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johnson 70 part throttle hesitation/tuning issues

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  • johnson 70 part throttle hesitation/tuning issues

    '82 Johson 70 3 cyl.. I had the original carbs cleaned out and rebuilt professionally, new fuel lines and all that. I'm pretty mechanically inclined but outboards are pretty foreign to me, but I do have the factory service manual for the motor. Its a little cryptic and conflicts itself here and there but my main issue is that my idle is a little rough when warm, and under part throttle the motor wants to die. At 5 mph heading out of the mooring field it runs perfect but add a little more throttle and the motor does not like it at all, but if I manipulate it a bit and push through the hesitation I can get the bow up and cruise no problem, and full throttle is no issue either.

    Boat doesn't have a tach so I'm doing most of this by ear. I keep reading posts on every forum about linking and syncing the carbs and how thats the issue but it seems like no matter how I set it up I get a stumble. I don't believe my base timing is off but I did not check the motor with a timing light. Another thought I had after putting the boat away after the weekend was maybe I'm getting a weak spark? I never checked the plugs for spark or to see if I was running rich or lean. These carbs don't have any adjustments on them. Comfortable cruising speed was ~24 mph and I was able to get the boat up to 30 just myself full throttle (1970 17').. not sure if thats where I should be at or not or if thats too slow and something is definitely wrong..

    Anyone with experience with these motors have any input?

  • #2
    have you checked the compression? is it firing on all tree cylinders?

    if that's ok you may have a vacuum leak or plugged mid jets and/or air bleeds in your carburetors

    you are going to want to install a tach, reasonably inexpensive and very useful
    76 25 \"Aenigma\"[br]73 17[br]Richland/Long Beach, MS[br]http://www.classicmako.com/forum/topic.asp?TOPIC_ID=14182[br][br]17 project[br]http://www.classicmako.com/forum/topic.asp?TOPIC_ID=56176

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    • #3
      Doesn't sound like theres a vacuum leak. I never did check for spark on all 3 cylinders, either. My father in law has an old tach and dwell meter I'll have to borrow to check it out. I'd be hard pressed to believe that the rebuilt carbs are clogged but it isn't impossible.

      Once I'm sure the motor is running good, the boats going to get a whole new gauge pack.

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      • #4
        Just my 2 cents, but it sounds like the same issue I had with my Mercury 800 1972 outboard. The merc had an adjustment screw on the carb for idle. I had the same hesitation at low idle and with giving it throttle. I readjusted the idle screws and the problem stopped. I think I ended up opening the screws 1/4 turn, and that did the trick. Not sure if Johnson has the same setup.
        Mark Kriz[br]Mako15 1974[br]Mercury Thunderbolt 800 1972[br]Maple Valley, WA[br][br]

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        • #5
          When it doubt, if it's firing on all cylinders (easy enough to test, pull each plug wire 1 at a time at idle and listen for RPM drop), it's the carbs.

          Go through them again, focusing as someone said on the mid-range jets. Use a wire and clean out all the orifices.

          The fact that it hesitates mid-range but clears up at WOT eliminates a fuel restriction before the carbs, methinks.
          ROGUE I[br]1978 235 CC[br]Newburyport, MA[br]ROGUE II[br]1987 17\' Montauk[br]Camden, ME[br]

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