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Thread: OMC Black top carbs for 56 cu in motors.

  1. #1
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    Default OMC Black top carbs for 56 cu in motors.

    Well today o managed to get off my fat arse and get a few bits n pieces done while it is dry outside..
    while doing that I took out the jets from my Black plastic top carbs, this is 70hp carbs for the omc 56 cu in OVAL port motor not the Bridgeport.
    these carbs only have the two jets in each one.

    I have seen many places showing the jet sizes for these and it all gets a bit strange by the different years.

    with same carb body and for the 70hp these are the years and what they normal fishing motor comes with as standard.


    70hp
    1993 55D top jet & 53D main jet .so the main is smaller than the jet at top of carb !!!
    ------------------------------------------
    1994 55D or 60D at the top with 53D mains depending on engine suffix ?

    1995 to 2001 60D top jet & 52D main jet ..these years the carbs have smaller main than the top's ?

    so I take mine out to check them and then find that I have a totally different set of jet numbers to confuse me even more.

    In my carbs I have 55D in the top and 50D in the mains ? wtf is going on here ?

    what do I realy need for more performance.
    with the above jets the spark plugs have the correct tan colouring to them, but I think the motor lacks a touch of snap on the top end.
    I am definitely not getting anywhere near the rpm I would expect from this motor.
    it runs fantastic starts as quick as you could ever wish for a motor to start, not too bad for smoke even on idle it dont smoke much at all.
    the motor is running nice and smooth with no spluttering.
    it picks up from zero to plane extremely quick, the mid range is also brilliant, but top end seem to be lacking as it dont like going over about
    6,000 rpm
    it had almost identical top end with the smaller 60hp carbs and much smaller jets ?

    timing is 17 deg compression 145-147-144 checked today.
    fuel pump is now the older style and running pre-mix at 50 to 1 with 91 octane( thats what you would call it we sell that as 95).

    any thoughts on what would be the best size jets to replace these with for a bit more top end . not worried about more bottom, thats fine.

  2. #2
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    if putting 55D as the main jet, what would roughly be needed for the top jet ? would this then need to be 63, 65 ?
    I am not getting my head around this jetting stuff to be honest.
    and prices over here for jets is not funny by a long way. so need to be getting it almost right due to ridiculous prices.

    I am thinking put the top jets 55D into the bottom mains, then either getting bigger jets for tops or drilling them out. (old spare set ).

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