatar
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    Quote Originally Posted by LittleCharger View Post
    I think based on what you have removed pocket depth is probably around 0.500, stocks heads vary so you could be on either side of this hence the reason Stephen refers to the pocket depth end result. I would suspect that Stephen's motor was decked to at least the sst60 specs as well.
    LC you are almost spot on. Chamber depth is 0.513 now. But you have to leave something material to the thermostat opening. How much is it, somewhere on Hpbc site is mensioned 0,1in before it starts warping more easily. But hardcore racers migth change the head more often than pleasure boaters, thats why I left 0,1in there.
    Combustionchamber is 5cc bigger than my small one so psi migth be somewhat lower.

  3. #183
    Team Member hupiveneilija's Avatar
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    Quote Originally Posted by Steves Jem View Post
    Rather than referring to the amount that is taken off, all should be referenced to the standard SST60 head which has a combustion chamber depth of 0.450 in and allowable limits of +or- 0.010. When I use to race the modified 56ci had two heads that measured 0.400 and 0.380. These had compression over 200 psi but trying to remember 20 years ago is a bit difficult. I think it was around 220 psi. While the .400 head was fine the .380 started to crack and warp.

    Stephen
    With so minimal depth what happens to that thermostat hole?
    I would like as much comp with my ex duration as I can.

    Youre not using normal pump gas on those psi readings, right?

  4. #184
    Team Member hupiveneilija's Avatar
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    Okay, question. Now with those "smallbigcarbs" i have 56 jets in use but with 1,50in bigcarbs should i start some 60 jet size? Or go as big as sst60?

    Does anyone know do all these carbs have same floatvalve seat diam? Because if main jet is bigger than floatvalve its supposed to empty bowl. Am i right?

  5. #185
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    Where the main jets are the seat is the same size.
    these do not fit like a adjustable needle, they thread in and just have the hole straight through them.

    with 1.5 inch throat at rear they are the carbs you should use... they have larger venturi.
    if using the older style big carbs then jet similar to sst60 if you have taken the porting to sst60 specification.
    you have basically turned the block into sst60 anyway. apart from not shaving the deck.

    so jet similar to SST60 if you have compression over 160. over 160 compression needs higher octane fuel. higher than you
    can buy in finland at car petrol stations.
    your car fuel is low octane 95 e10 and 98e5 .... 95 e10 for example is only realy about 89 usa octane and finland 98e5 is about 93 usa octane.
    you need real 100 octane or more if over 160 psi...
    if you get a reading of say 200 psi then you will be needing special race fuel, and that is going to cost you a fortune in finland..

    .
    Can you explain a bit more about thermostat problem ?..... I am not understanding what you mean
    .
    .

  6. #186
    Team Member hupiveneilija's Avatar
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    Quote Originally Posted by filthy phill View Post

    .
    Can you explain a bit more about thermostat problem ?..... I am not understanding what you mean
    .
    .
    You should open once one of these engines and have the parts on Your hands to look at them properly. Then You understand.
    Ive read things here and there with tought and understanding what Ive read.

    Oh btw Im wrenching my o/b at the time. Means that Im doing things.

  7. #187
    Team Member LittleCharger's Avatar
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    Quote Originally Posted by hupiveneilija View Post
    With so minimal depth what happens to that thermostat hole?
    I would like as much comp with my ex duration as I can.

    Youre not using normal pump gas on those psi readings, right?
    we don't run a thermostat but still enough meat in the head at the sst60 spec and even steve's depth's. I have one head that has 1.5mm at the thermostat hole

    if you run the bigger carbs step the jetting up to 72 or 73 and do a quick test pass check your jetting and work your way down if need be, fat is your friend.

  8. #188
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    Quote Originally Posted by LittleCharger View Post
    we don't run a thermostat but still enough meat in the head at the sst60 spec and even steve's depth's. I have one head that has 1.5mm at the thermostat hole

    if you run the bigger carbs step the jetting up to 72 or 73 and do a quick test pass check your jetting and work your way down if need be, fat is your friend.
    Well I have 2,6mm there and pocket depth is that 0,513=13mm. If I take it to the 0,440=11,2mm that leaves 0,8mm material thicknes there. Is that too little?
    I measured psi under 130 on every hole with 1,2mm gasket. Now my small head with 1mm gasket 145to147. Was with 1,5mm gasket, quite squish!
    Remember i have ex dur 180*.

    Think could use little more fuel?
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  9. #189
    Team Member Hevi Kevi's Avatar
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    I'm not sure what big bore carbs you're planning to use but I thought I should let you know the jetting could be different from one to the next. The SST-60 style carbs (which if I remember right are the same as the '86-'89 70hp carbs) and the older 75hp carbs are jetted about the same but the newer 70hp carbs have an extra adjustment screw on them up by the base and take a much smaller main jet. I don't know exact numbers but likely around 10 jet sizes.

    Kevin
    Critchfield 15" 56ci OMC
    RapidCraft Cyclone 15" 56ci OMC


  10. #190
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    Quote Originally Posted by Hevi Kevi View Post
    I'm not sure what big bore carbs you're planning to use but I thought I should let you know the jetting could be different from one to the next. The SST-60 style carbs (which if I remember right are the same as the '86-'89 70hp carbs) and the older 75hp carbs are jetted about the same but the newer 70hp carbs have an extra adjustment screw on them up by the base and take a much smaller main jet. I don't know exact numbers but likely around 10 jet sizes.

    Kevin
    Those are i think from older 2cyl 60hp. Have removed chokeflaps and modified needlescrew in place of idle jet. Original with 56 jets. Those up pipes inside the bowl has much to do every invidual carb model i guess. But what size to start?

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